diff --git a/.eslintrc.js b/.eslintrc.js
index 3d890dcfc8e9..b3ca14ff51e5 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -336,5 +336,13 @@ module.exports = {
     "rules": {
       "no-async-promise-executor": "off",
     }
+  },{
+    "files": [
+      "toolkit/**",
+      "browser/**",
+    ],
+    "rules": {
+      "mozilla/prefer-boolean-length-check": "error",
+    }
   }]
 };
diff --git a/browser/actors/PageStyleChild.jsm b/browser/actors/PageStyleChild.jsm
index d05acd9c654c..76149e7d31dc 100644
--- a/browser/actors/PageStyleChild.jsm
+++ b/browser/actors/PageStyleChild.jsm
@@ -104,7 +104,7 @@ class PageStyleChild extends ActorChild {
       }
 
       // Skip any stylesheets that don't match the screen media type.
-      if (currentStyleSheet.media.length > 0) {
+      if (currentStyleSheet.media.length) {
         let mediaQueryList = currentStyleSheet.media.mediaText;
         if (!content.matchMedia(mediaQueryList).matches) {
           continue;
diff --git a/browser/actors/PluginChild.jsm b/browser/actors/PluginChild.jsm
index e3bbe02f6a25..05b5040a6b9a 100644
--- a/browser/actors/PluginChild.jsm
+++ b/browser/actors/PluginChild.jsm
@@ -335,7 +335,7 @@ class PluginChild extends JSWindowActorChild {
           return;
         }
         evt.preventDefault();
-        if (callbackArgs.length == 0) {
+        if (!callbackArgs.length) {
           callbackArgs = [evt];
         }
         self[callbackName].apply(self, callbackArgs);
@@ -351,7 +351,7 @@ class PluginChild extends JSWindowActorChild {
         }
         if (evt.keyCode == evt.DOM_VK_RETURN) {
           evt.preventDefault();
-          if (callbackArgs.length == 0) {
+          if (!callbackArgs.length) {
             callbackArgs = [evt];
           }
           evt.preventDefault();
@@ -803,7 +803,7 @@ class PluginChild extends JSWindowActorChild {
           Ci.nsIObjectLoadingContent.TYPE_PLUGIN
       );
 
-      if (plugins.length == 0) {
+      if (!plugins.length) {
         this.removeNotification();
         return;
       }
diff --git a/browser/base/content/aboutNetError.js b/browser/base/content/aboutNetError.js
index 89b320543681..c89f7e760d72 100644
--- a/browser/base/content/aboutNetError.js
+++ b/browser/base/content/aboutNetError.js
@@ -863,7 +863,7 @@ function setTechnicalDetailsOnCertError() {
 
   if (failedCertInfo.isDomainMismatch) {
     let subjectAltNames = failedCertInfo.subjectAltNames.split(",");
-    subjectAltNames = subjectAltNames.filter(name => name.length > 0);
+    subjectAltNames = subjectAltNames.filter(name => !!name.length);
     let numSubjectAltNames = subjectAltNames.length;
 
     if (numSubjectAltNames != 0) {
diff --git a/browser/base/content/browser-addons.js b/browser/base/content/browser-addons.js
index 6ec1f988651a..6ecf3b5b866b 100644
--- a/browser/base/content/browser-addons.js
+++ b/browser/base/content/browser-addons.js
@@ -301,7 +301,7 @@ var gXPInstallObserver = {
       i => i.addon.signedState <= AddonManager.SIGNEDSTATE_MISSING
     );
     let someUnsigned =
-      unsigned.length > 0 && unsigned.length < installInfo.installs.length;
+      !!unsigned.length && unsigned.length < installInfo.installs.length;
 
     options.eventCallback = aEvent => {
       switch (aEvent) {
@@ -361,7 +361,7 @@ var gXPInstallObserver = {
       );
       notification.setAttribute("warning", "true");
       options.learnMoreURL += "unsigned-addons";
-    } else if (unsigned.length == 0) {
+    } else if (!unsigned.length) {
       // All add-ons are verified or don't need to be verified
       messageString = gNavigatorBundle.getString("addonConfirmInstall.message");
       notification.removeAttribute("warning");
@@ -450,7 +450,7 @@ var gXPInstallObserver = {
 
     PopupNotifications.remove(notifications, true);
 
-    return notifications.length > 0;
+    return !!notifications.length;
   },
 
   logWarningFullScreenInstallBlocked() {
diff --git a/browser/base/content/browser-fullScreenAndPointerLock.js b/browser/base/content/browser-fullScreenAndPointerLock.js
index 7e6309e917e4..28779f2d1809 100644
--- a/browser/base/content/browser-fullScreenAndPointerLock.js
+++ b/browser/base/content/browser-fullScreenAndPointerLock.js
@@ -436,7 +436,7 @@ var FullScreen = {
       window.fullScreen &&
       PopupNotifications.getNotification(
         this._permissionNotificationIDs
-      ).filter(n => !n.dismissed).length > 0
+      ).filter(n => !n.dismissed).length
     ) {
       this.exitDomFullScreen();
       this._logWarningPermissionPromptFS("fullScreenCanceled");
@@ -513,7 +513,7 @@ var FullScreen = {
         this._permissionNotificationIDs
       ).filter(n => !n.dismissed);
       PopupNotifications.remove(notifications, true);
-      if (notifications.length > 0) {
+      if (notifications.length) {
         this._logWarningPermissionPromptFS("promptCanceled");
       }
     }
diff --git a/browser/base/content/browser-pageActions.js b/browser/base/content/browser-pageActions.js
index ff33127353c7..1d0e011dbd22 100644
--- a/browser/base/content/browser-pageActions.js
+++ b/browser/base/content/browser-pageActions.js
@@ -1309,7 +1309,7 @@ BrowserPageActions.shareURL = {
 
     // We cache the providers + the UI if the user selects the share
     // panel multiple times while the panel is open.
-    if (this._cached && bodyNode.children.length > 0) {
+    if (this._cached && bodyNode.children.length) {
       return;
     }
 
diff --git a/browser/base/content/browser-siteProtections.js b/browser/base/content/browser-siteProtections.js
index a1709e5e57d5..01a7ab4d8aba 100644
--- a/browser/base/content/browser-siteProtections.js
+++ b/browser/base/content/browser-siteProtections.js
@@ -416,7 +416,7 @@ var TrackingProtection = {
     // not detect trackers on the page using the basic list, we currently
     // still show the panel. To reduce the confusion, tell the user that we have
     // not detected any tracker.
-    if (fragment.childNodes.length == 0) {
+    if (!fragment.childNodes.length) {
       let emptyBox = document.createXULElement("vbox");
       let emptyImage = document.createXULElement("image");
       emptyImage.classList.add("protections-popup-trackersView-empty-image");
diff --git a/browser/base/content/browser-sync.js b/browser/base/content/browser-sync.js
index 1e64fe448b40..d6b13705e8c5 100644
--- a/browser/base/content/browser-sync.js
+++ b/browser/base/content/browser-sync.js
@@ -795,7 +795,7 @@ var gSync = {
 
     const state = UIState.get();
     if (state.status == UIState.STATUS_SIGNED_IN) {
-      if (this.sendTabTargets.length > 0) {
+      if (this.sendTabTargets.length) {
         this._appendSendTabDeviceList(
           fragment,
           createDeviceNodeFn,
diff --git a/browser/base/content/browser-tabsintitlebar.js b/browser/base/content/browser-tabsintitlebar.js
index 2ede43f3eeb3..2e42ed100657 100644
--- a/browser/base/content/browser-tabsintitlebar.js
+++ b/browser/base/content/browser-tabsintitlebar.js
@@ -74,7 +74,7 @@ var TabsInTitlebar;
       let allowed =
         this.systemSupported &&
         !window.fullScreen &&
-        Object.keys(this._disallowed).length == 0;
+        !Object.keys(this._disallowed).length;
       if (allowed) {
         document.documentElement.setAttribute("tabsintitlebar", "true");
         if (AppConstants.platform == "macosx") {
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 576e1351415b..19ef05d5daa1 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -3882,7 +3882,7 @@ function BrowserReloadWithFlags(reloadFlags) {
     }
   }
 
-  if (unchangedRemoteness.length == 0) {
+  if (!unchangedRemoteness.length) {
     return;
   }
 
@@ -4687,7 +4687,7 @@ const BrowserSearch = {
     }
 
     var engines = gBrowser.selectedBrowser.engines;
-    if (engines && engines.length > 0) {
+    if (engines && engines.length) {
       searchBar.setAttribute("addengines", "true");
     } else {
       searchBar.removeAttribute("addengines");
diff --git a/browser/base/content/nsContextMenu.js b/browser/base/content/nsContextMenu.js
index 8a0bceb8c671..2d93901aa231 100644
--- a/browser/base/content/nsContextMenu.js
+++ b/browser/base/content/nsContextMenu.js
@@ -325,7 +325,7 @@ nsContextMenu.prototype = {
     const { gBrowser } = this.browser.ownerGlobal;
 
     this.textSelected = this.selectionInfo.text;
-    this.isTextSelected = this.textSelected.length != 0;
+    this.isTextSelected = !!this.textSelected.length;
     this.webExtBrowserType = this.browser.getAttribute(
       "webextension-view-type"
     );
diff --git a/browser/base/content/pageinfo/security.js b/browser/base/content/pageinfo/security.js
index 4c357425dee2..09419c37fb83 100644
--- a/browser/base/content/pageinfo/security.js
+++ b/browser/base/content/pageinfo/security.js
@@ -183,7 +183,7 @@ var security = {
     let usage = this.siteData.reduce((acc, site) => acc + site.usage, 0);
     if (usage > 0) {
       let size = DownloadUtils.convertByteUnits(usage);
-      let hasCookies = this.siteData.some(site => site.cookies.length > 0);
+      let hasCookies = this.siteData.some(site => !!site.cookies.length);
       if (hasCookies) {
         document.l10n.setAttributes(
           siteDataLabel,
diff --git a/browser/base/content/tabbrowser.js b/browser/base/content/tabbrowser.js
index 917333c0780e..16eb089a7c74 100644
--- a/browser/base/content/tabbrowser.js
+++ b/browser/base/content/tabbrowser.js
@@ -3320,11 +3320,7 @@
         // Closing the tab and replacing it with a blank one is notably slower
         // than closing the window right away. If the caller opts in, take
         // the fast path.
-        if (
-          closeWindow &&
-          closeWindowFastpath &&
-          this._removingTabs.length == 0
-        ) {
+        if (closeWindow && closeWindowFastpath && !this._removingTabs.length) {
           // This call actually closes the window, unless the user
           // cancels the operation.  We are finished here in both cases.
           this._windowIsClosing = window.closeWindow(
@@ -6279,8 +6275,9 @@ var TabContextMenu = {
 
     // Disable "Close Tabs to the Right" if there are no tabs
     // following it.
-    document.getElementById("context_closeTabsToTheEnd").disabled =
-      gBrowser.getTabsToTheEndFrom(this.contextTab).length == 0;
+    document.getElementById(
+      "context_closeTabsToTheEnd"
+    ).disabled = !gBrowser.getTabsToTheEndFrom(this.contextTab).length;
 
     // Disable "Close other Tabs" if there are no unpinned tabs.
     let unpinnedTabsToClose = multiselectionContext
diff --git a/browser/base/content/test/forms/browser_selectpopup.js b/browser/base/content/test/forms/browser_selectpopup.js
index e35978866bd6..55cfaf78d1da 100644
--- a/browser/base/content/test/forms/browser_selectpopup.js
+++ b/browser/base/content/test/forms/browser_selectpopup.js
@@ -797,7 +797,7 @@ async function performLargePopupTests(win) {
     );
 
     // Don't check the scroll position for the last step as the popup will be cut off.
-    if (positions.length > 0) {
+    if (positions.length) {
       let cs = win.getComputedStyle(selectPopup);
       let bpBottom =
         parseFloat(cs.paddingBottom) + parseFloat(cs.borderBottomWidth);
diff --git a/browser/base/content/test/general/browser_contentAreaClick.js b/browser/base/content/test/general/browser_contentAreaClick.js
index 60b934bce1b0..5fb8132012f9 100644
--- a/browser/base/content/test/general/browser_contentAreaClick.js
+++ b/browser/base/content/test/general/browser_contentAreaClick.js
@@ -291,11 +291,11 @@ function runNextTest() {
     gCurrentTest.setup();
   }
 
-  if (gCurrentTest.targets.length == 0) {
+  if (!gCurrentTest.targets.length) {
     info(gCurrentTest.desc + ": cleaning up...");
     gCurrentTest.clean();
 
-    if (gTests.length > 0) {
+    if (gTests.length) {
       gCurrentTest = gTests.shift();
       gCurrentTest.setup();
     } else {
diff --git a/browser/base/content/test/general/browser_contentSearchUI.js b/browser/base/content/test/general/browser_contentSearchUI.js
index 3c3e4049e3f1..6c8db64bdef1 100644
--- a/browser/base/content/test/general/browser_contentSearchUI.js
+++ b/browser/base/content/test/general/browser_contentSearchUI.js
@@ -746,10 +746,10 @@ function checkState(
       s => s.type == "formHistory"
     ),
 
-    tableHidden: expectedSuggestions.length == 0,
+    tableHidden: !expectedSuggestions.length,
 
     inputValue: expectedInputVal,
-    ariaExpanded: expectedSuggestions.length == 0 ? "false" : "true",
+    ariaExpanded: !expectedSuggestions.length ? "false" : "true",
   };
   if (expectedSelectedButtonIdx != undefined) {
     expectedState.selectedButtonIndex = expectedSelectedButtonIdx;
diff --git a/browser/base/content/test/performance/browser_startup_content_mainthreadio.js b/browser/base/content/test/performance/browser_startup_content_mainthreadio.js
index 836e991cbea7..e97a164479d4 100644
--- a/browser/base/content/test/performance/browser_startup_content_mainthreadio.js
+++ b/browser/base/content/test/performance/browser_startup_content_mainthreadio.js
@@ -400,7 +400,7 @@ add_task(async function() {
     // no I/O marker in that case, but it's good to keep the test running to check
     // that we are still able to produce startup profiles.
     is(
-      markers.length > 0,
+      !!markers.length,
       !AppConstants.RELEASE_OR_BETA,
       procName +
         " startup profiles should have IO markers in builds that are not RELEASE_OR_BETA"
diff --git a/browser/base/content/test/performance/browser_startup_flicker.js b/browser/base/content/test/performance/browser_startup_flicker.js
index 41e508540e42..5f70ba786641 100644
--- a/browser/base/content/test/performance/browser_startup_flicker.js
+++ b/browser/base/content/test/performance/browser_startup_flicker.js
@@ -16,7 +16,7 @@ add_task(async function() {
   // Ensure all the frame data is in the test compartment to avoid traversing
   // a cross compartment wrapper for each pixel.
   let frames = Cu.cloneInto(startupRecorder.data.frames, {});
-  ok(frames.length > 0, "Should have captured some frames.");
+  ok(!!frames.length, "Should have captured some frames.");
 
   let unexpectedRects = 0;
   let alreadyFocused = false;
diff --git a/browser/base/content/test/plugins/head.js b/browser/base/content/test/plugins/head.js
index 366efc00d2da..246e8c5cfbcd 100644
--- a/browser/base/content/test/plugins/head.js
+++ b/browser/base/content/test/plugins/head.js
@@ -471,7 +471,7 @@ function promiseUpdatePluginBindings(browser) {
     if (!elems || elems.length < 1) {
       elems = doc.getElementsByTagName("object");
     }
-    if (elems && elems.length > 0) {
+    if (elems && elems.length) {
       elems[0].clientTop;
     }
   });
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_4.js b/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
index 796508eec975..27375a172d0c 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_4.js
@@ -133,7 +133,7 @@ var tests = [
       let anchor = win.document.getElementById("default-notification-icon");
       win.PopupNotifications._reshowNotifications(anchor);
       ok(
-        win.PopupNotifications.panel.children.length == 0,
+        !win.PopupNotifications.panel.children.length,
         "no notification displayed in new window"
       );
       ok(
diff --git a/browser/base/content/test/popupNotifications/browser_popupNotification_5.js b/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
index 1302f8bb8ff9..11598445518d 100644
--- a/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
+++ b/browser/base/content/test/popupNotifications/browser_popupNotification_5.js
@@ -247,7 +247,7 @@ var tests = [
       let anchor = win.document.getElementById("default-notification-icon");
       win.PopupNotifications._reshowNotifications(anchor);
       ok(
-        win.PopupNotifications.panel.children.length == 0,
+        !win.PopupNotifications.panel.children.length,
         "no notification displayed in new window"
       );
 
diff --git a/browser/base/content/test/popupNotifications/head.js b/browser/base/content/test/popupNotifications/head.js
index a619c93b63da..4ae7657fe00f 100644
--- a/browser/base/content/test/popupNotifications/head.js
+++ b/browser/base/content/test/popupNotifications/head.js
@@ -69,7 +69,7 @@ function goNext() {
 }
 
 async function runNextTest() {
-  if (tests.length == 0) {
+  if (!tests.length) {
     executeSoon(finish);
     return;
   }
@@ -246,7 +246,7 @@ function checkPopup(popup, notifyObj) {
       "main action highlight matches"
     );
   }
-  if (notifyObj.secondaryActions && notifyObj.secondaryActions.length > 0) {
+  if (notifyObj.secondaryActions && notifyObj.secondaryActions.length) {
     let secondaryAction = notifyObj.secondaryActions[0];
     is(
       notification.getAttribute("secondarybuttonlabel"),
@@ -330,7 +330,7 @@ function waitForNotificationPanelHidden() {
 
 function triggerMainCommand(popup) {
   let notifications = popup.childNodes;
-  ok(notifications.length > 0, "at least one notification displayed");
+  ok(!!notifications.length, "at least one notification displayed");
   let notification = notifications[0];
   info("Triggering main command for notification " + notification.id);
   EventUtils.synthesizeMouseAtCenter(notification.button, {});
@@ -338,7 +338,7 @@ function triggerMainCommand(popup) {
 
 function triggerSecondaryCommand(popup, index) {
   let notifications = popup.childNodes;
-  ok(notifications.length > 0, "at least one notification displayed");
+  ok(!!notifications.length, "at least one notification displayed");
   let notification = notifications[0];
   info("Triggering secondary command for notification " + notification.id);
 
diff --git a/browser/base/content/test/webextensions/browser_permissions_local_file.js b/browser/base/content/test/webextensions/browser_permissions_local_file.js
index 689438f3c6f9..d675979ab0f2 100644
--- a/browser/base/content/test/webextensions/browser_permissions_local_file.js
+++ b/browser/base/content/test/webextensions/browser_permissions_local_file.js
@@ -51,7 +51,7 @@ add_task(async function test_install_extension_from_local_file() {
 
   // Make sure we got some data.
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
 
diff --git a/browser/base/content/test/webextensions/head.js b/browser/base/content/test/webextensions/head.js
index 000da75fa966..2e85d845bd30 100644
--- a/browser/base/content/test/webextensions/head.js
+++ b/browser/base/content/test/webextensions/head.js
@@ -255,7 +255,7 @@ function checkNotification(panel, checkIcon, permissions) {
     permissions.length,
     `Permissions list has ${permissions.length} entries`
   );
-  if (permissions.length == 0) {
+  if (!permissions.length) {
     is(header.getAttribute("hidden"), "true", "Permissions header is hidden");
     is(
       learnMoreLink.getAttribute("hidden"),
diff --git a/browser/base/content/test/webrtc/browser_devices_get_user_media_paused.js b/browser/base/content/test/webrtc/browser_devices_get_user_media_paused.js
index 16c29c81da5c..963078858bef 100644
--- a/browser/base/content/test/webrtc/browser_devices_get_user_media_paused.js
+++ b/browser/base/content/test/webrtc/browser_devices_get_user_media_paused.js
@@ -45,7 +45,7 @@ function stopClonedTracks(audio, video) {
       clones.filter(t => t.kind == "video").forEach(t => t.stop());
     }
     let liveClones = clones.filter(t => t.readyState == "live");
-    if (liveClones.length == 0) {
+    if (!liveClones.length) {
       delete content.wrappedJSObject.gClones;
     } else {
       content.wrappedJSObject.gClones = liveClones;
diff --git a/browser/base/content/utilityOverlay.js b/browser/base/content/utilityOverlay.js
index 7c868ef75262..246a99eae8ff 100644
--- a/browser/base/content/utilityOverlay.js
+++ b/browser/base/content/utilityOverlay.js
@@ -888,7 +888,7 @@ function eventMatchesKey(aEvent, aKey) {
     aEvent.getModifierState(modifier)
   );
   // Check if aEvent has a modifier and aKey doesn't
-  if (eventModifiers.length > 0 && keyModifiers.length == 0) {
+  if (eventModifiers.length && !keyModifiers.length) {
     return false;
   }
   // Check whether aKey's modifiers match aEvent's modifiers
diff --git a/browser/components/about/AboutPrivateBrowsingHandler.jsm b/browser/components/about/AboutPrivateBrowsingHandler.jsm
index ff38c7f0d14f..58d9370435b2 100644
--- a/browser/components/about/AboutPrivateBrowsingHandler.jsm
+++ b/browser/components/about/AboutPrivateBrowsingHandler.jsm
@@ -47,7 +47,7 @@ var AboutPrivateBrowsingHandler = {
         let searchAlias = "";
         let searchAliases =
           Services.search.defaultEngine.wrappedJSObject.__internalAliases;
-        if (searchAliases && searchAliases.length > 0) {
+        if (searchAliases && searchAliases.length) {
           searchAlias = `${searchAliases[0]} `;
         }
         let urlBar = aMessage.target.browser.ownerGlobal.gURLBar;
diff --git a/browser/components/aboutlogins/tests/browser/browser_loginListChanges.js b/browser/components/aboutlogins/tests/browser/browser_loginListChanges.js
index 21758d297cd6..5a1c9281c944 100644
--- a/browser/components/aboutlogins/tests/browser/browser_loginListChanges.js
+++ b/browser/components/aboutlogins/tests/browser/browser_loginListChanges.js
@@ -70,7 +70,7 @@ add_task(async function test_login_removed() {
   await ContentTask.spawn(browser, login, async removedLogin => {
     let loginList = Cu.waiveXrays(content.document.querySelector("login-list"));
     let loginRemoved = await ContentTaskUtils.waitForCondition(() => {
-      return loginList._loginGuidsSortedOrder.length == 0;
+      return !loginList._loginGuidsSortedOrder.length;
     }, "Waiting for login to get removed");
     ok(loginRemoved, "The login should be removed from the page");
   });
@@ -121,7 +121,7 @@ add_task(async function test_all_logins_removed() {
   await ContentTask.spawn(browser, null, async () => {
     let loginList = Cu.waiveXrays(content.document.querySelector("login-list"));
     let loginFound = await ContentTaskUtils.waitForCondition(() => {
-      return loginList._loginGuidsSortedOrder.length == 0;
+      return !loginList._loginGuidsSortedOrder.length;
     }, "Waiting for logins to be cleared");
     ok(loginFound, "Logins should be cleared");
     ok(
diff --git a/browser/components/contextualidentity/test/browser/file_reflect_cookie_into_title.html b/browser/components/contextualidentity/test/browser/file_reflect_cookie_into_title.html
index b04f3fd5ca81..849916d28a0a 100644
--- a/browser/components/contextualidentity/test/browser/file_reflect_cookie_into_title.html
+++ b/browser/components/contextualidentity/test/browser/file_reflect_cookie_into_title.html
@@ -4,7 +4,7 @@
     <title>title not set</title>
     <script>
       // if we have a query string, use it to set the cookie and localStorage
-      if (window.location.search.length > 0) {
+      if (window.location.search.length) {
         let context_name = window.location.search.substr(1);
         document.cookie = "userContextId=" + context_name;
         localStorage.setItem("userContext", context_name);
diff --git a/browser/components/contextualidentity/test/browser/file_set_storages.html b/browser/components/contextualidentity/test/browser/file_set_storages.html
index 8f2f1df93840..96c46f90627c 100644
--- a/browser/components/contextualidentity/test/browser/file_set_storages.html
+++ b/browser/components/contextualidentity/test/browser/file_set_storages.html
@@ -8,7 +8,7 @@
       "use strict";
 
       // if we have a query string, use it to set storages
-      if (window.location.search.length > 0) {
+      if (window.location.search.length) {
         let context_name = window.location.search.substr(1);
         localStorage.setItem("userContext", context_name);
         sessionStorage.setItem("userContext", context_name);
diff --git a/browser/components/customizableui/CustomizableWidgets.jsm b/browser/components/customizableui/CustomizableWidgets.jsm
index ee53715094f6..85c889471670 100644
--- a/browser/components/customizableui/CustomizableWidgets.jsm
+++ b/browser/components/customizableui/CustomizableWidgets.jsm
@@ -767,7 +767,7 @@ if (Services.prefs.getBoolPref("identity.fxaccounts.enabled")) {
 
       attachFragment.appendChild(clientItem);
 
-      if (client.tabs.length == 0) {
+      if (!client.tabs.length) {
         let label = this._appendMessageLabel(
           "notabsforclientlabel",
           attachFragment
diff --git a/browser/components/customizableui/content/panelUI.js b/browser/components/customizableui/content/panelUI.js
index 4f9444b921aa..2c10e9bdff2f 100644
--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -820,7 +820,7 @@ const PanelUI = {
       if (!notifications[0].options.badgeOnly) {
         this._showBannerItem(notifications[0]);
       }
-    } else if (doorhangers.length > 0) {
+    } else if (doorhangers.length) {
       // Only show the doorhanger if the window is focused and not fullscreen
       if (
         (window.fullScreen && this.autoHideToolbarInFullScreen) ||
diff --git a/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js b/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js
index 66fc65141753..c9ac81b81395 100644
--- a/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js
+++ b/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js
@@ -123,7 +123,7 @@ add_task(async function() {
     "toolbarbutton.customization-lwtheme-menu-theme[active]"
   );
   is(activeThemes.length, 1, "Exactly 1 theme should be selected");
-  if (activeThemes.length > 0) {
+  if (activeThemes.length) {
     is(
       activeThemes[0].theme.id,
       LIGHT_THEME_ID,
@@ -155,7 +155,7 @@ add_task(async function() {
     "toolbarbutton.customization-lwtheme-menu-theme[active]"
   );
   is(activeThemes.length, 1, "Exactly 1 theme should be selected");
-  if (activeThemes.length > 0) {
+  if (activeThemes.length) {
     is(
       activeThemes[0].theme.id,
       "my-theme-5@example.com",
@@ -183,7 +183,7 @@ add_task(async function() {
     "toolbarbutton.customization-lwtheme-menu-theme[active]"
   );
   is(activeThemes.length, 1, "Exactly 1 theme should be selected");
-  if (activeThemes.length > 0) {
+  if (activeThemes.length) {
     is(
       activeThemes[0].theme.id,
       firstLWThemeId,
diff --git a/browser/components/customizableui/test/browser_lwt_telemetry.js b/browser/components/customizableui/test/browser_lwt_telemetry.js
index 65fc2dd663a9..330004aa0ec2 100644
--- a/browser/components/customizableui/test/browser_lwt_telemetry.js
+++ b/browser/components/customizableui/test/browser_lwt_telemetry.js
@@ -41,7 +41,7 @@ add_task(async function testCustomize() {
 
   // Make sure we got some data.
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
 
diff --git a/browser/components/downloads/DownloadsCommon.jsm b/browser/components/downloads/DownloadsCommon.jsm
index d8de4ab919f0..66fc11bc8077 100644
--- a/browser/components/downloads/DownloadsCommon.jsm
+++ b/browser/components/downloads/DownloadsCommon.jsm
@@ -983,7 +983,7 @@ const DownloadsViewPrototype = {
    */
   addView(aView) {
     // Start receiving events when the first of our views is registered.
-    if (this._views.length == 0) {
+    if (!this._views.length) {
       if (this._isPrivate) {
         PrivateDownloadsData.addView(this);
       } else {
@@ -1021,7 +1021,7 @@ const DownloadsViewPrototype = {
     }
 
     // Stop receiving events when the last of our views is unregistered.
-    if (this._views.length == 0) {
+    if (!this._views.length) {
       if (this._isPrivate) {
         PrivateDownloadsData.removeView(this);
       } else {
@@ -1175,7 +1175,7 @@ DownloadsIndicatorDataCtor.prototype = {
   removeView(aView) {
     DownloadsViewPrototype.removeView.call(this, aView);
 
-    if (this._views.length == 0) {
+    if (!this._views.length) {
       this._itemCount = 0;
     }
   },
@@ -1393,7 +1393,7 @@ DownloadsSummaryData.prototype = {
   removeView(aView) {
     DownloadsViewPrototype.removeView.call(this, aView);
 
-    if (this._views.length == 0) {
+    if (!this._views.length) {
       // Clear out our collection of Download objects. If we ever have
       // another view registered with us, this will get re-populated.
       this._downloads = [];
@@ -1448,7 +1448,7 @@ DownloadsSummaryData.prototype = {
    * which was set when constructing this DownloadsSummaryData instance.
    */
   *_downloadsForSummary() {
-    if (this._downloads.length > 0) {
+    if (this._downloads.length) {
       for (let i = this._numToExclude; i < this._downloads.length; ++i) {
         yield this._downloads[i];
       }
diff --git a/browser/components/downloads/content/allDownloadsView.js b/browser/components/downloads/content/allDownloadsView.js
index 549be20e8d08..9cb0eca193be 100644
--- a/browser/components/downloads/content/allDownloadsView.js
+++ b/browser/components/downloads/content/allDownloadsView.js
@@ -337,7 +337,7 @@ DownloadsPlacesView.prototype = {
   },
 
   get hasSelection() {
-    return this.selectedNodes.length > 0;
+    return !!this.selectedNodes.length;
   },
 
   get controller() {
diff --git a/browser/components/enterprisepolicies/Policies.jsm b/browser/components/enterprisepolicies/Policies.jsm
index bbdc6e73979c..26c3f94b8940 100644
--- a/browser/components/enterprisepolicies/Policies.jsm
+++ b/browser/components/enterprisepolicies/Policies.jsm
@@ -869,7 +869,7 @@ var Policies = {
       // (and therefore what the pref |browser.startup.homepage|) accepts.
       if (param.URL) {
         let homepages = param.URL.href;
-        if (param.Additional && param.Additional.length > 0) {
+        if (param.Additional && param.Additional.length) {
           homepages += "|" + param.Additional.map(url => url.href).join("|");
         }
         setDefaultPref("browser.startup.homepage", homepages, param.Locked);
diff --git a/browser/components/enterprisepolicies/content/aboutPolicies.js b/browser/components/enterprisepolicies/content/aboutPolicies.js
index b7ee279cd78f..c37df4f94bc5 100644
--- a/browser/components/enterprisepolicies/content/aboutPolicies.js
+++ b/browser/components/enterprisepolicies/content/aboutPolicies.js
@@ -169,7 +169,7 @@ function generatePolicy(data, row, depth, new_cont, islast, arr_sep = false) {
         generatePolicy(data[count], new_row, depth + 1, new_cont, false, false);
       }
     }
-  } else if (typeof data == "object" && Object.keys(data).length > 0) {
+  } else if (typeof data == "object" && Object.keys(data).length) {
     let count = 0;
     for (let obj in data) {
       if (count == 0) {
diff --git a/browser/components/extensions/parent/ext-chrome-settings-overrides.js b/browser/components/extensions/parent/ext-chrome-settings-overrides.js
index acf9f2576c50..acf70fd55542 100644
--- a/browser/components/extensions/parent/ext-chrome-settings-overrides.js
+++ b/browser/components/extensions/parent/ext-chrome-settings-overrides.js
@@ -236,7 +236,7 @@ this.chrome_settings_overrides = class extends ExtensionAPI {
 
     try {
       let engines = await Services.search.getEnginesByExtensionID(id);
-      if (engines.length > 0) {
+      if (engines.length) {
         await Services.search.removeWebExtensionEngine(id);
       }
     } catch (e) {
@@ -455,7 +455,7 @@ this.chrome_settings_overrides = class extends ExtensionAPI {
       !extension.addonData.builtIn
     ) {
       let engines = await Services.search.getEnginesByExtensionID(extension.id);
-      if (engines.length > 0) {
+      if (engines.length) {
         let firstEngine = engines[0];
         let firstEngineName = firstEngine.name;
         // There can be only one engine right now
@@ -470,7 +470,7 @@ this.chrome_settings_overrides = class extends ExtensionAPI {
     }
     try {
       let engines = await Services.search.addEnginesFromExtension(extension);
-      if (engines.length > 0) {
+      if (engines.length) {
         await ExtensionSettingsStore.initialize();
         await ExtensionSettingsStore.addSetting(
           extension.id,
diff --git a/browser/components/extensions/parent/ext-menus.js b/browser/components/extensions/parent/ext-menus.js
index 78b5547ceea4..983fa7a1b086 100644
--- a/browser/components/extensions/parent/ext-menus.js
+++ b/browser/components/extensions/parent/ext-menus.js
@@ -312,7 +312,7 @@ var gMenuBuilder = {
   buildSingleElement(item, contextData) {
     let doc = contextData.menu.ownerDocument;
     let element;
-    if (item.children.length > 0) {
+    if (item.children.length) {
       element = this.createMenuElement(doc, item);
     } else if (item.type == "separator") {
       element = doc.createXULElement("menuseparator");
diff --git a/browser/components/extensions/parent/ext-sessions.js b/browser/components/extensions/parent/ext-sessions.js
index cdae464c80f8..7a399076c099 100644
--- a/browser/components/extensions/parent/ext-sessions.js
+++ b/browser/components/extensions/parent/ext-sessions.js
@@ -180,7 +180,7 @@ this.sessions = class extends ExtensionAPI {
               }
             }
 
-            if (recentlyClosedTabs.length > 0) {
+            if (recentlyClosedTabs.length) {
               // Sort the tabs.
               recentlyClosedTabs.sort((a, b) => b.closedAt - a.closedAt);
 
diff --git a/browser/components/extensions/parent/ext-tabs.js b/browser/components/extensions/parent/ext-tabs.js
index 68bfc7bc0a29..b117d8477e10 100644
--- a/browser/components/extensions/parent/ext-tabs.js
+++ b/browser/components/extensions/parent/ext-tabs.js
@@ -1592,7 +1592,7 @@ this.tabs = class extends ExtensionAPI {
               }
             }
           }
-          if (hidden.length > 0) {
+          if (hidden.length) {
             let win = Services.wm.getMostRecentWindow("navigator:browser");
             tabHidePopup.open(win, extension.id);
           }
@@ -1616,7 +1616,7 @@ this.tabs = class extends ExtensionAPI {
 
           if (!Array.isArray(tabs)) {
             tabs = [tabs];
-          } else if (tabs.length == 0) {
+          } else if (!tabs.length) {
             throw new ExtensionError("No highlighted tab.");
           }
           window.gBrowser.selectedTabs = tabs.map(tabIndex => {
diff --git a/browser/components/extensions/test/browser/browser_ext_addon_debugging_netmonitor.js b/browser/components/extensions/test/browser/browser_ext_addon_debugging_netmonitor.js
index 54add1b7be82..8dcd65757e58 100644
--- a/browser/components/extensions/test/browser/browser_ext_addon_debugging_netmonitor.js
+++ b/browser/components/extensions/test/browser/browser_ext_addon_debugging_netmonitor.js
@@ -63,7 +63,7 @@ async function setupToolboxTest(extensionId) {
       filterRequest
     );
 
-    return requests.length > 0;
+    return !!requests.length;
   });
 
   // Call a function defined in the target extension to make assertions
diff --git a/browser/components/extensions/test/browser/browser_ext_browsingData_pluginData.js b/browser/components/extensions/test/browser/browser_ext_browsingData_pluginData.js
index b303e43c5432..a5567f8c91cf 100644
--- a/browser/components/extensions/test/browser/browser_ext_browsingData_pluginData.js
+++ b/browser/components/extensions/test/browser/browser_ext_browsingData_pluginData.js
@@ -36,7 +36,7 @@ function promiseUpdatePluginBindings(browser) {
   return ContentTask.spawn(browser, {}, async function() {
     let doc = content.document;
     let elems = doc.getElementsByTagName("embed");
-    if (elems && elems.length > 0) {
+    if (elems && elems.length) {
       elems[0].clientTop; // eslint-disable-line no-unused-expressions
     }
   });
diff --git a/browser/components/extensions/test/browser/browser_ext_browsingData_serviceWorkers.js b/browser/components/extensions/test/browser/browser_ext_browsingData_serviceWorkers.js
index 706cae062d1c..395e70119d15 100644
--- a/browser/components/extensions/test/browser/browser_ext_browsingData_serviceWorkers.js
+++ b/browser/components/extensions/test/browser/browser_ext_browsingData_serviceWorkers.js
@@ -90,7 +90,7 @@ add_task(async function testServiceWorkers() {
   await extension.awaitMessage("serviceWorkersRemoved");
 
   // The serviceWorkers and not necessarily removed immediately.
-  while (serviceWorkers.length > 0) {
+  while (serviceWorkers.length) {
     serviceWorkers = serviceWorkerManager.getAllRegistrations();
     await new Promise(resolve => setTimeout(resolve, 1));
   }
diff --git a/browser/components/extensions/test/browser/browser_ext_devtools_panel.js b/browser/components/extensions/test/browser/browser_ext_devtools_panel.js
index 0626017c739d..44a5b9b35f81 100644
--- a/browser/components/extensions/test/browser/browser_ext_devtools_panel.js
+++ b/browser/components/extensions/test/browser/browser_ext_devtools_panel.js
@@ -440,7 +440,7 @@ add_task(async function test_devtools_page_panels_create() {
   );
   const panelInfo = getPanelInfo(toolbox);
   ok(
-    panelInfo.panelLabel && panelInfo.panelLabel.length > 0,
+    panelInfo.panelLabel && !!panelInfo.panelLabel.length,
     "Expect the registered panel to include a non empty panelLabel property"
   );
   is(
diff --git a/browser/components/extensions/test/browser/browser_ext_sidebarAction_windows.js b/browser/components/extensions/test/browser/browser_ext_sidebarAction_windows.js
index 99612be15876..35b70866caf0 100644
--- a/browser/components/extensions/test/browser/browser_ext_sidebarAction_windows.js
+++ b/browser/components/extensions/test/browser/browser_ext_sidebarAction_windows.js
@@ -43,7 +43,7 @@ add_task(async function sidebar_windows() {
   let elements = document.getElementsByClassName("webextension-menuitem");
   // ui is in flux, at time of writing we potentially have 3 menuitems, later
   // it may be two or one, just make sure one is there.
-  ok(elements.length > 0, "have a menuitem");
+  ok(!!elements.length, "have a menuitem");
   let style = elements[0].getAttribute("style");
   ok(style.includes("webextension-menuitem-image"), "this menu has style");
 
@@ -60,7 +60,7 @@ add_task(async function sidebar_windows() {
   );
   // Check that the menuitem has our image styling.
   elements = win.document.getElementsByClassName("webextension-menuitem");
-  ok(elements.length > 0, "have a menuitem");
+  ok(!!elements.length, "have a menuitem");
   style = elements[0].getAttribute("style");
   ok(style.includes("webextension-menuitem-image"), "this menu has style");
 
diff --git a/browser/components/extensions/test/browser/browser_ext_webNavigation_getFrames.js b/browser/components/extensions/test/browser/browser_ext_webNavigation_getFrames.js
index f0215f292607..889157412e96 100644
--- a/browser/components/extensions/test/browser/browser_ext_webNavigation_getFrames.js
+++ b/browser/components/extensions/test/browser/browser_ext_webNavigation_getFrames.js
@@ -83,7 +83,7 @@ add_task(async function testWebNavigationFrames() {
         while (
           getAllFramesDetails.filter(
             details => details.frameId == nonExistentFrameId
-          ).length > 0
+          ).length
         ) {
           nonExistentFrameId += 1;
         }
diff --git a/browser/components/extensions/test/browser/head.js b/browser/components/extensions/test/browser/head.js
index b7d515645a08..b1b4f6bc49bb 100644
--- a/browser/components/extensions/test/browser/head.js
+++ b/browser/components/extensions/test/browser/head.js
@@ -502,7 +502,7 @@ async function openExtensionContextMenu(selector = "#img1") {
   );
 
   // Return null if the extension only has one item and therefore no extension menu.
-  if (topLevelMenu.length == 0) {
+  if (!topLevelMenu.length) {
     return null;
   }
 
diff --git a/browser/components/migration/360seProfileMigrator.jsm b/browser/components/migration/360seProfileMigrator.jsm
index 67e94a812790..1793dc65d44a 100644
--- a/browser/components/migration/360seProfileMigrator.jsm
+++ b/browser/components/migration/360seProfileMigrator.jsm
@@ -317,7 +317,7 @@ Qihoo360seProfileMigrator.prototype.getSourceProfiles = function() {
 
   this.__sourceProfiles = profiles.filter(profile => {
     let resources = this.getResources(profile);
-    return resources && resources.length > 0;
+    return resources && !!resources.length;
   });
   return this.__sourceProfiles;
 };
diff --git a/browser/components/migration/ChromeProfileMigrator.jsm b/browser/components/migration/ChromeProfileMigrator.jsm
index 10d03d033fe3..08cca1fd2af6 100644
--- a/browser/components/migration/ChromeProfileMigrator.jsm
+++ b/browser/components/migration/ChromeProfileMigrator.jsm
@@ -185,7 +185,7 @@ ChromeProfileMigrator.prototype.getSourceProfiles = async function Chrome_getSou
   // Only list profiles from which any data can be imported
   this.__sourceProfiles = profileResources
     .filter(({ resources }) => {
-      return resources && resources.length > 0;
+      return resources && !!resources.length;
     }, this)
     .map(({ profile }) => profile);
   return this.__sourceProfiles;
@@ -220,10 +220,7 @@ async function GetBookmarksResource(aProfileFolder) {
         let roots = JSON.parse(bookmarkJSON).roots;
 
         // Importing bookmark bar items
-        if (
-          roots.bookmark_bar.children &&
-          roots.bookmark_bar.children.length > 0
-        ) {
+        if (roots.bookmark_bar.children && roots.bookmark_bar.children.length) {
           // Toolbar
           let parentGuid = PlacesUtils.bookmarks.toolbarGuid;
           let bookmarks = convertBookmarks(
@@ -243,7 +240,7 @@ async function GetBookmarksResource(aProfileFolder) {
         }
 
         // Importing bookmark menu items
-        if (roots.other.children && roots.other.children.length > 0) {
+        if (roots.other.children && roots.other.children.length) {
           // Bookmark menu
           let parentGuid = PlacesUtils.bookmarks.menuGuid;
           let bookmarks = convertBookmarks(roots.other.children, errorGatherer);
@@ -329,7 +326,7 @@ async function GetHistoryResource(aProfileFolder) {
           }
         }
 
-        if (pageInfos.length > 0) {
+        if (pageInfos.length) {
           await MigrationUtils.insertVisitsWrapper(pageInfos);
         }
       })().then(
@@ -520,7 +517,7 @@ async function GetWindowsPasswordsResource(aProfileFolder) {
         }
       }
       try {
-        if (logins.length > 0) {
+        if (logins.length) {
           await MigrationUtils.insertLoginsWrapper(logins);
         }
       } catch (e) {
diff --git a/browser/components/migration/EdgeProfileMigrator.jsm b/browser/components/migration/EdgeProfileMigrator.jsm
index 989b20708e5f..0b7fae209904 100644
--- a/browser/components/migration/EdgeProfileMigrator.jsm
+++ b/browser/components/migration/EdgeProfileMigrator.jsm
@@ -163,7 +163,7 @@ EdgeTypedURLMigrator.prototype = {
       });
     }
 
-    if (pageInfos.length == 0) {
+    if (!pageInfos.length) {
       aCallback(typedURLs.size == 0);
       return;
     }
diff --git a/browser/components/migration/IEProfileMigrator.jsm b/browser/components/migration/IEProfileMigrator.jsm
index dcfe709d0a8b..ca02b40acf83 100644
--- a/browser/components/migration/IEProfileMigrator.jsm
+++ b/browser/components/migration/IEProfileMigrator.jsm
@@ -69,7 +69,7 @@ History.prototype = {
 
       let title = entry.get("title");
       // Embed visits have no title and don't need to be imported.
-      if (title.length == 0) {
+      if (!title.length) {
         continue;
       }
 
@@ -93,7 +93,7 @@ History.prototype = {
     }
 
     // Check whether there is any history to import.
-    if (pageInfos.length == 0) {
+    if (!pageInfos.length) {
       aCallback(true);
       return;
     }
@@ -235,7 +235,7 @@ IE7FormPasswords.prototype = {
       }
     }
 
-    if (logins.length > 0) {
+    if (logins.length) {
       await MigrationUtils.insertLoginsWrapper(logins);
     }
 
diff --git a/browser/components/migration/MSMigrationUtils.jsm b/browser/components/migration/MSMigrationUtils.jsm
index f0d1e5b4ba62..dd8529628ab7 100644
--- a/browser/components/migration/MSMigrationUtils.jsm
+++ b/browser/components/migration/MSMigrationUtils.jsm
@@ -669,7 +669,7 @@ Cookies.prototype = {
     // Note the last record is an empty string...
     let records = [];
     let lines = aTextBuffer.split("\n");
-    while (lines.length > 0) {
+    while (lines.length) {
       let record = lines.splice(0, 9);
       // ... which means this is going to be a 1-element array for that record
       if (record.length > 1) {
@@ -680,7 +680,7 @@ Cookies.prototype = {
       let [name, value, hostpath, flags, expireTimeLo, expireTimeHi] = record;
 
       // IE stores deleted cookies with a zero-length value, skip them.
-      if (value.length == 0) {
+      if (!value.length) {
         continue;
       }
 
@@ -696,7 +696,7 @@ Cookies.prototype = {
 
       // For a non-null domain, assume it's what Mozilla considers
       // a domain cookie.  See bug 222343.
-      if (host.length > 0) {
+      if (host.length) {
         // Fist delete any possible extant matching host cookie.
         Services.cookies.remove(host, name, path, false, {});
         // Now make it a domain cookie.
@@ -970,7 +970,7 @@ WindowsVaultFormPasswords.prototype = {
         }
       }
 
-      if (logins.length > 0) {
+      if (logins.length) {
         await MigrationUtils.insertLoginsWrapper(logins);
       }
     } catch (e) {
diff --git a/browser/components/migration/MigrationUtils.jsm b/browser/components/migration/MigrationUtils.jsm
index 7d782873d470..0244e932df5b 100644
--- a/browser/components/migration/MigrationUtils.jsm
+++ b/browser/components/migration/MigrationUtils.jsm
@@ -244,7 +244,7 @@ var MigratorPrototype = {
    */
   migrate: async function MP_migrate(aItems, aStartup, aProfile) {
     let resources = await this._getMaybeCachedResources(aProfile);
-    if (resources.length == 0) {
+    if (!resources.length) {
       throw new Error("migrate called for a non-existent source");
     }
 
@@ -481,11 +481,11 @@ var MigratorPrototype = {
       let profiles = await this.getSourceProfiles();
       if (!profiles) {
         let resources = await this._getMaybeCachedResources("");
-        if (resources && resources.length > 0) {
+        if (resources && resources.length) {
           exists = true;
         }
       } else {
-        exists = profiles.length > 0;
+        exists = !!profiles.length;
       }
     } catch (ex) {
       Cu.reportError(ex);
diff --git a/browser/components/migration/SafariProfileMigrator.jsm b/browser/components/migration/SafariProfileMigrator.jsm
index 8d9a5e3d99b3..2c980428520d 100644
--- a/browser/components/migration/SafariProfileMigrator.jsm
+++ b/browser/components/migration/SafariProfileMigrator.jsm
@@ -113,7 +113,7 @@ Bookmarks.prototype = {
       entriesFiltered = aEntries;
     }
 
-    if (entriesFiltered.length == 0) {
+    if (!entriesFiltered.length) {
       return;
     }
 
@@ -271,7 +271,7 @@ History.prototype = {
             }
           }
         }
-        if (pageInfos.length == 0) {
+        if (!pageInfos.length) {
           // If we failed at least once, then we didn't succeed in importing,
           // otherwise we didn't actually have anything to import, so we'll
           // report it as a success.
@@ -313,7 +313,7 @@ MainPreferencesPropertyList.prototype = {
       return;
     }
 
-    let alreadyReading = this._callbacks.length > 0;
+    let alreadyReading = !!this._callbacks.length;
     this._callbacks.push(aCallback);
     if (!alreadyReading) {
       PropertyListUtils.read(this._file, aDict => {
@@ -346,7 +346,7 @@ SearchStrings.prototype = {
 
         if (aDict.has("RecentSearchStrings")) {
           let recentSearchStrings = aDict.get("RecentSearchStrings");
-          if (recentSearchStrings && recentSearchStrings.length > 0) {
+          if (recentSearchStrings && recentSearchStrings.length) {
             let changes = recentSearchStrings.map(searchString => ({
               op: "add",
               fieldname: "searchbar-history",
diff --git a/browser/components/migration/tests/unit/test_360se_bookmarks.js b/browser/components/migration/tests/unit/test_360se_bookmarks.js
index d4c1a4c580ba..6da76f6fc42c 100644
--- a/browser/components/migration/tests/unit/test_360se_bookmarks.js
+++ b/browser/components/migration/tests/unit/test_360se_bookmarks.js
@@ -42,7 +42,7 @@ add_task(async function() {
       ) {
         gotFolder = true;
       }
-      if (expectedParents.length > 0 && event.title == label) {
+      if (expectedParents.length && event.title == label) {
         let index = expectedParents.indexOf(event.parentId);
         Assert.ok(index != -1, "Found expected parent");
         expectedParents.splice(index, 1);
diff --git a/browser/components/migration/tests/unit/test_IE_bookmarks.js b/browser/components/migration/tests/unit/test_IE_bookmarks.js
index 1b4643ed2ce9..7ffeead4b126 100644
--- a/browser/components/migration/tests/unit/test_IE_bookmarks.js
+++ b/browser/components/migration/tests/unit/test_IE_bookmarks.js
@@ -23,7 +23,7 @@ add_task(async function() {
       if (event.title != label) {
         itemCount++;
       }
-      if (expectedParents.length > 0 && event.title == label) {
+      if (expectedParents.length && event.title == label) {
         let index = expectedParents.indexOf(event.parentId);
         Assert.notEqual(index, -1);
         expectedParents.splice(index, 1);
diff --git a/browser/components/migration/tests/unit/test_IE_history.js b/browser/components/migration/tests/unit/test_IE_history.js
index 9dceb08357a9..fd803f460773 100644
--- a/browser/components/migration/tests/unit/test_IE_history.js
+++ b/browser/components/migration/tests/unit/test_IE_history.js
@@ -45,6 +45,6 @@ add_task(async function test_IE_history() {
     let entry = await PlacesUtils.history.fetch(url, { includeVisits: true });
     Assert.equal(entry.url, url, "Should have the correct URL");
     Assert.equal(entry.title, title, "Should have the correct title");
-    Assert.ok(entry.visits.length > 0, "Should have some visits");
+    Assert.ok(!!entry.visits.length, "Should have some visits");
   }
 });
diff --git a/browser/components/migration/tests/unit/test_Safari_bookmarks.js b/browser/components/migration/tests/unit/test_Safari_bookmarks.js
index 7f1a5bdd2d5f..1c3c2e0302bc 100644
--- a/browser/components/migration/tests/unit/test_Safari_bookmarks.js
+++ b/browser/components/migration/tests/unit/test_Safari_bookmarks.js
@@ -29,7 +29,7 @@ add_task(async function() {
       ) {
         gotFolder = true;
       }
-      if (expectedParents.length > 0 && event.title == label) {
+      if (expectedParents.length && event.title == label) {
         let index = expectedParents.indexOf(event.parentId);
         Assert.ok(index != -1, "Found expected parent");
         expectedParents.splice(index, 1);
diff --git a/browser/components/newtab/common/Reducers.jsm b/browser/components/newtab/common/Reducers.jsm
index b34f309acd2f..61dd7540e640 100644
--- a/browser/components/newtab/common/Reducers.jsm
+++ b/browser/components/newtab/common/Reducers.jsm
@@ -307,7 +307,7 @@ function Sections(prevState = INITIAL_STATE.Sections, action) {
       });
       // Otherwise, append it
       if (!hasMatch) {
-        const initialized = !!(action.data.rows && action.data.rows.length > 0);
+        const initialized = !!(action.data.rows && !!action.data.rows.length);
         const section = Object.assign(
           { title: "", rows: [], enabled: false },
           action.data,
@@ -327,7 +327,7 @@ function Sections(prevState = INITIAL_STATE.Sections, action) {
           // Disabling a section (SECTION_UPDATE with empty rows) does not retain pinned cards.
           if (
             action.data.rows &&
-            action.data.rows.length > 0 &&
+            !!action.data.rows.length &&
             section.rows.find(card => card.pinned)
           ) {
             const rows = Array.from(action.data.rows);
diff --git a/browser/components/newtab/content-src/asrouter/templates/StartupOverlay/StartupOverlay.jsx b/browser/components/newtab/content-src/asrouter/templates/StartupOverlay/StartupOverlay.jsx
index 5f0fe10859b9..765b0634cdb2 100644
--- a/browser/components/newtab/content-src/asrouter/templates/StartupOverlay/StartupOverlay.jsx
+++ b/browser/components/newtab/content-src/asrouter/templates/StartupOverlay/StartupOverlay.jsx
@@ -82,7 +82,7 @@ export class StartupOverlay extends React.PureComponent {
    */
   _getFormInfo() {
     const value = {
-      has_flow_params: this.props.flowParams.flowId.length > 0,
+      has_flow_params: !!this.props.flowParams.flowId.length,
     };
     return { value };
   }
diff --git a/browser/components/newtab/content-src/asrouter/templates/Trailhead/Trailhead.jsx b/browser/components/newtab/content-src/asrouter/templates/Trailhead/Trailhead.jsx
index 571efe2621ec..f730e59e284b 100644
--- a/browser/components/newtab/content-src/asrouter/templates/Trailhead/Trailhead.jsx
+++ b/browser/components/newtab/content-src/asrouter/templates/Trailhead/Trailhead.jsx
@@ -112,7 +112,7 @@ export class Trailhead extends React.PureComponent {
    * Report to telemetry additional information about the form submission.
    */
   _getFormInfo() {
-    const value = { has_flow_params: this.props.flowParams.flowId.length > 0 };
+    const value = { has_flow_params: !!this.props.flowParams.flowId.length };
     return { value };
   }
 
diff --git a/browser/components/newtab/content-src/components/DiscoveryStreamBase/DiscoveryStreamBase.jsx b/browser/components/newtab/content-src/components/DiscoveryStreamBase/DiscoveryStreamBase.jsx
index ab5e87c92084..f2f54de3e3a6 100644
--- a/browser/components/newtab/content-src/components/DiscoveryStreamBase/DiscoveryStreamBase.jsx
+++ b/browser/components/newtab/content-src/components/DiscoveryStreamBase/DiscoveryStreamBase.jsx
@@ -335,7 +335,7 @@ export class _DiscoveryStreamBase extends React.PureComponent {
               components: [topSites],
             },
           ])}
-        {layoutRender.length > 0 && (
+        {!!layoutRender.length && (
           <CollapsibleSection
             className="ds-layout"
             collapsed={topStories.pref.collapsed}
diff --git a/browser/components/newtab/content-src/components/Sections/Sections.jsx b/browser/components/newtab/content-src/components/Sections/Sections.jsx
index 39473a573e1a..cc00e93b9493 100644
--- a/browser/components/newtab/content-src/components/Sections/Sections.jsx
+++ b/browser/components/newtab/content-src/components/Sections/Sections.jsx
@@ -200,7 +200,7 @@ export class Section extends React.PureComponent {
     const isPocketLoggedInDefined =
       isUserLoggedIn === true || isUserLoggedIn === false;
 
-    const hasTopics = topics && topics.length > 0;
+    const hasTopics = topics && !!topics.length;
 
     const shouldShowPocketCta =
       id === "topstories" && useCta && isUserLoggedIn === false;
diff --git a/browser/components/newtab/lib/ASRouterTargeting.jsm b/browser/components/newtab/lib/ASRouterTargeting.jsm
index 50171ca084ff..1c88755a55d2 100644
--- a/browser/components/newtab/lib/ASRouterTargeting.jsm
+++ b/browser/components/newtab/lib/ASRouterTargeting.jsm
@@ -96,7 +96,7 @@ function CheckBrowserNeedsUpdate(
         const now = Date.now();
         const updateServiceListener = {
           onCheckComplete(request, updates) {
-            checker._value = updates.length > 0;
+            checker._value = !!updates.length;
             resolve(checker._value);
           },
           onError(request, update) {
diff --git a/browser/components/newtab/lib/CFRPageActions.jsm b/browser/components/newtab/lib/CFRPageActions.jsm
index 1f4e2ddf8915..e7d3dbb02c31 100644
--- a/browser/components/newtab/lib/CFRPageActions.jsm
+++ b/browser/components/newtab/lib/CFRPageActions.jsm
@@ -257,7 +257,7 @@ class PageAction {
   }
 
   _clearScheduledStateChanges() {
-    while (this.stateTransitionTimeoutIDs.length > 0) {
+    while (this.stateTransitionTimeoutIDs.length) {
       // clearTimeout is safe even with invalid/expired IDs
       this.window.clearTimeout(this.stateTransitionTimeoutIDs.pop());
     }
diff --git a/browser/components/newtab/lib/PlacesFeed.jsm b/browser/components/newtab/lib/PlacesFeed.jsm
index b4c573344ce1..2652af6cfdc2 100644
--- a/browser/components/newtab/lib/PlacesFeed.jsm
+++ b/browser/components/newtab/lib/PlacesFeed.jsm
@@ -366,7 +366,7 @@ class PlacesFeed {
   _getSearchPrefix() {
     const searchAliases =
       Services.search.defaultEngine.wrappedJSObject.__internalAliases;
-    if (searchAliases && searchAliases.length > 0) {
+    if (searchAliases && searchAliases.length) {
       return `${searchAliases[0]} `;
     }
     return "";
diff --git a/browser/components/newtab/lib/ShortURL.jsm b/browser/components/newtab/lib/ShortURL.jsm
index 0826fd1994ee..aa9257601244 100644
--- a/browser/components/newtab/lib/ShortURL.jsm
+++ b/browser/components/newtab/lib/ShortURL.jsm
@@ -71,7 +71,7 @@ function shortURL({ url }) {
 
   // Remove the eTLD (e.g., com, net) and the preceding period from the hostname
   const eTLD = getETLD(hostname);
-  const eTLDExtra = eTLD.length > 0 ? -(eTLD.length + 1) : Infinity;
+  const eTLDExtra = eTLD.length ? -(eTLD.length + 1) : Infinity;
 
   // Ideally get the short eTLD-less host but fall back to longer url parts
   return (
diff --git a/browser/components/newtab/lib/TopStoriesFeed.jsm b/browser/components/newtab/lib/TopStoriesFeed.jsm
index 0d6ae6305626..d2cce07bc63e 100644
--- a/browser/components/newtab/lib/TopStoriesFeed.jsm
+++ b/browser/components/newtab/lib/TopStoriesFeed.jsm
@@ -314,7 +314,7 @@ this.TopStoriesFeed = class TopStoriesFeed {
       );
       this.domainAffinitiesLastUpdated = affinities._timestamp;
     }
-    if (stories && stories.length > 0 && this.storiesLastUpdated === 0) {
+    if (stories && !!stories.length && this.storiesLastUpdated === 0) {
       this.updateSettings(data.stories.settings);
       this.stories = this.rotate(this.transform(stories));
       this.storiesLastUpdated = data.stories._timestamp;
@@ -328,7 +328,7 @@ this.TopStoriesFeed = class TopStoriesFeed {
         this.cleanUpCampaignImpressionPref();
       }
     }
-    if (topics && topics.length > 0 && this.topicsLastUpdated === 0) {
+    if (topics && !!topics.length && this.topicsLastUpdated === 0) {
       this.topics = topics;
       this.topicsLastUpdated = data.topics._timestamp;
     }
diff --git a/browser/components/newtab/test/unit/asrouter/CFRMessageProvider.test.js b/browser/components/newtab/test/unit/asrouter/CFRMessageProvider.test.js
index 41e220abbcb0..41980ed40eb0 100644
--- a/browser/components/newtab/test/unit/asrouter/CFRMessageProvider.test.js
+++ b/browser/components/newtab/test/unit/asrouter/CFRMessageProvider.test.js
@@ -51,8 +51,8 @@ describe("CFRMessageProvider", () => {
     const pinTabMessage = messages.find(m => m.id === "PIN_TAB");
 
     assert.isTrue(
-      pinTabMessage.trigger.params.filter(host => host.startsWith("www."))
-        .length > 0
+      !!pinTabMessage.trigger.params.filter(host => host.startsWith("www."))
+        .length
     );
   });
 });
diff --git a/browser/components/newtab/test/unit/content-src/components/TopSites.test.jsx b/browser/components/newtab/test/unit/content-src/components/TopSites.test.jsx
index b2eb4721b746..f094b40c22a5 100644
--- a/browser/components/newtab/test/unit/content-src/components/TopSites.test.jsx
+++ b/browser/components/newtab/test/unit/content-src/components/TopSites.test.jsx
@@ -463,7 +463,7 @@ describe("<TopSiteLink>", () => {
   it("should have rtl direction automatically set for text", () => {
     const wrapper = shallow(<TopSiteLink link={link} />);
 
-    assert.isTrue(wrapper.find("[dir='auto']").length > 0);
+    assert.isTrue(!!wrapper.find("[dir='auto']").length);
   });
   it("should render a title", () => {
     const wrapper = shallow(<TopSiteLink link={link} title="foobar" />);
diff --git a/browser/components/originattributes/test/browser/file_windowOpenerRestrictionTarget.html b/browser/components/originattributes/test/browser/file_windowOpenerRestrictionTarget.html
index 89814b02d871..5a1483489704 100644
--- a/browser/components/originattributes/test/browser/file_windowOpenerRestrictionTarget.html
+++ b/browser/components/originattributes/test/browser/file_windowOpenerRestrictionTarget.html
@@ -10,7 +10,7 @@
     // Therefore, it should fail at this setting.
     let openerRestriction = true;
     let cookieValue;
-    if (window.location.search.length > 0) {
+    if (window.location.search.length) {
       cookieValue = window.location.search.substr(1);
       openerRestriction = false;
     }
diff --git a/browser/components/payments/PaymentUIService.jsm b/browser/components/payments/PaymentUIService.jsm
index ce48988bcbf8..b996ad134118 100644
--- a/browser/components/payments/PaymentUIService.jsm
+++ b/browser/components/payments/PaymentUIService.jsm
@@ -245,7 +245,7 @@ PaymentUIService.prototype = {
 
       if (
         !browser.tabModalPromptBox ||
-        browser.tabModalPromptBox.listPrompts().length == 0
+        !browser.tabModalPromptBox.listPrompts().length
       ) {
         browser.removeAttribute("tabmodalPromptShowing");
       }
diff --git a/browser/components/payments/res/components/address-option.js b/browser/components/payments/res/components/address-option.js
index f0c06a026866..2661f3e2319e 100644
--- a/browser/components/payments/res/components/address-option.js
+++ b/browser/components/payments/res/components/address-option.js
@@ -143,7 +143,7 @@ export default class AddressOption extends ObservedPropertiesMixin(RichOption) {
         continue;
       }
 
-      if (lineEl.children.length > 0) {
+      if (lineEl.children.length) {
         lineEl.append(this.dataset.fieldSeparator);
       }
       lineEl.appendChild(fieldEl);
diff --git a/browser/components/payments/res/paymentRequest.js b/browser/components/payments/res/paymentRequest.js
index da769b29f1e0..1a0818461595 100644
--- a/browser/components/payments/res/paymentRequest.js
+++ b/browser/components/payments/res/paymentRequest.js
@@ -143,8 +143,8 @@ var paymentRequest = {
       },
     };
 
-    let hasSavedAddresses = Object.keys(this.getAddresses(state)).length != 0;
-    let hasSavedCards = Object.keys(this.getBasicCards(state)).length != 0;
+    let hasSavedAddresses = !!Object.keys(this.getAddresses(state)).length;
+    let hasSavedCards = !!Object.keys(this.getBasicCards(state)).length;
     let shippingRequested = state.request.paymentOptions.requestShipping;
 
     // Onboarding wizard flow.
@@ -282,7 +282,7 @@ var paymentRequest = {
       let supportedNetworks =
         (modifier.data && modifier.data.supportedNetworks) || [];
       return (
-        supportedNetworks.length == 0 ||
+        !supportedNetworks.length ||
         supportedNetworks.includes(selectedMethod["cc-type"])
       );
     });
diff --git a/browser/components/payments/test/mochitest/test_address_form.html b/browser/components/payments/test/mochitest/test_address_form.html
index ccf4c528a488..906e5cc0f5d8 100644
--- a/browser/components/payments/test/mochitest/test_address_form.html
+++ b/browser/components/payments/test/mochitest/test_address_form.html
@@ -582,7 +582,7 @@ add_task(async function test_customMerchantValidity_reset() {
   display.appendChild(form);
   await asyncElementRendered();
 
-  ok(form.querySelectorAll(":-moz-ui-invalid").length > 0, "Check fields are visibly invalid");
+  ok(!!form.querySelectorAll(":-moz-ui-invalid").length, "Check fields are visibly invalid");
   info("merchant cleared the errors");
   await form.requestStore.setState({
     request: {
diff --git a/browser/components/payments/test/mochitest/test_basic_card_form.html b/browser/components/payments/test/mochitest/test_basic_card_form.html
index 2ff9fd9efdc5..239e3b013dc3 100644
--- a/browser/components/payments/test/mochitest/test_basic_card_form.html
+++ b/browser/components/payments/test/mochitest/test_basic_card_form.html
@@ -445,7 +445,7 @@ add_task(async function test_edit() {
     },
   });
   await asyncElementRendered();
-  ok(form.querySelectorAll(":-moz-ui-invalid").length > 0,
+  ok(!!form.querySelectorAll(":-moz-ui-invalid").length,
      "Check fields are visibly invalid on an 'edit' form with missing fields");
   checkCCForm(form, minimalCard);
 
diff --git a/browser/components/places/PlacesUIUtils.jsm b/browser/components/places/PlacesUIUtils.jsm
index 51cf6fb691e2..50f2cf76d14b 100644
--- a/browser/components/places/PlacesUIUtils.jsm
+++ b/browser/components/places/PlacesUIUtils.jsm
@@ -920,7 +920,7 @@ var PlacesUIUtils = {
           return Ci.nsINavHistoryResultNode.RESULT_TYPE_FOLDER;
         }
 
-        if (this.uri.length == 0) {
+        if (!this.uri.length) {
           throw new Error("Unexpected item type");
         }
 
diff --git a/browser/components/places/content/browserPlacesViews.js b/browser/components/places/content/browserPlacesViews.js
index 84aceab596de..4c288a02ad99 100644
--- a/browser/components/places/content/browserPlacesViews.js
+++ b/browser/components/places/content/browserPlacesViews.js
@@ -820,7 +820,7 @@ PlacesViewBase.prototype = {
 
     // Remove any delayed element, see _cleanPopup for details.
     if ("_delayedRemovals" in popup) {
-      while (popup._delayedRemovals.length > 0) {
+      while (popup._delayedRemovals.length) {
         popup.removeChild(popup._delayedRemovals.shift());
       }
     }
diff --git a/browser/components/places/content/controller.js b/browser/components/places/content/controller.js
index ecbdfd7efe15..0a532ea7d6ae 100644
--- a/browser/components/places/content/controller.js
+++ b/browser/components/places/content/controller.js
@@ -939,7 +939,7 @@ PlacesController.prototype = {
       );
     }
 
-    if (transactions.length > 0) {
+    if (transactions.length) {
       await PlacesUIUtils.batchUpdatesForNode(
         this._view.result,
         totalItems,
@@ -1114,7 +1114,7 @@ PlacesController.prototype = {
   },
 
   _releaseClipboardOwnership: function PC__releaseClipboardOwnership() {
-    if (this.cutNodes.length > 0) {
+    if (this.cutNodes.length) {
       // This clears the logical clipboard, doesn't remove data.
       this.clipboard.emptyClipboard(Ci.nsIClipboard.kGlobalClipboard);
     }
@@ -1175,7 +1175,7 @@ PlacesController.prototype = {
     // This order matters here!  It controls how this and other applications
     // select data to be inserted based on type.
     contents.forEach(function(content) {
-      if (content.entries.length > 0) {
+      if (content.entries.length) {
         hasData = true;
         let glue =
           content.type == PlacesUtils.TYPE_X_MOZ_PLACE ? "," : PlacesUtils.endl;
@@ -1309,7 +1309,7 @@ PlacesController.prototype = {
       this._clearClipboard();
     }
 
-    if (itemsToSelect.length > 0) {
+    if (itemsToSelect.length) {
       this._view.selectItems(itemsToSelect, false);
     }
   },
diff --git a/browser/components/places/content/editBookmark.js b/browser/components/places/content/editBookmark.js
index c3bf6e7072a5..cff6e83bb4a9 100644
--- a/browser/components/places/content/editBookmark.js
+++ b/browser/components/places/content/editBookmark.js
@@ -188,7 +188,7 @@ var gEditItemOverlay = {
       await PlacesUtils.keywords.fetch({ url: this._paneInfo.uri.spec }, e =>
         entries.push(e)
       );
-      if (entries.length > 0) {
+      if (entries.length) {
         // We show an existing keyword if either POST data was not provided, or
         // if the POST data is the same.
         let existingKeyword = entries[0].keyword;
@@ -580,13 +580,13 @@ var gEditItemOverlay = {
     let inputTags = this._getTagsArrayFromTagsInputField();
 
     // Optimize the trivial cases (which are actually the most common).
-    if (inputTags.length == 0 && aCurrentTags.length == 0) {
+    if (!inputTags.length && !aCurrentTags.length) {
       return { newTags: [], removedTags: [] };
     }
-    if (inputTags.length == 0) {
+    if (!inputTags.length) {
       return { newTags: [], removedTags: aCurrentTags };
     }
-    if (aCurrentTags.length == 0) {
+    if (!aCurrentTags.length) {
       return { newTags: inputTags, removedTags: [] };
     }
 
@@ -609,7 +609,7 @@ var gEditItemOverlay = {
 
     let setTags = async () => {
       let promises = [];
-      if (removedTags.length > 0) {
+      if (removedTags.length) {
         let promise = PlacesTransactions.Untag({
           urls: aURIs,
           tags: removedTags,
@@ -619,7 +619,7 @@ var gEditItemOverlay = {
         this.transactionPromises.push(promise);
         promises.push(promise);
       }
-      if (newTags.length > 0) {
+      if (newTags.length) {
         let promise = PlacesTransactions.Tag({ urls: aURIs, tags: newTags })
           .transact()
           .catch(Cu.reportError);
@@ -995,7 +995,7 @@ var gEditItemOverlay = {
     return tags
       .trim()
       .split(/\s*,\s*/) // Split on commas and remove spaces.
-      .filter(tag => tag.length > 0); // Kill empty tags.
+      .filter(tag => !!tag.length); // Kill empty tags.
   },
 
   async newFolder() {
diff --git a/browser/components/places/content/places-tree.js b/browser/components/places/content/places-tree.js
index 419bdd8784b2..7ee4d25d61eb 100644
--- a/browser/components/places/content/places-tree.js
+++ b/browser/components/places/content/places-tree.js
@@ -741,7 +741,7 @@
 
         var concreteGuid = PlacesUtils.getConcreteItemGuid(node);
         if (
-          guids.length == 0 ||
+          !guids.length ||
           !PlacesUtils.nodeIsContainer(node) ||
           checkedGuidsSet.has(concreteGuid)
         ) {
@@ -768,11 +768,7 @@
         // this node if we don't find any additional results here.
         var previousOpenness = node.containerOpen;
         node.containerOpen = true;
-        for (
-          var child = 0;
-          child < node.childCount && guids.length > 0;
-          child++
-        ) {
+        for (var child = 0; child < node.childCount && guids.length; child++) {
           var childNode = node.getChild(child);
           var found = findNodes(childNode);
           if (!foundOne) {
diff --git a/browser/components/places/content/places.js b/browser/components/places/content/places.js
index 29f23078b7e1..eb9036b4222f 100644
--- a/browser/components/places/content/places.js
+++ b/browser/components/places/content/places.js
@@ -255,12 +255,12 @@ var PlacesOrganizer = {
     aEvent.stopPropagation();
     switch (aEvent.command) {
       case "Back":
-        if (this._backHistory.length > 0) {
+        if (this._backHistory.length) {
           this.back();
         }
         break;
       case "Forward":
-        if (this._forwardHistory.length > 0) {
+        if (this._forwardHistory.length) {
           this.forward();
         }
         break;
@@ -297,7 +297,7 @@ var PlacesOrganizer = {
     this.updateDetailsPane();
 
     // update navigation commands
-    if (this._backHistory.length == 0) {
+    if (!this._backHistory.length) {
       document
         .getElementById("OrganizerCommand:Back")
         .setAttribute("disabled", true);
@@ -306,7 +306,7 @@ var PlacesOrganizer = {
         .getElementById("OrganizerCommand:Back")
         .removeAttribute("disabled");
     }
-    if (this._forwardHistory.length == 0) {
+    if (!this._forwardHistory.length) {
       document
         .getElementById("OrganizerCommand:Forward")
         .setAttribute("disabled", true);
@@ -543,7 +543,7 @@ var PlacesOrganizer = {
 
     (async function() {
       let backupFiles = await PlacesBackups.getBackupFiles();
-      if (backupFiles.length == 0) {
+      if (!backupFiles.length) {
         return;
       }
 
@@ -829,7 +829,7 @@ var PlacesSearchBox = {
    */
   _folders: [],
   get folders() {
-    if (this._folders.length == 0) {
+    if (!this._folders.length) {
       this._folders = PlacesUtils.bookmarks.userContentRoots;
     }
     return this._folders;
diff --git a/browser/components/places/content/treeView.js b/browser/components/places/content/treeView.js
index e61af1c09443..fb62d2c8ff9a 100644
--- a/browser/components/places/content/treeView.js
+++ b/browser/components/places/content/treeView.js
@@ -175,7 +175,7 @@ PlacesTreeView.prototype = {
     // for which nodeRemoved was called).
     let ancestors = Array.from(PlacesUtils.nodeAncestors(aNode));
     if (
-      ancestors.length == 0 ||
+      !ancestors.length ||
       ancestors[ancestors.length - 1] != this._rootNode
     ) {
       throw new Error("Removed node passed to _getRowForNode");
@@ -522,7 +522,7 @@ PlacesTreeView.prototype = {
     aNodesInfo,
     aUpdatedContainer
   ) {
-    if (aNodesInfo.length == 0) {
+    if (!aNodesInfo.length) {
       return;
     }
 
@@ -855,7 +855,7 @@ PlacesTreeView.prototype = {
 
     // Persist selection state.
     let nodesToReselect = this._getSelectedNodesInRange(oldRow, oldRow + count);
-    if (nodesToReselect.length > 0) {
+    if (nodesToReselect.length) {
       this.selection.selectEventsSuppressed = true;
     }
 
@@ -875,7 +875,7 @@ PlacesTreeView.prototype = {
     this.nodeInserted(aNewParent, aNode, aNewIndex);
 
     // Restore selection.
-    if (nodesToReselect.length > 0) {
+    if (nodesToReselect.length) {
       this._restoreSelection(nodesToReselect, aNewParent);
       this.selection.selectEventsSuppressed = false;
     }
@@ -1061,7 +1061,7 @@ PlacesTreeView.prototype = {
       // Select the row next to the closed container if any of its
       // children were selected, and nothing else is selected.
       if (
-        nodesToReselect.length > 0 &&
+        nodesToReselect.length &&
         nodesToReselect.length == oldSelectionCount
       ) {
         this.selection.rangedSelect(startReplacement, startReplacement, true);
diff --git a/browser/components/places/tests/browser/browser_paste_into_tags.js b/browser/components/places/tests/browser/browser_paste_into_tags.js
index c06a5e83af0a..f6e98edd4cd2 100644
--- a/browser/components/places/tests/browser/browser_paste_into_tags.js
+++ b/browser/components/places/tests/browser/browser_paste_into_tags.js
@@ -66,7 +66,7 @@ add_task(async function() {
   await PlacesUtils.bookmarks.fetch({ url: MOZURISPEC }, bm => {
     bookmarks.push(bm);
   });
-  ok(bookmarks.length > 0, "bookmark exists for the tagged history item");
+  ok(!!bookmarks.length, "bookmark exists for the tagged history item");
 
   // is the bookmark visible in the UI?
   // get the Unsorted Bookmarks node
diff --git a/browser/components/places/tests/browser/browser_sort_in_library.js b/browser/components/places/tests/browser/browser_sort_in_library.js
index 7a52f36cafc7..2f467ae98dec 100644
--- a/browser/components/places/tests/browser/browser_sort_in_library.js
+++ b/browser/components/places/tests/browser/browser_sort_in_library.js
@@ -170,7 +170,7 @@ function testInvalid(aOrganizerWin, aPlaceContentTree) {
  */
 function testSortByColAndDir(aOrganizerWin, aPlaceContentTree, aUnsortFirst) {
   let cols = aPlaceContentTree.getElementsByTagName("treecol");
-  ok(cols.length > 0, "sanity check: placeContent should contain columns");
+  ok(!!cols.length, "sanity check: placeContent should contain columns");
 
   for (let i = 0; i < cols.length; i++) {
     let col = cols.item(i);
diff --git a/browser/components/preferences/browserLanguages.js b/browser/components/preferences/browserLanguages.js
index 1d12125d6f66..7811a24031b6 100644
--- a/browser/components/preferences/browserLanguages.js
+++ b/browser/components/preferences/browserLanguages.js
@@ -66,7 +66,7 @@ async function dictionaryIdsForLocale(locale) {
   let entries = await RemoteSettings("language-dictionaries").get({
     filters: { id: locale },
   });
-  if (entries.length > 0) {
+  if (entries.length) {
     return entries[0].dictionaries;
   }
   return [];
@@ -512,7 +512,7 @@ var gBrowserLanguagesDialog = {
 
   async loadLocalesFromInstalled(available) {
     let items;
-    if (available.length > 0) {
+    if (available.length) {
       items = await getLocaleDisplayInfo(available);
       items.push(await this.createInstalledLabel());
     } else {
diff --git a/browser/components/preferences/in-content/findInPage.js b/browser/components/preferences/in-content/findInPage.js
index c3a86c209f2a..3929a50ad515 100644
--- a/browser/components/preferences/in-content/findInPage.js
+++ b/browser/components/preferences/in-content/findInPage.js
@@ -194,7 +194,7 @@ var gSearchResultsPane = {
       this.getFindSelection(startNode.ownerGlobal).addRange(range);
     }
 
-    return indices.length > 0;
+    return !!indices.length;
   },
 
   /**
@@ -567,7 +567,7 @@ var gSearchResultsPane = {
         .getAttribute("search-l10n-ids")
         .split(",")
         .map(s => s.trim().split("."))
-        .filter(s => s[0].length > 0);
+        .filter(s => !!s[0].length);
 
       const messages = await document.l10n.formatMessages(
         refs.map(ref => ({ id: ref[0] }))
diff --git a/browser/components/preferences/in-content/main.js b/browser/components/preferences/in-content/main.js
index 0b1ee155ac4c..f2f91ec219e3 100644
--- a/browser/components/preferences/in-content/main.js
+++ b/browser/components/preferences/in-content/main.js
@@ -1062,7 +1062,7 @@ var gMainPane = {
   /* Confirm the locale change and restart the browser in the new locale. */
   confirmBrowserLanguageChange(event) {
     let localesString = (event.target.getAttribute("locales") || "").trim();
-    if (!localesString || localesString.length == 0) {
+    if (!localesString || !localesString.length) {
       return;
     }
     let locales = localesString.split(",");
diff --git a/browser/components/preferences/in-content/subdialogs.js b/browser/components/preferences/in-content/subdialogs.js
index 773887d20f8e..3956358657d6 100644
--- a/browser/components/preferences/in-content/subdialogs.js
+++ b/browser/components/preferences/in-content/subdialogs.js
@@ -618,7 +618,7 @@ var gSubDialog = {
   _nextDialogID: 0,
   _preloadDialog: null,
   get _topDialog() {
-    return this._dialogs.length > 0
+    return this._dialogs.length
       ? this._dialogs[this._dialogs.length - 1]
       : undefined;
   },
@@ -639,7 +639,7 @@ var gSubDialog = {
       return;
     }
 
-    if (this._dialogs.length == 0) {
+    if (!this._dialogs.length) {
       // When opening the first dialog, show the dialog stack to make sure
       // the browser binding can be constructed.
       this._dialogStack.hidden = false;
diff --git a/browser/components/preferences/in-content/tests/browser_browser_languages_subdialog.js b/browser/components/preferences/in-content/tests/browser_browser_languages_subdialog.js
index 5968cd9d9701..62483c9dbc9b 100644
--- a/browser/components/preferences/in-content/tests/browser_browser_languages_subdialog.js
+++ b/browser/components/preferences/in-content/tests/browser_browser_languages_subdialog.js
@@ -184,7 +184,7 @@ function assertTelemetryRecorded(events) {
 
   // Make sure we got some data.
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
 
@@ -603,7 +603,7 @@ add_task(async function testInstallFromAMO() {
 
   await BrowserTestUtils.waitForCondition(async () => {
     let newDicts = await AddonManager.getAddonsByTypes(["dictionary"]);
-    let done = newDicts.length != 0;
+    let done = !!newDicts.length;
 
     if (done) {
       is(
diff --git a/browser/components/preferences/in-content/tests/browser_bug410900.js b/browser/components/preferences/in-content/tests/browser_bug410900.js
index cb074be50c23..4fd4bb21f52e 100644
--- a/browser/components/preferences/in-content/tests/browser_bug410900.js
+++ b/browser/components/preferences/in-content/tests/browser_bug410900.js
@@ -32,7 +32,7 @@ function runTest(win) {
   ok(rbox, "handlersView is present");
 
   var items = rbox && rbox.getElementsByTagName("richlistitem");
-  ok(items && items.length > 0, "App handler list populated");
+  ok(items && !!items.length, "App handler list populated");
 
   var handlerAdded = false;
   for (let i = 0; i < items.length; i++) {
diff --git a/browser/components/preferences/in-content/tests/browser_cookies_exceptions.js b/browser/components/preferences/in-content/tests/browser_cookies_exceptions.js
index b99b72d1e216..16ffb9a86ca1 100644
--- a/browser/components/preferences/in-content/tests/browser_cookies_exceptions.js
+++ b/browser/components/preferences/in-content/tests/browser_cookies_exceptions.js
@@ -546,7 +546,7 @@ function createObserveAllPromise(observances) {
           return;
         }
 
-        if (observances.length == 0) {
+        if (!observances.length) {
           // Should fail here as we are not expecting a notification, but we
           // don't. See bug 1063410.
           return;
@@ -576,7 +576,7 @@ function createObserveAllPromise(observances) {
           );
         }
 
-        if (observances.length == 0) {
+        if (!observances.length) {
           Services.obs.removeObserver(permObserver, "perm-changed");
           executeSoon(resolve);
         }
diff --git a/browser/components/preferences/in-content/tests/browser_permissions_checkPermissionsWereAdded.js b/browser/components/preferences/in-content/tests/browser_permissions_checkPermissionsWereAdded.js
index 3cc4cf117d62..cf9567b97100 100644
--- a/browser/components/preferences/in-content/tests/browser_permissions_checkPermissionsWereAdded.js
+++ b/browser/components/preferences/in-content/tests/browser_permissions_checkPermissionsWereAdded.js
@@ -49,7 +49,7 @@ const _addWebsiteAddressToPermissionBox = (
   buttonDialog.click();
   let permissionsBox = dialog.document.getElementById("permissionsBox");
   let children = permissionsBox.getElementsByAttribute("origin", "*");
-  is(children.length == 0, false, "Website added in url should be in the list");
+  is(!children.length, false, "Website added in url should be in the list");
 };
 
 const _checkIfPermissionsWereAdded = (dialog, expectedResult) => {
diff --git a/browser/components/preferences/in-content/tests/siteData/head.js b/browser/components/preferences/in-content/tests/siteData/head.js
index ca8ab9c03d36..a4e7c13ad36c 100644
--- a/browser/components/preferences/in-content/tests/siteData/head.js
+++ b/browser/components/preferences/in-content/tests/siteData/head.js
@@ -262,7 +262,7 @@ async function loadServiceWorkerTestPage(url) {
 function promiseServiceWorkersCleared() {
   return BrowserTestUtils.waitForCondition(() => {
     let serviceWorkers = serviceWorkerManager.getAllRegistrations();
-    if (serviceWorkers.length == 0) {
+    if (!serviceWorkers.length) {
       ok(true, "Cleared all service workers");
       return true;
     }
diff --git a/browser/components/preferences/languages.js b/browser/components/preferences/languages.js
index 1f14233fe94e..123fef0ade2e 100644
--- a/browser/components/preferences/languages.js
+++ b/browser/components/preferences/languages.js
@@ -186,7 +186,7 @@ var gLanguagesDialog = {
     // result in overflow.
     await document.l10n.translateFragment(this._activeLanguages);
 
-    if (this._activeLanguages.childNodes.length > 0) {
+    if (this._activeLanguages.childNodes.length) {
       this._activeLanguages.ensureIndexIsVisible(selectedIndex);
       this._activeLanguages.selectedIndex = selectedIndex;
     }
diff --git a/browser/components/preferences/siteDataSettings.js b/browser/components/preferences/siteDataSettings.js
index a40cdf3eac69..24fa68d017cc 100644
--- a/browser/components/preferences/siteDataSettings.js
+++ b/browser/components/preferences/siteDataSettings.js
@@ -148,8 +148,8 @@ let gSiteDataSettings = {
     let items = this._list.getElementsByTagName("richlistitem");
     let removeSelectedBtn = document.getElementById("removeSelected");
     let removeAllBtn = document.getElementById("removeAll");
-    removeSelectedBtn.disabled = this._list.selectedItems.length == 0;
-    removeAllBtn.disabled = items.length == 0;
+    removeSelectedBtn.disabled = !this._list.selectedItems.length;
+    removeAllBtn.disabled = !items.length;
 
     let l10nId = this._searchBox.value
       ? "site-data-remove-shown"
@@ -259,7 +259,7 @@ let gSiteDataSettings = {
       .filter(site => site.userAction == "remove")
       .map(site => site.host);
 
-    if (removals.length > 0) {
+    if (removals.length) {
       if (this._sites.length == removals.length) {
         allowed = SiteDataManager.promptSiteDataRemoval(window);
         if (allowed) {
@@ -323,7 +323,7 @@ let gSiteDataSettings = {
 
   onClickRemoveAll() {
     let siteItems = this._list.getElementsByTagName("richlistitem");
-    if (siteItems.length > 0) {
+    if (siteItems.length) {
       this._removeSiteItems(siteItems);
     }
   },
diff --git a/browser/components/search/test/browser/browser_searchTelemetry.js b/browser/components/search/test/browser/browser_searchTelemetry.js
index d62304f6d92d..af92f72898b1 100644
--- a/browser/components/search/test/browser/browser_searchTelemetry.js
+++ b/browser/components/search/test/browser/browser_searchTelemetry.js
@@ -55,7 +55,7 @@ async function assertTelemetry(expectedHistograms, expectedScalars) {
     );
   }, "should have the correct number of histograms");
 
-  if (Object.entries(expectedScalars).length > 0) {
+  if (Object.entries(expectedScalars).length) {
     await TestUtils.waitForCondition(() => {
       scalars =
         Services.telemetry.getSnapshotForKeyedScalars("main", false).parent ||
diff --git a/browser/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm b/browser/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm
index 06644f232180..b1ee8252da06 100644
--- a/browser/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm
+++ b/browser/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm
@@ -92,7 +92,7 @@ var RecentlyClosedTabsAndWindowsMenuUtils = {
     let closedWindowData = SessionStore.getClosedWindowData(false);
     let doc = aWindow.document;
     let fragment = doc.createDocumentFragment();
-    if (closedWindowData.length != 0) {
+    if (closedWindowData.length) {
       let menuLabelString = navigatorBundle.GetStringFromName(
         "menuUndoCloseWindowLabel"
       );
diff --git a/browser/components/sessionstore/SessionStore.jsm b/browser/components/sessionstore/SessionStore.jsm
index c6ad52e63327..07b693fb42d8 100644
--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -2069,7 +2069,7 @@ var SessionStoreInternal = {
           }
         }
       }
-      if (openTabs.length == 0) {
+      if (!openTabs.length) {
         this._closedWindows.splice(ix, 1);
       } else if (openTabs.length != openTabCount) {
         // Adjust the window's title if we removed an open tab
@@ -3822,7 +3822,7 @@ var SessionStoreInternal = {
     var hidden = WINDOW_HIDEABLE_FEATURES.filter(function(aItem) {
       return aWindow[aItem] && !aWindow[aItem].visible;
     });
-    if (hidden.length != 0) {
+    if (hidden.length) {
       winData.hidden = hidden.join(",");
     } else if (winData.hidden) {
       delete winData.hidden;
@@ -3913,14 +3913,14 @@ var SessionStoreInternal = {
       //        its own check for popups. c.f. bug 597619
       if (
         nonPopupCount == 0 &&
-        lastClosedWindowsCopy.length > 0 &&
+        !!lastClosedWindowsCopy.length &&
         RunState.isQuitting
       ) {
         // prepend the last non-popup browser window, so that if the user loads more tabs
         // at startup we don't accidentally add them to a popup window
         do {
           total.unshift(lastClosedWindowsCopy.shift());
-        } while (total[0].isPopup && lastClosedWindowsCopy.length > 0);
+        } while (total[0].isPopup && lastClosedWindowsCopy.length);
       }
     }
 
@@ -4085,7 +4085,7 @@ var SessionStoreInternal = {
       firstWindow &&
       !overwriteTabs &&
       winData.tabs.length == 1 &&
-      (!winData.tabs[0].entries || winData.tabs[0].entries.length == 0)
+      (!winData.tabs[0].entries || !winData.tabs[0].entries.length)
     ) {
       winData.tabs = [];
     }
@@ -5299,7 +5299,7 @@ var SessionStoreInternal = {
 
     // don't display the page when there's nothing to restore
     let winData = aState.windows || null;
-    if (!winData || winData.length == 0) {
+    if (!winData || !winData.length) {
       return false;
     }
 
diff --git a/browser/components/sessionstore/test/browser_393716.js b/browser/components/sessionstore/test/browser_393716.js
index eb65ef79f3cc..c27a977ad7a9 100644
--- a/browser/components/sessionstore/test/browser_393716.js
+++ b/browser/components/sessionstore/test/browser_393716.js
@@ -37,7 +37,7 @@ add_task(async function test_set_tabstate() {
   ok(
     state instanceof Object &&
       state.entries instanceof Array &&
-      state.entries.length > 0,
+      !!state.entries.length,
     "state object seems valid"
   );
   ok(
diff --git a/browser/components/sessionstore/test/browser_background_tab_crash.js b/browser/components/sessionstore/test/browser_background_tab_crash.js
index 0091aa8a685b..00baaeb1e40f 100644
--- a/browser/components/sessionstore/test/browser_background_tab_crash.js
+++ b/browser/components/sessionstore/test/browser_background_tab_crash.js
@@ -80,7 +80,7 @@ async function setupBackgroundTabs(testFn) {
  *        background state.
  */
 async function crashBackgroundTabs(tabs) {
-  Assert.ok(tabs.length > 0, "Need to crash at least one tab.");
+  Assert.ok(!!tabs.length, "Need to crash at least one tab.");
   for (let tab of tabs) {
     Assert.ok(tab.linkedBrowser.isRemoteBrowser, "tab is remote");
   }
diff --git a/browser/components/syncedtabs/TabListView.js b/browser/components/syncedtabs/TabListView.js
index 341b8050fab1..c09c4fe6dd13 100644
--- a/browser/components/syncedtabs/TabListView.js
+++ b/browser/components/syncedtabs/TabListView.js
@@ -580,7 +580,7 @@ TabListView.prototype = {
         }
       } else if (el.getAttribute("id") == "syncedTabsOpenAllInTabs") {
         const tabs = item.querySelectorAll(".item-tabs-list > .item.tab");
-        show = tabs.length > 0;
+        show = !!tabs.length;
       } else if (el.getAttribute("id") == "syncedTabsRefresh") {
         show = true;
       } else if (el.getAttribute("id") == "syncedTabsManageDevices") {
diff --git a/browser/components/translation/TranslationDocument.jsm b/browser/components/translation/TranslationDocument.jsm
index af1d1f726db9..f3ffd5594709 100644
--- a/browser/components/translation/TranslationDocument.jsm
+++ b/browser/components/translation/TranslationDocument.jsm
@@ -72,7 +72,7 @@ this.TranslationDocument.prototype = {
     // we are able to reduce their data payload sent to the translation service.
 
     for (let root of this.roots) {
-      if (root.children.length == 0 && root.nodeRef.childElementCount == 0) {
+      if (!root.children.length && root.nodeRef.childElementCount == 0) {
         root.isSimpleRoot = true;
       }
     }
@@ -497,7 +497,7 @@ function swapTextForItem(item, target) {
   // Let's start the process by adding the root item.
   let visitStack = [item];
 
-  while (visitStack.length > 0) {
+  while (visitStack.length) {
     let curItem = visitStack.shift();
 
     let domNode = curItem.nodeRef;
diff --git a/browser/components/uitour/UITour.jsm b/browser/components/uitour/UITour.jsm
index f11b524e1db8..19e64fbe7074 100644
--- a/browser/components/uitour/UITour.jsm
+++ b/browser/components/uitour/UITour.jsm
@@ -416,7 +416,7 @@ var UITour = {
             }
 
             let buttons = [];
-            if (Array.isArray(data.buttons) && data.buttons.length > 0) {
+            if (Array.isArray(data.buttons) && data.buttons.length) {
               for (let buttonData of data.buttons) {
                 if (
                   typeof buttonData == "object" &&
@@ -848,7 +848,7 @@ var UITour = {
           typeof name != "string" ||
           typeof value != "string" ||
           !name.startsWith("utm_") ||
-          value.length == 0 ||
+          !value.length ||
           !reSimpleString.test(name)
         ) {
           log.warn("_populateCampaignParams: invalid campaign param specified");
diff --git a/browser/components/uitour/test/head.js b/browser/components/uitour/test/head.js
index e928ffc1fd69..a88739125472 100644
--- a/browser/components/uitour/test/head.js
+++ b/browser/components/uitour/test/head.js
@@ -534,7 +534,7 @@ function done(usingAddTask = false) {
 }
 
 function nextTest() {
-  if (tests.length == 0) {
+  if (!tests.length) {
     info("finished tests in this file");
     finish();
     return;
diff --git a/browser/components/urlbar/UrlbarController.jsm b/browser/components/urlbar/UrlbarController.jsm
index 3256a6d18f4e..68fc90c758b9 100644
--- a/browser/components/urlbar/UrlbarController.jsm
+++ b/browser/components/urlbar/UrlbarController.jsm
@@ -392,7 +392,7 @@ class UrlbarController {
    */
   speculativeConnect(result, context, reason) {
     // Never speculative connect in private contexts.
-    if (!this.input || context.isPrivate || context.results.length == 0) {
+    if (!this.input || context.isPrivate || !context.results.length) {
       return;
     }
     let { url } = UrlbarUtils.getUrlFromResult(result);
diff --git a/browser/components/urlbar/UrlbarEventBufferer.jsm b/browser/components/urlbar/UrlbarEventBufferer.jsm
index d142a547d7e9..7054b768cc1d 100644
--- a/browser/components/urlbar/UrlbarEventBufferer.jsm
+++ b/browser/components/urlbar/UrlbarEventBufferer.jsm
@@ -231,7 +231,7 @@ class UrlbarEventBufferer {
     // If any event has been deferred for this search, then defer all subsequent
     // events so that the user does not experience them out of order.
     // All events will be replayed when _deferringTimeout fires.
-    if (this._eventsQueue.length > 0) {
+    if (this._eventsQueue.length) {
       return true;
     }
 
diff --git a/browser/components/urlbar/UrlbarInput.jsm b/browser/components/urlbar/UrlbarInput.jsm
index 07daa2fe4bd9..83e760b159b1 100644
--- a/browser/components/urlbar/UrlbarInput.jsm
+++ b/browser/components/urlbar/UrlbarInput.jsm
@@ -1963,7 +1963,7 @@ function getDroppableData(event) {
   }
   // The URL bar automatically handles inputs with newline characters,
   // so we can get away with treating text/x-moz-url flavours as text/plain.
-  if (links.length > 0 && links[0].url) {
+  if (links.length && links[0].url) {
     event.preventDefault();
     let href = links[0].url;
     if (UrlbarUtils.stripUnsafeProtocolOnPaste(href) != href) {
diff --git a/browser/components/urlbar/UrlbarTokenizer.jsm b/browser/components/urlbar/UrlbarTokenizer.jsm
index f1c6394c1b39..0340c36de2ba 100644
--- a/browser/components/urlbar/UrlbarTokenizer.jsm
+++ b/browser/components/urlbar/UrlbarTokenizer.jsm
@@ -158,7 +158,7 @@ var UrlbarTokenizer = {
    * @returns {boolean} whether the token looks like an origin.
    */
   looksLikeOrigin(token) {
-    if (token.length == 0) {
+    if (!token.length) {
       return false;
     }
     let atIndex = token.indexOf("@");
@@ -312,7 +312,7 @@ function filterTokens(tokens) {
   }
 
   // Handle restriction characters.
-  if (restrictions.length > 0) {
+  if (restrictions.length) {
     // We can apply two kind of restrictions: type (bookmark, search, ...) and
     // matching (url, title). These kind of restrictions can be combined, but we
     // can only have one restriction per kind.
diff --git a/browser/components/urlbar/UrlbarView.jsm b/browser/components/urlbar/UrlbarView.jsm
index 8fd12b297ddd..63e07b383930 100644
--- a/browser/components/urlbar/UrlbarView.jsm
+++ b/browser/components/urlbar/UrlbarView.jsm
@@ -762,7 +762,7 @@ class UrlbarView {
 
     let tagsContainer = item._elements.get("tagsContainer");
     tagsContainer.textContent = "";
-    if (result.payload.tags && result.payload.tags.length > 0) {
+    if (result.payload.tags && result.payload.tags.length) {
       tagsContainer.append(
         ...result.payload.tags.map((tag, i) => {
           const element = this._createElement("span");
diff --git a/browser/components/urlbar/tests/UrlbarTestUtils.jsm b/browser/components/urlbar/tests/UrlbarTestUtils.jsm
index a4eb223976af..72b164c10a33 100644
--- a/browser/components/urlbar/tests/UrlbarTestUtils.jsm
+++ b/browser/components/urlbar/tests/UrlbarTestUtils.jsm
@@ -141,8 +141,8 @@ var UrlbarTestUtils = {
     let typeIconStyle = win.getComputedStyle(typeIcon);
     details.displayed = {
       title: element.getElementsByClassName("urlbarView-title")[0].textContent,
-      action: actions.length > 0 ? actions[0].textContent : null,
-      url: urls.length > 0 ? urls[0].textContent : null,
+      action: actions.length ? actions[0].textContent : null,
+      url: urls.length ? urls[0].textContent : null,
       typeIcon: typeIconStyle["background-image"],
     };
     details.element = {
diff --git a/browser/components/urlbar/tests/browser/browser_autocomplete_a11y_label.js b/browser/components/urlbar/tests/browser/browser_autocomplete_a11y_label.js
index 83c2951df3c5..31034afab315 100644
--- a/browser/components/urlbar/tests/browser/browser_autocomplete_a11y_label.js
+++ b/browser/components/urlbar/tests/browser/browser_autocomplete_a11y_label.js
@@ -135,5 +135,5 @@ add_task(async function searchSuggestions() {
       }
     }
   }
-  Assert.ok(expectedSearches.length == 0);
+  Assert.ok(!expectedSearches.length);
 });
diff --git a/browser/components/urlbar/tests/browser/browser_urlbarCopying.js b/browser/components/urlbar/tests/browser/browser_urlbarCopying.js
index b5d8fb5da0db..ea2fc18ae671 100644
--- a/browser/components/urlbar/tests/browser/browser_urlbarCopying.js
+++ b/browser/components/urlbar/tests/browser/browser_urlbarCopying.js
@@ -241,7 +241,7 @@ var tests = [
 
 function nextTest() {
   let testCase = tests.shift();
-  if (tests.length == 0) {
+  if (!tests.length) {
     runTest(testCase, finish);
   } else {
     runTest(testCase, nextTest);
@@ -292,7 +292,7 @@ function testCopy(copyVal, targetValue, cb) {
       offsets.push([startBracket, endBracket - 1]);
       copyVal = copyVal.replace("<", "").replace(">", "");
     }
-    if (offsets.length == 0 || copyVal != gURLBar.value) {
+    if (!offsets.length || copyVal != gURLBar.value) {
       ok(false, "invalid copyVal: " + copyVal);
     }
     gURLBar.selectionStart = offsets[0][0];
diff --git a/browser/components/urlbar/tests/browser/browser_urlbar_speculative_connect_not_with_client_cert.js b/browser/components/urlbar/tests/browser/browser_urlbar_speculative_connect_not_with_client_cert.js
index f3507ee6af90..16ed4f85686f 100644
--- a/browser/components/urlbar/tests/browser/browser_urlbar_speculative_connect_not_with_client_cert.js
+++ b/browser/components/urlbar/tests/browser/browser_urlbar_speculative_connect_not_with_client_cert.js
@@ -40,7 +40,7 @@ const clientAuthDialogs = {
         expectingChooseCertificate ? "" : "not "
       }expecting chooseCertificate to be called`
     );
-    ok(certList.length > 0, "Should have at least one certificate");
+    ok(!!certList.length, "Should have at least one certificate");
     selectedIndex.value = 0;
     rememberClientAuthCertificate.value = false;
     chooseCertificateCalled = true;
diff --git a/browser/extensions/formautofill/FormAutofillHandler.jsm b/browser/extensions/formautofill/FormAutofillHandler.jsm
index d350578e1d65..ce1ee71f2936 100644
--- a/browser/extensions/formautofill/FormAutofillHandler.jsm
+++ b/browser/extensions/formautofill/FormAutofillHandler.jsm
@@ -517,7 +517,7 @@ class FormAutofillSection {
    */
   createRecord() {
     let details = this.fieldDetails;
-    if (!this.isEnabled() || !details || details.length == 0) {
+    if (!this.isEnabled() || !details || !details.length) {
       return null;
     }
 
diff --git a/browser/extensions/formautofill/FormAutofillHeuristics.jsm b/browser/extensions/formautofill/FormAutofillHeuristics.jsm
index 5108dcc10be1..0e061bda2ef3 100644
--- a/browser/extensions/formautofill/FormAutofillHeuristics.jsm
+++ b/browser/extensions/formautofill/FormAutofillHeuristics.jsm
@@ -167,7 +167,7 @@ class FieldScanner {
     if (!this._sectionEnabled) {
       return this._getFinalDetails(this.fieldDetails);
     }
-    if (this._sections.length == 0) {
+    if (!this._sections.length) {
       return [];
     }
     if (
@@ -304,7 +304,7 @@ class FieldScanner {
         });
         return section;
       })
-      .filter(section => section.fieldDetails.length > 0);
+      .filter(section => !!section.fieldDetails.length);
   }
 
   elementExisting(index) {
@@ -352,7 +352,7 @@ var LabelUtils = {
         return;
       }
 
-      if (el.nodeType == el.TEXT_NODE || el.childNodes.length == 0) {
+      if (el.nodeType == el.TEXT_NODE || !el.childNodes.length) {
         let trimmedText = el.textContent.trim();
         if (trimmedText) {
           strings.push(trimmedText);
@@ -1000,7 +1000,7 @@ this.FormAutofillHeuristics = {
     }
 
     let regexps = this._getRegExpList(isAutoCompleteOff, element.tagName);
-    if (regexps.length == 0) {
+    if (!regexps.length) {
       return null;
     }
 
diff --git a/browser/extensions/formautofill/ProfileAutoCompleteResult.jsm b/browser/extensions/formautofill/ProfileAutoCompleteResult.jsm
index 4f44512c5339..505abfe7c3c7 100644
--- a/browser/extensions/formautofill/ProfileAutoCompleteResult.jsm
+++ b/browser/extensions/formautofill/ProfileAutoCompleteResult.jsm
@@ -78,7 +78,7 @@ class ProfileAutoCompleteResult {
     // The result code of this result object.
     if (resultCode) {
       this.searchResult = resultCode;
-    } else if (matchingProfiles.length > 0) {
+    } else if (matchingProfiles.length) {
       this.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
     } else {
       this.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/browser/extensions/formautofill/content/editDialog.js b/browser/extensions/formautofill/content/editDialog.js
index 16dd6bcfee06..5aadb845d645 100644
--- a/browser/extensions/formautofill/content/editDialog.js
+++ b/browser/extensions/formautofill/content/editDialog.js
@@ -132,9 +132,7 @@ class AutofillEditDialog {
   updateSaveButtonState() {
     // Toggle disabled attribute on the save button based on
     // whether the form is filled or empty.
-    if (
-      Object.keys(this._elements.fieldContainer.buildFormObject()).length == 0
-    ) {
+    if (!Object.keys(this._elements.fieldContainer.buildFormObject()).length) {
       this._elements.save.setAttribute("disabled", true);
     } else {
       this._elements.save.removeAttribute("disabled");
diff --git a/browser/extensions/formautofill/test/browser/head.js b/browser/extensions/formautofill/test/browser/head.js
index 4977b9c7a218..aa2558b265d6 100644
--- a/browser/extensions/formautofill/test/browser/head.js
+++ b/browser/extensions/formautofill/test/browser/head.js
@@ -243,7 +243,7 @@ async function expectPopupOpen(browser) {
   await BrowserTestUtils.waitForCondition(() => {
     const listItemElems = getDisplayedPopupItems(browser);
     return (
-      [...listItemElems].length > 0 &&
+      !![...listItemElems].length &&
       [...listItemElems].every(item => {
         return (
           (item.getAttribute("originaltype") == "autofill-profile" ||
@@ -329,7 +329,7 @@ function removeCreditCards(guids) {
 
 function getNotification(index = 0) {
   let notifications = PopupNotifications.panel.childNodes;
-  ok(notifications.length > 0, "at least one notification displayed");
+  ok(!!notifications.length, "at least one notification displayed");
   ok(true, notifications.length + " notification(s)");
   return notifications[index];
 }
diff --git a/browser/extensions/formautofill/test/mochitest/formautofill_parent_utils.js b/browser/extensions/formautofill/test/mochitest/formautofill_parent_utils.js
index f48d80a4cfa8..5fb4d2f22138 100644
--- a/browser/extensions/formautofill/test/mochitest/formautofill_parent_utils.js
+++ b/browser/extensions/formautofill/test/mochitest/formautofill_parent_utils.js
@@ -142,7 +142,7 @@ var ParentUtils = {
       record => record.guid
     );
 
-    if (guids.length == 0) {
+    if (!guids.length) {
       return;
     }
 
@@ -161,7 +161,7 @@ var ParentUtils = {
       record => record.guid
     );
 
-    if (guids.length == 0) {
+    if (!guids.length) {
       return;
     }
 
diff --git a/browser/extensions/formautofill/test/unit/test_profileAutocompleteResult.js b/browser/extensions/formautofill/test/unit/test_profileAutocompleteResult.js
index c0e2b01f13f6..c6efff3998b3 100644
--- a/browser/extensions/formautofill/test/unit/test_profileAutocompleteResult.js
+++ b/browser/extensions/formautofill/test/unit/test_profileAutocompleteResult.js
@@ -422,7 +422,7 @@ add_task(async function test_all_patterns() {
         equal(actual.getImageAt(index), item.image);
       });
 
-      if (expectedValue.items.length != 0) {
+      if (expectedValue.items.length) {
         Assert.throws(
           () => actual.getValueAt(expectedItemLength),
           /Index out of range\./
diff --git a/browser/extensions/pdfjs/test/browser_pdfjs_main.js b/browser/extensions/pdfjs/test/browser_pdfjs_main.js
index 2de656df49c2..483aef4f993c 100644
--- a/browser/extensions/pdfjs/test/browser_pdfjs_main.js
+++ b/browser/extensions/pdfjs/test/browser_pdfjs_main.js
@@ -62,7 +62,7 @@ add_task(async function test() {
         var viewBookmark = content.document.querySelector("a#viewBookmark");
         viewBookmark.click();
 
-        Assert.ok(viewBookmark.href.length > 0, "viewBookmark button has href");
+        Assert.ok(!!viewBookmark.href.length, "viewBookmark button has href");
 
         var viewer = content.wrappedJSObject.PDFViewerApplication;
         await viewer.close();
diff --git a/browser/modules/AsyncTabSwitcher.jsm b/browser/modules/AsyncTabSwitcher.jsm
index e8684ab81f2f..e03b3555bad8 100644
--- a/browser/modules/AsyncTabSwitcher.jsm
+++ b/browser/modules/AsyncTabSwitcher.jsm
@@ -1308,7 +1308,7 @@ class AsyncTabSwitcher {
 
   addLogFlag(flag, ...subFlags) {
     if (this.logging()) {
-      if (subFlags.length > 0) {
+      if (subFlags.length) {
         flag += `(${subFlags.map(f => (f ? 1 : 0)).join("")})`;
       }
       this._logFlags.push(flag);
@@ -1420,13 +1420,13 @@ class AsyncTabSwitcher {
       lastMatch = i;
     }
 
-    if (unloadedTabsStrings.length > 0) {
+    if (unloadedTabsStrings.length) {
       accum += `${unloadedTabsStrings.join(",")}:(unloaded) `;
     }
 
     accum += "cached: " + this.tabLayerCache.length + " ";
 
-    if (this._logFlags.length > 0) {
+    if (this._logFlags.length) {
       accum += `[${this._logFlags.join(",")}] `;
       this._logFlags = [];
     }
diff --git a/browser/modules/ExtensionsUI.jsm b/browser/modules/ExtensionsUI.jsm
index c894cfa2a614..00888248ed86 100644
--- a/browser/modules/ExtensionsUI.jsm
+++ b/browser/modules/ExtensionsUI.jsm
@@ -240,7 +240,7 @@ var ExtensionsUI = {
       let strings = this._buildStrings(info);
 
       // If this is an update with no promptable permissions, just apply it
-      if (info.type == "update" && strings.msgs.length == 0) {
+      if (info.type == "update" && !strings.msgs.length) {
         info.resolve();
         return;
       }
@@ -288,7 +288,7 @@ var ExtensionsUI = {
       let strings = this._buildStrings(info);
 
       // If we don't prompt for any new permissions, just apply it
-      if (strings.msgs.length == 0) {
+      if (!strings.msgs.length) {
         info.resolve();
         return;
       }
@@ -326,7 +326,7 @@ var ExtensionsUI = {
       });
 
       // If we don't have any promptable permissions, just proceed
-      if (strings.msgs.length == 0) {
+      if (!strings.msgs.length) {
         resolve(true);
         return;
       }
@@ -399,14 +399,14 @@ var ExtensionsUI = {
 
           let listIntroEl = doc.getElementById("addon-webext-perm-intro");
           listIntroEl.textContent = strings.listIntro;
-          listIntroEl.hidden = strings.msgs.length == 0;
+          listIntroEl.hidden = !strings.msgs.length;
 
           let listInfoEl = doc.getElementById("addon-webext-perm-info");
           listInfoEl.textContent = strings.learnMore;
           listInfoEl.href =
             Services.urlFormatter.formatURLPref("app.support.baseURL") +
             "extension-permissions";
-          listInfoEl.hidden = strings.msgs.length == 0;
+          listInfoEl.hidden = !strings.msgs.length;
 
           let list = doc.getElementById("addon-webext-perm-list");
           while (list.firstChild) {
diff --git a/browser/modules/FaviconLoader.jsm b/browser/modules/FaviconLoader.jsm
index 132d4f97a116..fb003e0d3bdb 100644
--- a/browser/modules/FaviconLoader.jsm
+++ b/browser/modules/FaviconLoader.jsm
@@ -426,7 +426,7 @@ function guessType(icon) {
  * @param {integer} preferredWidth The preferred width for tab icons.
  */
 function selectIcons(iconInfos, preferredWidth) {
-  if (iconInfos.length == 0) {
+  if (!iconInfos.length) {
     return {
       richIcon: null,
       tabIcon: null,
@@ -598,7 +598,7 @@ class FaviconLoader {
     // we can still attempt to load icons, which will fail since the content
     // window is no longer available. Checking if iconInfos has been cleared
     // allows us to bail out early in this case.
-    if (this.iconInfos.length == 0) {
+    if (!this.iconInfos.length) {
       return;
     }
 
diff --git a/browser/modules/PermissionUI.jsm b/browser/modules/PermissionUI.jsm
index cbae0fb6e47f..03cc9a34d656 100644
--- a/browser/modules/PermissionUI.jsm
+++ b/browser/modules/PermissionUI.jsm
@@ -1062,7 +1062,7 @@ function MIDIPermissionPrompt(request) {
   let types = request.types.QueryInterface(Ci.nsIArray);
   let perm = types.queryElementAt(0, Ci.nsIContentPermissionType);
   this.isSysexPerm =
-    perm.options.length > 0 &&
+    !!perm.options.length &&
     perm.options.queryElementAt(0, Ci.nsISupportsString) == "sysex";
   this.permName = "midi";
   if (this.isSysexPerm) {
diff --git a/browser/modules/Sanitizer.jsm b/browser/modules/Sanitizer.jsm
index 9b6f71fbfc6b..507844071915 100644
--- a/browser/modules/Sanitizer.jsm
+++ b/browser/modules/Sanitizer.jsm
@@ -552,7 +552,7 @@ var Sanitizer = {
           }
         }
 
-        if (windowList.length == 0) {
+        if (!windowList.length) {
           return;
         }
 
diff --git a/browser/modules/WindowsJumpLists.jsm b/browser/modules/WindowsJumpLists.jsm
index db3ce9bc16c8..506cf807197d 100644
--- a/browser/modules/WindowsJumpLists.jsm
+++ b/browser/modules/WindowsJumpLists.jsm
@@ -218,7 +218,7 @@ var WinTaskbarJumpList = {
 
   _pendingStatements: {},
   _hasPendingStatements: function WTBJL__hasPendingStatements() {
-    return Object.keys(this._pendingStatements).length > 0;
+    return !!Object.keys(this._pendingStatements).length;
   },
 
   async _buildList() {
@@ -265,7 +265,7 @@ var WinTaskbarJumpList = {
   async _startBuild() {
     this._builder.abortListBuild();
     let URIsToRemove = await this._builder.initListBuild();
-    if (URIsToRemove.length > 0) {
+    if (URIsToRemove.length) {
       // Prior to building, delete removed items from history.
       this._clearHistory(URIsToRemove);
     }
@@ -302,7 +302,7 @@ var WinTaskbarJumpList = {
       items.appendElement(item);
     }, this);
 
-    if (items.length > 0) {
+    if (items.length) {
       this._builder.addListToBuild(
         this._builder.JUMPLIST_CATEGORY_TASKS,
         items
@@ -311,7 +311,7 @@ var WinTaskbarJumpList = {
   },
 
   _buildCustom: function WTBJL__buildCustom(title, items) {
-    if (items.length > 0) {
+    if (items.length) {
       this._builder.addListToBuild(
         this._builder.JUMPLIST_CATEGORY_CUSTOMLIST,
         items,
@@ -427,7 +427,7 @@ var WinTaskbarJumpList = {
     ].createInstance(Ci.nsILocalHandlerApp);
     handlerApp.executable = file;
     // handlers default to the leaf name if a name is not specified
-    if (name && name.length != 0) {
+    if (name && name.length) {
       handlerApp.name = name;
     }
     handlerApp.detailedDescription = description;
@@ -499,7 +499,7 @@ var WinTaskbarJumpList = {
       })
       .filter(uri => !!uri);
 
-    if (URIsToRemove.length > 0) {
+    if (URIsToRemove.length) {
       PlacesUtils.history.remove(URIsToRemove).catch(Cu.reportError);
     }
   },
diff --git a/browser/modules/WindowsPreviewPerTab.jsm b/browser/modules/WindowsPreviewPerTab.jsm
index 762648f57020..49b07b7660c3 100644
--- a/browser/modules/WindowsPreviewPerTab.jsm
+++ b/browser/modules/WindowsPreviewPerTab.jsm
@@ -839,7 +839,7 @@ var AeroPeek = {
     win.gTaskbarTabGroup.destroy();
     delete win.gTaskbarTabGroup;
 
-    if (this.windows.length == 0) {
+    if (!this.windows.length) {
       this.destroy();
     }
   },
diff --git a/browser/modules/test/browser/browser_PageActions.js b/browser/modules/test/browser/browser_PageActions.js
index d4d0f59a697d..68cde2379a3f 100644
--- a/browser/modules/test/browser/browser_PageActions.js
+++ b/browser/modules/test/browser/browser_PageActions.js
@@ -1396,7 +1396,7 @@ add_task(async function removeRetainState() {
   // Get the list of actions initially in the urlbar.
   let initialActionsInUrlbar = PageActions.actionsInUrlbar(window);
   Assert.ok(
-    initialActionsInUrlbar.length > 0,
+    !!initialActionsInUrlbar.length,
     "This test expects there to be at least one action in the urlbar initially (like the bookmark star)"
   );
 
@@ -1754,7 +1754,7 @@ add_task(async function contextMenu() {
     true
   );
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
   let relatedEvents = snapshot.parent
diff --git a/browser/modules/webrtcUI.jsm b/browser/modules/webrtcUI.jsm
index 06fcce92d876..06235e3599d4 100644
--- a/browser/modules/webrtcUI.jsm
+++ b/browser/modules/webrtcUI.jsm
@@ -699,8 +699,8 @@ function prompt(aBrowser, aRequest) {
             browser._devicePermissionPrincipals || [];
           browser._devicePermissionPrincipals.push(principal);
 
-          let camNeeded = videoDevices.length > 0;
-          let micNeeded = audioDevices.length > 0;
+          let camNeeded = !!videoDevices.length;
+          let micNeeded = !!audioDevices.length;
           checkOSPermission(camNeeded, micNeeded).then(havePermission => {
             if (havePermission) {
               let mm = browser.messageManager;
@@ -1067,8 +1067,8 @@ function prompt(aBrowser, aRequest) {
           aBrowser._devicePermissionPrincipals.push(principal);
         }
 
-        let camNeeded = videoDevices.length > 0;
-        let micNeeded = audioDevices.length > 0;
+        let camNeeded = !!videoDevices.length;
+        let micNeeded = !!audioDevices.length;
         let havePermission = await checkOSPermission(camNeeded, micNeeded);
         if (!havePermission) {
           denyRequestNoPermission(notification.browser, aRequest);
diff --git a/toolkit/actors/DateTimePickerChild.jsm b/toolkit/actors/DateTimePickerChild.jsm
index 7e41ea6ceeb7..25bbae801060 100644
--- a/toolkit/actors/DateTimePickerChild.jsm
+++ b/toolkit/actors/DateTimePickerChild.jsm
@@ -176,8 +176,7 @@ class DateTimePickerChild extends JSWindowActorChild {
           detail: {
             // Pass partial value if it's available, otherwise pass input
             // element's value.
-            value:
-              Object.keys(value).length > 0 ? value : this._inputElement.value,
+            value: Object.keys(value).length ? value : this._inputElement.value,
             min: this._inputElement.getMinimum(),
             max: this._inputElement.getMaximum(),
             step: this._inputElement.getStep(),
diff --git a/toolkit/actors/PopupBlockingChild.jsm b/toolkit/actors/PopupBlockingChild.jsm
index 85d0ce3d648f..b2bbafbc6903 100644
--- a/toolkit/actors/PopupBlockingChild.jsm
+++ b/toolkit/actors/PopupBlockingChild.jsm
@@ -140,7 +140,7 @@ class PopupBlockingChild extends ActorChild {
           this.popupDataInternal.splice(i, 1);
         }
       }
-      if (this.popupData.length == 0) {
+      if (!this.popupData.length) {
         this.popupData = null;
         this.popupDataInternal = null;
       }
diff --git a/toolkit/components/aboutmemory/content/aboutMemory.js b/toolkit/components/aboutmemory/content/aboutMemory.js
index ae16cf3ec91d..6c28ec862b57 100644
--- a/toolkit/components/aboutmemory/content/aboutMemory.js
+++ b/toolkit/components/aboutmemory/content/aboutMemory.js
@@ -1747,7 +1747,7 @@ function appendWarningElements(
     );
   }
 
-  if (gUnsafePathsWithInvalidValuesForThisProcess.length > 0) {
+  if (gUnsafePathsWithInvalidValuesForThisProcess.length) {
     let div = appendElement(aP, "div");
     appendElementWithText(
       div,
diff --git a/toolkit/components/aboutperformance/content/aboutPerformance.js b/toolkit/components/aboutperformance/content/aboutPerformance.js
index a083962db1fa..96a177d1cddf 100644
--- a/toolkit/components/aboutperformance/content/aboutPerformance.js
+++ b/toolkit/components/aboutperformance/content/aboutPerformance.js
@@ -269,7 +269,7 @@ var State = {
    */
   async update() {
     // If the buffer is empty, add one value for bootstraping purposes.
-    if (this._buffer.length == 0) {
+    if (!this._buffer.length) {
       this._latest = await this._promiseSnapshot();
       this._buffer.push(this._latest);
       await wait(BUFFER_SAMPLING_RATE_MS * 1.1);
diff --git a/toolkit/components/antitracking/test/browser/antitracking_head.js b/toolkit/components/antitracking/test/browser/antitracking_head.js
index ef82c2564854..75e4c62d15de 100644
--- a/toolkit/components/antitracking/test/browser/antitracking_head.js
+++ b/toolkit/components/antitracking/test/browser/antitracking_head.js
@@ -671,7 +671,7 @@ this.AntiTracking = {
       if (expectedCategory == "") {
         is(allMessages.length, 0, "No console messages should be generated");
       } else {
-        ok(allMessages.length != 0, "Some console message should be generated");
+        ok(!!allMessages.length, "Some console message should be generated");
       }
       for (let msg of allMessages) {
         is(
diff --git a/toolkit/components/apppicker/content/appPicker.js b/toolkit/components/apppicker/content/appPicker.js
index b1505c82f7f5..34e7d33063fe 100644
--- a/toolkit/components/apppicker/content/appPicker.js
+++ b/toolkit/components/apppicker/content/appPicker.js
@@ -78,7 +78,7 @@ AppPicker.prototype = {
 
     var primaryCount = 0;
 
-    if (!fileList || fileList.length == 0) {
+    if (!fileList || !fileList.length) {
       // display a message saying nothing is configured
       document.getElementById("app-picker-notfound").removeAttribute("hidden");
       return;
diff --git a/toolkit/components/autocomplete/tests/unit/test_378079.js b/toolkit/components/autocomplete/tests/unit/test_378079.js
index ffa927f5a721..5c680affbe08 100644
--- a/toolkit/components/autocomplete/tests/unit/test_378079.js
+++ b/toolkit/components/autocomplete/tests/unit/test_378079.js
@@ -64,7 +64,7 @@ function AutoCompleteResult(aValues, aComments, aStyles) {
   this._comments = aComments;
   this._styles = aStyles;
 
-  if (this._values.length > 0) {
+  if (this._values.length) {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
   } else {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/toolkit/components/autocomplete/tests/unit/test_393191.js b/toolkit/components/autocomplete/tests/unit/test_393191.js
index 25a98ca4d0db..094be86da73e 100644
--- a/toolkit/components/autocomplete/tests/unit/test_393191.js
+++ b/toolkit/components/autocomplete/tests/unit/test_393191.js
@@ -63,7 +63,7 @@ function AutoCompleteResult(aValues, aComments, aStyles) {
   this._comments = aComments;
   this._styles = aStyles;
 
-  if (this._values.length > 0) {
+  if (this._values.length) {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
   } else {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/toolkit/components/autocomplete/tests/unit/test_440866.js b/toolkit/components/autocomplete/tests/unit/test_440866.js
index 4262368616f0..9ab0b4fba338 100644
--- a/toolkit/components/autocomplete/tests/unit/test_440866.js
+++ b/toolkit/components/autocomplete/tests/unit/test_440866.js
@@ -62,7 +62,7 @@ function AutoCompleteResult(aValues, aComments, aStyles) {
   this._comments = aComments;
   this._styles = aStyles;
 
-  if (this._values.length > 0) {
+  if (this._values.length) {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
   } else {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/toolkit/components/autocomplete/tests/unit/test_autocomplete_multiple.js b/toolkit/components/autocomplete/tests/unit/test_autocomplete_multiple.js
index f1e4c6b16ab2..c7a39ead6071 100644
--- a/toolkit/components/autocomplete/tests/unit/test_autocomplete_multiple.js
+++ b/toolkit/components/autocomplete/tests/unit/test_autocomplete_multiple.js
@@ -128,14 +128,14 @@ AutoCompleteSearch.prototype = {
    */
   startSearch(aSearchString, aSearchParam, aPreviousResult, aListener) {
     var result = this._result;
-    if (result._values.length > 0) {
+    if (result._values.length) {
       result.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS_ONGOING;
     } else {
       result.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH_ONGOING;
     }
     aListener.onSearchResult(this, result);
 
-    if (result._values.length > 0) {
+    if (result._values.length) {
       result.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
     } else {
       result.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/toolkit/components/autocomplete/tests/unit/test_previousResult.js b/toolkit/components/autocomplete/tests/unit/test_previousResult.js
index 53dd97a01ca4..cb206ff49e42 100644
--- a/toolkit/components/autocomplete/tests/unit/test_previousResult.js
+++ b/toolkit/components/autocomplete/tests/unit/test_previousResult.js
@@ -63,7 +63,7 @@ function AutoCompleteResult(aValues, aComments, aStyles) {
   this._values = aValues;
   this._comments = aComments;
   this._styles = aStyles;
-  if (this._values.length > 0) {
+  if (this._values.length) {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
   } else {
     this.searchResult = Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
diff --git a/toolkit/components/backgroundhangmonitor/tests/test_BHRObserver.js b/toolkit/components/backgroundhangmonitor/tests/test_BHRObserver.js
index b00f68578d19..d5189586aa27 100644
--- a/toolkit/components/backgroundhangmonitor/tests/test_BHRObserver.js
+++ b/toolkit/components/backgroundhangmonitor/tests/test_BHRObserver.js
@@ -94,7 +94,7 @@ add_task(async function test_BHRObserver() {
 
     // hang.stack
     ok(Array.isArray(hang.stack));
-    ok(hang.stack.length > 0);
+    ok(!!hang.stack.length);
     hang.stack.forEach(entry => {
       // Each stack frame entry is either a native or pseudostack entry. A
       // native stack entry is an array with module index (number), and offset
@@ -141,7 +141,7 @@ add_task(async function test_BHRObserver() {
 
     // hang.stack
     ok(Array.isArray(hang.stack));
-    ok(hang.stack.length > 0);
+    ok(!!hang.stack.length);
     hang.stack.forEach(entry => {
       // Each stack frame entry is either a native or pseudostack entry. A
       // native stack entry is an array with module index (number), and offset
diff --git a/toolkit/components/certviewer/content/certviewer.js b/toolkit/components/certviewer/content/certviewer.js
index eaa0feb50841..411d56236105 100644
--- a/toolkit/components/certviewer/content/certviewer.js
+++ b/toolkit/components/certviewer/content/certviewer.js
@@ -47,7 +47,7 @@ const createEntryItem = (label, info) => {
 
 const addToResultUsing = (callback, certItems, sectionTitle, Critical) => {
   let items = callback();
-  if (items.length > 0) {
+  if (items.length) {
     certItems.push({
       sectionTitle,
       sectionItems: items,
diff --git a/toolkit/components/contentprefs/ContentPrefService2.jsm b/toolkit/components/contentprefs/ContentPrefService2.jsm
index 9d958c16eff1..58cc908be5c8 100644
--- a/toolkit/components/contentprefs/ContentPrefService2.jsm
+++ b/toolkit/components/contentprefs/ContentPrefService2.jsm
@@ -940,7 +940,7 @@ ContentPrefService2.prototype = {
           ? Ci.nsIContentPrefCallback2.COMPLETE_OK
           : Ci.nsIContentPrefCallback2.COMPLETE_ERROR,
         ok,
-        rows && rows.length > 0
+        rows && !!rows.length
       );
     } catch (e) {
       Cu.reportError(e);
diff --git a/toolkit/components/crashes/tests/xpcshell/test_crash_service.js b/toolkit/components/crashes/tests/xpcshell/test_crash_service.js
index c1453ec22d72..564691405136 100644
--- a/toolkit/components/crashes/tests/xpcshell/test_crash_service.js
+++ b/toolkit/components/crashes/tests/xpcshell/test_crash_service.js
@@ -93,11 +93,11 @@ async function test_addCrashBase(crashId, allThreads) {
     Assert.equal(stackTraces.status, "OK");
     Assert.ok(stackTraces.crash_info, "The crash_info field is populated.");
     Assert.ok(
-      stackTraces.modules && stackTraces.modules.length > 0,
+      stackTraces.modules && !!stackTraces.modules.length,
       "The module list is populated."
     );
     Assert.ok(
-      stackTraces.threads && stackTraces.threads.length > 0,
+      stackTraces.threads && !!stackTraces.threads.length,
       "The thread list is populated."
     );
 
@@ -114,7 +114,7 @@ async function test_addCrashBase(crashId, allThreads) {
     }
 
     let frames = stackTraces.threads[0].frames;
-    Assert.ok(frames && frames.length > 0, "The stack trace is present.\n");
+    Assert.ok(frames && !!frames.length, "The stack trace is present.\n");
   } catch (e) {
     Assert.ok(false, "StackTraces does not contain valid JSON.");
   }
diff --git a/toolkit/components/ctypes/tests/unit/test_jsctypes.js b/toolkit/components/ctypes/tests/unit/test_jsctypes.js
index e2fc99f7a2cc..770adad7cbd3 100644
--- a/toolkit/components/ctypes/tests/unit/test_jsctypes.js
+++ b/toolkit/components/ctypes/tests/unit/test_jsctypes.js
@@ -2853,7 +2853,7 @@ function run_FunctionType_tests() {
   Assert.equal(f_t.size, undefined);
   Assert.ok(f_t.abi === ctypes.default_abi);
   Assert.ok(f_t.returnType === g_t);
-  Assert.ok(f_t.argTypes.length == 0);
+  Assert.ok(!f_t.argTypes.length);
 
   Assert.equal(f_t.toString(), "type " + name);
   Assert.equal(f_t.toSource(), "ctypes.FunctionType(ctypes.default_abi, g_t)");
diff --git a/toolkit/components/enterprisepolicies/EnterprisePolicies.js b/toolkit/components/enterprisepolicies/EnterprisePolicies.js
index 84e9227acd0f..67aafa791dbf 100644
--- a/toolkit/components/enterprisepolicies/EnterprisePolicies.js
+++ b/toolkit/components/enterprisepolicies/EnterprisePolicies.js
@@ -188,7 +188,7 @@ EnterprisePoliciesManager.prototype = {
 
   _runPoliciesCallbacks(timing) {
     let callbacks = this._callbacks[timing];
-    while (callbacks.length > 0) {
+    while (callbacks.length) {
       let callback = callbacks.shift();
       try {
         callback();
diff --git a/toolkit/components/extensions/Extension.jsm b/toolkit/components/extensions/Extension.jsm
index 5e4948b871fd..75d5c9012b1c 100644
--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -939,7 +939,7 @@ class ExtensionData {
       // Check if there's a root directory `/localization` in the langpack.
       // If there is one, add it with the name `toolkit` as a FileSource.
       const entries = await this.readDirectory("localization");
-      if (entries.length > 0) {
+      if (entries.length) {
         l10nRegistrySources.toolkit = "";
       }
 
@@ -1404,10 +1404,9 @@ class ExtensionData {
         "webextPerms.sideloadHeader",
         ["<>"]
       );
-      let key =
-        result.msgs.length == 0
-          ? "webextPerms.sideloadTextNoPerms"
-          : "webextPerms.sideloadText2";
+      let key = !result.msgs.length
+        ? "webextPerms.sideloadTextNoPerms"
+        : "webextPerms.sideloadText2";
       result.text = bundle.GetStringFromName(key);
       result.acceptText = bundle.GetStringFromName(
         "webextPerms.sideloadEnable.label"
@@ -1652,7 +1651,7 @@ class Extension extends ExtensionData {
         this.permissions.add(perm);
       }
 
-      if (permissions.origins.length > 0) {
+      if (permissions.origins.length) {
         let patterns = this.whiteListedHosts.patterns.map(host => host.pattern);
 
         this.whiteListedHosts = new MatchPatternSet(
@@ -2498,7 +2497,7 @@ class Langpack extends ExtensionData {
 
   async startup(reason) {
     this.chromeRegistryHandle = null;
-    if (this.startupData.chromeEntries.length > 0) {
+    if (this.startupData.chromeEntries.length) {
       const manifestURI = Services.io.newURI(
         "manifest.json",
         null,
diff --git a/toolkit/components/extensions/ExtensionChild.jsm b/toolkit/components/extensions/ExtensionChild.jsm
index 7a7ce112e09b..f427a2a93134 100644
--- a/toolkit/components/extensions/ExtensionChild.jsm
+++ b/toolkit/components/extensions/ExtensionChild.jsm
@@ -839,7 +839,7 @@ class BrowserExtensionContent extends EventEmitter {
 
     /* eslint-disable mozilla/balanced-listeners */
     this.on("add-permissions", (ignoreEvent, permissions) => {
-      if (permissions.permissions.length > 0) {
+      if (permissions.permissions.length) {
         let perms = new Set(this.policy.permissions);
         for (let perm of permissions.permissions) {
           perms.add(perm);
@@ -847,7 +847,7 @@ class BrowserExtensionContent extends EventEmitter {
         this.policy.permissions = perms;
       }
 
-      if (permissions.origins.length > 0) {
+      if (permissions.origins.length) {
         let patterns = this.whiteListedHosts.patterns.map(host => host.pattern);
 
         this.policy.allowedOrigins = new MatchPatternSet(
@@ -858,7 +858,7 @@ class BrowserExtensionContent extends EventEmitter {
     });
 
     this.on("remove-permissions", (ignoreEvent, permissions) => {
-      if (permissions.permissions.length > 0) {
+      if (permissions.permissions.length) {
         let perms = new Set(this.policy.permissions);
         for (let perm of permissions.permissions) {
           perms.delete(perm);
@@ -866,7 +866,7 @@ class BrowserExtensionContent extends EventEmitter {
         this.policy.permissions = perms;
       }
 
-      if (permissions.origins.length > 0) {
+      if (permissions.origins.length) {
         let origins = permissions.origins.map(
           origin => new MatchPattern(origin, { ignorePath: true }).pattern
         );
@@ -1198,7 +1198,7 @@ class ChildAPIManager {
 
     this.permissionsChangedCallbacks = new Set();
     this.updatePermissions = null;
-    if (this.context.extension.optionalPermissions.length > 0) {
+    if (this.context.extension.optionalPermissions.length) {
       this.updatePermissions = () => {
         for (let callback of this.permissionsChangedCallbacks) {
           try {
diff --git a/toolkit/components/extensions/ExtensionContent.jsm b/toolkit/components/extensions/ExtensionContent.jsm
index 6533f9c2f003..3d37060a5713 100644
--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -355,7 +355,7 @@ class Script {
   }
 
   get requiresCleanup() {
-    return !this.removeCSS && (this.css.length > 0 || this.cssCodeHash);
+    return !this.removeCSS && (!!this.css.length || this.cssCodeHash);
   }
 
   async addCSSCode(cssCode) {
@@ -531,7 +531,7 @@ class Script {
         // problem since there are no network loads involved, and since we cache
         // the stylesheets on first load. We should fix this up if it does becomes
         // a problem.
-        if (this.css.length > 0) {
+        if (this.css.length) {
           context.contentWindow.document.blockParsing(cssPromise, {
             blockScriptCreated: false,
           });
diff --git a/toolkit/components/extensions/ExtensionParent.jsm b/toolkit/components/extensions/ExtensionParent.jsm
index 2424e38e130c..9c4e0e9f27f1 100644
--- a/toolkit/components/extensions/ExtensionParent.jsm
+++ b/toolkit/components/extensions/ExtensionParent.jsm
@@ -135,14 +135,14 @@ let apiManager = new (class extends SchemaAPIManager {
     let disabledIds = AddonManager.getStartupChanges(
       AddonManager.STARTUP_CHANGE_DISABLED
     );
-    if (disabledIds.length > 0) {
+    if (disabledIds.length) {
       this._callHandlers(disabledIds, "disable", "onDisable");
     }
 
     let uninstalledIds = AddonManager.getStartupChanges(
       AddonManager.STARTUP_CHANGE_UNINSTALLED
     );
-    if (uninstalledIds.length > 0) {
+    if (uninstalledIds.length) {
       this._callHandlers(uninstalledIds, "uninstall", "onUninstall");
     }
   }
diff --git a/toolkit/components/extensions/ExtensionPermissions.jsm b/toolkit/components/extensions/ExtensionPermissions.jsm
index 746a60b13c14..0e64ec87052d 100644
--- a/toolkit/components/extensions/ExtensionPermissions.jsm
+++ b/toolkit/components/extensions/ExtensionPermissions.jsm
@@ -121,7 +121,7 @@ var ExtensionPermissions = {
       }
     }
 
-    if (added.permissions.length > 0 || added.origins.length > 0) {
+    if (added.permissions.length || added.origins.length) {
       await this._update(extensionId, { permissions, origins });
       if (emitter) {
         emitter.emit("add-permissions", added);
@@ -160,7 +160,7 @@ var ExtensionPermissions = {
       }
     }
 
-    if (removed.permissions.length > 0 || removed.origins.length > 0) {
+    if (removed.permissions.length || removed.origins.length) {
       await this._update(extensionId, { permissions, origins });
       if (emitter) {
         emitter.emit("remove-permissions", removed);
diff --git a/toolkit/components/extensions/ExtensionStorageSync.jsm b/toolkit/components/extensions/ExtensionStorageSync.jsm
index 2d56a989ec33..cd1f2ab18c82 100644
--- a/toolkit/components/extensions/ExtensionStorageSync.jsm
+++ b/toolkit/components/extensions/ExtensionStorageSync.jsm
@@ -819,7 +819,7 @@ class ExtensionStorageSync {
     const extensions = await this.getExtensions();
     const extIds = Array.from(extensions, extension => extension.id);
     log.debug(`Syncing extension settings for ${JSON.stringify(extIds)}`);
-    if (extIds.length == 0) {
+    if (!extIds.length) {
       // No extensions to sync. Get out.
       return;
     }
@@ -908,7 +908,7 @@ class ExtensionStorageSync {
         newValue: accepted.data,
       };
     }
-    if (Object.keys(changes).length > 0) {
+    if (Object.keys(changes).length) {
       this.notifyListeners(extension, changes);
     }
     log.info(`Successfully synced '${collection.name}'`);
@@ -1059,7 +1059,7 @@ class ExtensionStorageSync {
     };
     await this.cryptoCollection.upsert(newRecord);
     const result = await this._syncKeyRing(newRecord);
-    if (result.resolved.length != 0) {
+    if (result.resolved.length) {
       // We had a conflict which was automatically resolved. We now
       // have a new keyring which might have keys for the
       // collections. Recurse.
@@ -1142,7 +1142,7 @@ class ExtensionStorageSync {
       // UUID. If so, reset our sync status, so that we'll reupload
       // everything.
       const result = await this.cryptoCollection.sync(this);
-      if (result.resolved.length > 0) {
+      if (result.resolved.length) {
         // Automatically-resolved conflict. It should
         // be for the keys record.
         const resolutionIds = result.resolved.map(resolution => resolution.id);
@@ -1319,7 +1319,7 @@ class ExtensionStorageSync {
       },
       { preloadIds: ids }
     );
-    if (Object.keys(changes).length > 0) {
+    if (Object.keys(changes).length) {
       this.notifyListeners(extension, changes);
     }
     const histogram = this._telemetry.getKeyedHistogramById(
diff --git a/toolkit/components/extensions/MatchURLFilters.jsm b/toolkit/components/extensions/MatchURLFilters.jsm
index be5189fb7195..53dc66ca0b2c 100644
--- a/toolkit/components/extensions/MatchURLFilters.jsm
+++ b/toolkit/components/extensions/MatchURLFilters.jsm
@@ -21,7 +21,7 @@ class MatchURLFilters {
       throw new TypeError("filters should be an array");
     }
 
-    if (filters.length == 0) {
+    if (!filters.length) {
       throw new Error("filters array should not be empty");
     }
 
diff --git a/toolkit/components/extensions/MessageChannel.jsm b/toolkit/components/extensions/MessageChannel.jsm
index 8059de407286..a86abd5721f8 100644
--- a/toolkit/components/extensions/MessageChannel.jsm
+++ b/toolkit/components/extensions/MessageChannel.jsm
@@ -892,7 +892,7 @@ this.MessageChannel = {
 
     // At least one handler is required for all response types but
     // RESPONSE_ALL.
-    if (handlers.length == 0 && responseType != this.RESPONSE_ALL) {
+    if (!handlers.length && responseType != this.RESPONSE_ALL) {
       return Promise.reject({
         result: MessageChannel.RESULT_NO_HANDLER,
         message: "No matching message handler",
@@ -927,7 +927,7 @@ this.MessageChannel = {
 
     switch (responseType) {
       case this.RESPONSE_FIRST:
-        if (responses.length == 0) {
+        if (!responses.length) {
           return Promise.reject({
             result: MessageChannel.RESULT_NO_RESPONSE,
             message: "No handler returned a response",
diff --git a/toolkit/components/extensions/NativeMessaging.jsm b/toolkit/components/extensions/NativeMessaging.jsm
index e6e6f651364c..b13a03968984 100644
--- a/toolkit/components/extensions/NativeMessaging.jsm
+++ b/toolkit/components/extensions/NativeMessaging.jsm
@@ -198,7 +198,7 @@ var NativeApp = class extends EventEmitter {
   }
 
   _startWrite() {
-    if (this.sendQueue.length == 0) {
+    if (!this.sendQueue.length) {
       return;
     }
 
@@ -230,7 +230,7 @@ var NativeApp = class extends EventEmitter {
       let partial = "";
       while (true) {
         let data = await proc.stderr.readString();
-        if (data.length == 0) {
+        if (!data.length) {
           // We have hit EOF, just stop reading
           if (partial) {
             Services.console.logStringMessage(
diff --git a/toolkit/components/extensions/ProxyScriptContext.jsm b/toolkit/components/extensions/ProxyScriptContext.jsm
index 71256c1cb388..b8dc8e0c683d 100644
--- a/toolkit/components/extensions/ProxyScriptContext.jsm
+++ b/toolkit/components/extensions/ProxyScriptContext.jsm
@@ -343,7 +343,7 @@ const ProxyInfoData = {
         proxyData = proxyRules;
       // fall through
       case "object":
-        if (Array.isArray(proxyData) && proxyData.length > 0) {
+        if (Array.isArray(proxyData) && proxyData.length) {
           return ProxyInfoData.createProxyInfoFromData(
             proxyData,
             defaultProxyInfo
diff --git a/toolkit/components/extensions/Schemas.jsm b/toolkit/components/extensions/Schemas.jsm
index dfa0adc7b735..3f63416339a3 100644
--- a/toolkit/components/extensions/Schemas.jsm
+++ b/toolkit/components/extensions/Schemas.jsm
@@ -3191,7 +3191,7 @@ class SchemaRoots extends Namespaces {
       return results[0];
     }
 
-    if (results.length > 0) {
+    if (results.length) {
       return new Namespaces(this.root, name, name.split("."), results);
     }
     return null;
diff --git a/toolkit/components/extensions/parent/ext-contentScripts.js b/toolkit/components/extensions/parent/ext-contentScripts.js
index 3bb0937cead9..ae31a69f97fc 100644
--- a/toolkit/components/extensions/parent/ext-contentScripts.js
+++ b/toolkit/components/extensions/parent/ext-contentScripts.js
@@ -85,7 +85,7 @@ class ContentScriptParent {
       return blobURL;
     };
 
-    if (details.js && details.js.length > 0) {
+    if (details.js && details.js.length) {
       options.jsPaths = details.js.map(data => {
         if (data.file) {
           return data.file;
@@ -95,7 +95,7 @@ class ContentScriptParent {
       });
     }
 
-    if (details.css && details.css.length > 0) {
+    if (details.css && details.css.length) {
       options.cssPaths = details.css.map(data => {
         if (data.file) {
           return data.file;
diff --git a/toolkit/components/extensions/parent/ext-downloads.js b/toolkit/components/extensions/parent/ext-downloads.js
index d29d5f851feb..8d3502573b26 100644
--- a/toolkit/components/extensions/parent/ext-downloads.js
+++ b/toolkit/components/extensions/parent/ext-downloads.js
@@ -558,7 +558,7 @@ this.downloads = class extends ExtensionAPI {
           }
 
           if (filename != null) {
-            if (filename.length == 0) {
+            if (!filename.length) {
               return Promise.reject({ message: "filename must not be empty" });
             }
 
@@ -1065,7 +1065,7 @@ this.downloads = class extends ExtensionAPI {
                 };
               }
             });
-            if (Object.keys(changes).length > 0) {
+            if (Object.keys(changes).length) {
               changes.id = item.id;
               fire.async(changes);
             }
diff --git a/toolkit/components/extensions/parent/ext-permissions.js b/toolkit/components/extensions/parent/ext-permissions.js
index 21d762e5164e..d7f4a5d26e01 100644
--- a/toolkit/components/extensions/parent/ext-permissions.js
+++ b/toolkit/components/extensions/parent/ext-permissions.js
@@ -61,7 +61,7 @@ this.permissions = class extends ExtensionAPI {
                 )
             );
 
-            if (permissions.length == 0 && origins.length == 0) {
+            if (!permissions.length && !origins.length) {
               return true;
             }
 
diff --git a/toolkit/components/extensions/test/mochitest/test_chrome_ext_webnavigation_resolved_urls.html b/toolkit/components/extensions/test/mochitest/test_chrome_ext_webnavigation_resolved_urls.html
index 3b9920e69225..a06709d807c3 100644
--- a/toolkit/components/extensions/test/mochitest/test_chrome_ext_webnavigation_resolved_urls.html
+++ b/toolkit/components/extensions/test/mochitest/test_chrome_ext_webnavigation_resolved_urls.html
@@ -17,7 +17,7 @@ add_task(async function webnav_unresolved_uri_on_expected_URI_scheme() {
     let checkURLs;
 
     browser.webNavigation.onCompleted.addListener(async msg => {
-      if (checkURLs.length > 0) {
+      if (checkURLs.length) {
         let expectedURL = checkURLs.shift();
         browser.test.assertEq(expectedURL, msg.url, "Got the expected URL");
         await browser.tabs.remove(msg.tabId);
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_activityLog.html b/toolkit/components/extensions/test/mochitest/test_ext_activityLog.html
index c2dbeaa4b6fc..d3a7cebdaa01 100644
--- a/toolkit/components/extensions/test/mochitest/test_ext_activityLog.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_activityLog.html
@@ -265,7 +265,7 @@ add_task(async function test_api() {
         JSON.stringify(details.data),
         "message matches"
       );
-      if (expecting.length == 0) {
+      if (!expecting.length) {
         await browser.tabs.remove(tab.id);
         browser.test.notifyPass("activity");
       }
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_async_clipboard.html b/toolkit/components/extensions/test/mochitest/test_ext_async_clipboard.html
index 7121b2adf921..d6e4734888e0 100644
--- a/toolkit/components/extensions/test/mochitest/test_ext_async_clipboard.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_async_clipboard.html
@@ -331,7 +331,7 @@ add_task(async function test_contentscript_clipboard_nocontents_read() {
       // and read from it, the data transfer object contains an item of type
       // text/plain and kind string, but we can't call getAsString on it to verify
       // that at least it is an empty string because the callback never gets invoked.
-      if (dataT.items.length == 0 ||
+      if (!dataT.items.length ||
         (dataT.items.length == 1 && dataT.items[0].type == "text/plain" &&
          dataT.items[0].kind == "string")) {
         browser.test.succeed("Read promise successfully resolved");
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_hsts.html b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_hsts.html
index 34b123eff81f..603dcc6192f1 100644
--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_hsts.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_hsts.html
@@ -44,7 +44,7 @@ function getExtension() {
       browser.test.assertTrue(now < cert.validity.end, "cert end validity is correct");
       if (options.rawDER) {
         for (let cert of securityInfo.certificates) {
-          browser.test.assertTrue(cert.rawDER.length > 0, "have rawDER");
+          browser.test.assertTrue(!!cert.rawDER.length, "have rawDER");
         }
       }
     }
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_dns.js b/toolkit/components/extensions/test/xpcshell/test_ext_dns.js
index e2363734d919..4daf2dc535d0 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_dns.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_dns.js
@@ -106,7 +106,7 @@ add_task(async function test_dns_resolve() {
         result.addresses.length >= test.expect.addresses.length,
         "expected number of addresses returned"
       );
-      if (test.expect.addresses.length > 0 && result.addresses.length > 0) {
+      if (test.expect.addresses.length && result.addresses.length) {
         ok(
           result.addresses.includes(test.expect.addresses[0]),
           "got expected ip address"
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_downloads_misc.js b/toolkit/components/extensions/test/xpcshell/test_ext_downloads_misc.js
index 071f9e07fdb8..89aa892f6a18 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_downloads_misc.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_downloads_misc.js
@@ -169,7 +169,7 @@ function backgroundScript() {
         // expected events and we're not looking for an exact match,
         // then we just may not have seen the event yet, so return without
         // failing and check() will be called again when a new event arrives.
-        if (expected.length == 0) {
+        if (!expected.length) {
           events = remaining;
           eventWaiter = null;
           resolve();
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_incognito.js b/toolkit/components/extensions/test/xpcshell/test_ext_incognito.js
index 707a0b9da667..9b17a633e90b 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_incognito.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_incognito.js
@@ -29,7 +29,7 @@ function assertActionAMTelemetryEvent(
   );
 
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
 
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js
index c3efabc05bb8..f023e12df5f4 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js
@@ -101,7 +101,7 @@ class SocksClient {
   }
 
   checkSocksGreeting() {
-    if (this.inbuf.length == 0) {
+    if (!this.inbuf.length) {
       return;
     }
 
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
index f60d925f8bb7..5c8ef748f8e5 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
@@ -204,7 +204,7 @@ class KintoServer {
         }),
       };
 
-      if (this.conflicts.length > 0) {
+      if (this.conflicts.length) {
         const nextConflict = this.conflicts.shift();
         if (!nextConflict.transient) {
           this.records.push(nextConflict);
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_urlclassification.js b/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_urlclassification.js
index c4eae7af824b..06a644c1a05e 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_urlclassification.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_urlclassification.js
@@ -14,7 +14,7 @@ add_task(async function test_webrequest_url_classification_enum() {
   let ns = Schemas.getNamespace("webRequest");
   let schema_enum = ns.get("UrlClassificationFlags").enumeration;
   ok(
-    schema_enum.length > 0,
+    !!schema_enum.length,
     `UrlClassificationFlags: ${JSON.stringify(schema_enum)}`
   );
 
diff --git a/toolkit/components/extensions/webrequest/WebRequestContent.js b/toolkit/components/extensions/webrequest/WebRequestContent.js
index ff103ed184f0..38043aac2112 100644
--- a/toolkit/components/extensions/webrequest/WebRequestContent.js
+++ b/toolkit/components/extensions/webrequest/WebRequestContent.js
@@ -200,7 +200,7 @@ var ContentPolicy = {
       windowId,
       parentWindowId,
     };
-    if (frameAncestors.length > 0) {
+    if (frameAncestors.length) {
       data.frameAncestors = frameAncestors;
     }
     if (requestOrigin) {
diff --git a/toolkit/components/mozintl/test/test_mozintl_getLocaleDisplayNames.js b/toolkit/components/mozintl/test/test_mozintl_getLocaleDisplayNames.js
index 8caa2541ad1b..2e7c08992546 100644
--- a/toolkit/components/mozintl/test/test_mozintl_getLocaleDisplayNames.js
+++ b/toolkit/components/mozintl/test/test_mozintl_getLocaleDisplayNames.js
@@ -122,15 +122,11 @@ add_test(function test_invalid_regions() {
 add_test(function test_availableLocaleDisplayNames() {
   let langCodes = gAvLocDN("language");
   equal(
-    langCodes.length > 0,
+    !!langCodes.length,
     true,
     "There should be some language codes available"
   );
   let regCodes = gAvLocDN("region");
-  equal(
-    regCodes.length > 0,
-    true,
-    "There should be some region codes available"
-  );
+  equal(!!regCodes.length, true, "There should be some region codes available");
   run_next_test();
 });
diff --git a/toolkit/components/normandy/actions/PreferenceExperimentAction.jsm b/toolkit/components/normandy/actions/PreferenceExperimentAction.jsm
index 89248200288c..5a1ee0603065 100644
--- a/toolkit/components/normandy/actions/PreferenceExperimentAction.jsm
+++ b/toolkit/components/normandy/actions/PreferenceExperimentAction.jsm
@@ -71,7 +71,7 @@ class PreferenceExperimentAction extends BaseStudyAction {
             );
           }
         );
-        if (conflictingPrefs.length > 0) {
+        if (conflictingPrefs.length) {
           throw new Error(
             `Experiment ${slug} ignored; another active experiment is already using the
             ${conflictingPrefs[0]} preference.`
diff --git a/toolkit/components/normandy/lib/PreferenceExperiments.jsm b/toolkit/components/normandy/lib/PreferenceExperiments.jsm
index 240ce2931a5b..b4c399f8e9c7 100644
--- a/toolkit/components/normandy/lib/PreferenceExperiments.jsm
+++ b/toolkit/components/normandy/lib/PreferenceExperiments.jsm
@@ -426,7 +426,7 @@ var PreferenceExperiments = {
       }
     );
 
-    if (preferencesWithConflicts.length > 0) {
+    if (preferencesWithConflicts.length) {
       TelemetryEvents.sendEvent("enrollFailed", "preference_study", slug, {
         reason: "pref-conflict",
       });
diff --git a/toolkit/components/normandy/test/browser/browser_LogManager.js b/toolkit/components/normandy/test/browser/browser_LogManager.js
index 179eb0834e28..e87963f4f635 100644
--- a/toolkit/components/normandy/test/browser/browser_LogManager.js
+++ b/toolkit/components/normandy/test/browser/browser_LogManager.js
@@ -14,7 +14,7 @@ add_task(async function() {
   // Ensure that our loggers have at least one appender.
   LogManager.configure(Log.Level.Warn);
   const logger = LogManager.getLogger("test");
-  ok(logger.appenders.length > 0, "Loggers have at least one appender.");
+  ok(!!logger.appenders.length, "Loggers have at least one appender.");
 
   // Ensure our loggers log to the console.
   await new Promise(resolve => {
diff --git a/toolkit/components/normandy/test/browser/browser_actions_PreferenceExperimentAction.js b/toolkit/components/normandy/test/browser/browser_actions_PreferenceExperimentAction.js
index 2b6b599e6341..9f0f47789d17 100644
--- a/toolkit/components/normandy/test/browser/browser_actions_PreferenceExperimentAction.js
+++ b/toolkit/components/normandy/test/browser/browser_actions_PreferenceExperimentAction.js
@@ -444,7 +444,7 @@ decorate_task(
     await action.finalize();
 
     const activeExperiments = await PreferenceExperiments.getAllActive();
-    ok(activeExperiments.length > 0);
+    ok(!!activeExperiments.length);
     Assert.deepEqual(activeExperiments, [
       {
         slug: "integration test experiment",
diff --git a/toolkit/components/osfile/modules/osfile_async_front.jsm b/toolkit/components/osfile/modules/osfile_async_front.jsm
index 775c940cd538..6e958e88acc7 100644
--- a/toolkit/components/osfile/modules/osfile_async_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_async_front.jsm
@@ -355,11 +355,11 @@ var Scheduler = (this.Scheduler = {
           // The worker still holds resources. Report them.
 
           let msg = "";
-          if (openedFiles.length > 0) {
+          if (openedFiles.length) {
             msg +=
               "The following files are still open:\n" + openedFiles.join("\n");
           }
-          if (openedDirectoryIterators.length > 0) {
+          if (openedDirectoryIterators.length) {
             msg +=
               "The following directory iterators are still open:\n" +
               openedDirectoryIterators.join("\n");
diff --git a/toolkit/components/osfile/modules/osfile_async_worker.js b/toolkit/components/osfile/modules/osfile_async_worker.js
index f6cafe4d8067..a37436947cf8 100644
--- a/toolkit/components/osfile/modules/osfile_async_worker.js
+++ b/toolkit/components/osfile/modules/osfile_async_worker.js
@@ -179,8 +179,7 @@ if (this.Components) {
 
       // Is it safe to kill the worker?
       let safe =
-        result.openedFiles.length == 0 &&
-        result.openedDirectoryIterators.length == 0;
+        !result.openedFiles.length && !result.openedDirectoryIterators.length;
       result.killed = safe && kill;
 
       return new Meta(result, { shutdown: result.killed });
diff --git a/toolkit/components/osfile/modules/ospath_unix.jsm b/toolkit/components/osfile/modules/ospath_unix.jsm
index 8460e57ba7a8..75f51cbc11c5 100644
--- a/toolkit/components/osfile/modules/ospath_unix.jsm
+++ b/toolkit/components/osfile/modules/ospath_unix.jsm
@@ -88,7 +88,7 @@ var join = function(...path) {
     if (subpath == null) {
       throw new TypeError("invalid path component");
     }
-    if (subpath.length == 0) {
+    if (!subpath.length) {
       continue;
     } else if (subpath[0] == "/") {
       paths = [subpath];
@@ -117,7 +117,7 @@ var normalize = function(path) {
       case ".": // fallthrough
         break;
       case "..":
-        if (stack.length == 0) {
+        if (!stack.length) {
           if (absolute) {
             throw new Error("Path is ill-formed: attempting to go past root");
           } else {
diff --git a/toolkit/components/osfile/modules/ospath_win.jsm b/toolkit/components/osfile/modules/ospath_win.jsm
index 19e4868e5d36..922901229320 100644
--- a/toolkit/components/osfile/modules/ospath_win.jsm
+++ b/toolkit/components/osfile/modules/ospath_win.jsm
@@ -255,7 +255,7 @@ var normalize = function(path) {
       case ".": // Ignore
         break;
       case "..":
-        if (stack.length == 0) {
+        if (!stack.length) {
           if (absolute) {
             throw new Error("Path is ill-formed: attempting to go past root");
           } else {
diff --git a/toolkit/components/passwordmgr/LoginHelper.jsm b/toolkit/components/passwordmgr/LoginHelper.jsm
index ca9824bf6d32..1c47b3856e90 100644
--- a/toolkit/components/passwordmgr/LoginHelper.jsm
+++ b/toolkit/components/passwordmgr/LoginHelper.jsm
@@ -475,7 +475,7 @@ this.LoginHelper = {
     }
 
     // Sanity check the login
-    if (newLogin.origin == null || newLogin.origin.length == 0) {
+    if (newLogin.origin == null || !newLogin.origin.length) {
       throw new Error("Can't add a login with a null or empty origin.");
     }
 
@@ -484,7 +484,7 @@ this.LoginHelper = {
       throw new Error("Can't add a login with a null username.");
     }
 
-    if (newLogin.password == null || newLogin.password.length == 0) {
+    if (newLogin.password == null || !newLogin.password.length) {
       throw new Error("Can't add a login with a null or empty password.");
     }
 
@@ -645,7 +645,7 @@ this.LoginHelper = {
      * over the existingLogin.
      */
     function isLoginPreferred(existingLogin, login) {
-      if (!resolveBy || resolveBy.length == 0) {
+      if (!resolveBy || !resolveBy.length) {
         // If there is no preference, prefer the existing login.
         return false;
       }
diff --git a/toolkit/components/passwordmgr/LoginImport.jsm b/toolkit/components/passwordmgr/LoginImport.jsm
index c3d839aa07d8..982502bb5f2c 100644
--- a/toolkit/components/passwordmgr/LoginImport.jsm
+++ b/toolkit/components/passwordmgr/LoginImport.jsm
@@ -63,7 +63,7 @@ this.LoginImport.prototype = {
     // Thus, merging with existing data is not a use case we support.  This
     // restriction might be removed to support re-importing passwords set by an
     // old version by flipping the import preference and restarting.
-    if (this.store.data.logins.length > 0) {
+    if (this.store.data.logins.length) {
       throw new Error(
         "Unable to import saved passwords because some data " +
           "has already been imported or saved."
diff --git a/toolkit/components/passwordmgr/LoginManager.jsm b/toolkit/components/passwordmgr/LoginManager.jsm
index 0fb4381effe9..9edd81ead09b 100644
--- a/toolkit/components/passwordmgr/LoginManager.jsm
+++ b/toolkit/components/passwordmgr/LoginManager.jsm
@@ -255,7 +255,7 @@ LoginManager.prototype = {
    */
   _checkLogin(login) {
     // Sanity check the login
-    if (login.origin == null || login.origin.length == 0) {
+    if (login.origin == null || !login.origin.length) {
       throw new Error("Can't add a login with a null or empty origin.");
     }
 
@@ -264,7 +264,7 @@ LoginManager.prototype = {
       throw new Error("Can't add a login with a null username.");
     }
 
-    if (login.password == null || login.password.length == 0) {
+    if (login.password == null || !login.password.length) {
       throw new Error("Can't add a login with a null or empty password.");
     }
 
diff --git a/toolkit/components/passwordmgr/LoginManagerContent.jsm b/toolkit/components/passwordmgr/LoginManagerContent.jsm
index d2937d3a3e88..a995d4b60459 100644
--- a/toolkit/components/passwordmgr/LoginManagerContent.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerContent.jsm
@@ -835,10 +835,9 @@ this.LoginManagerContent = {
       let rootElsWeakSet = LoginFormFactory.getRootElementsWeakSetForDocument(
         doc
       );
-      let hasLoginForm =
-        ChromeUtils.nondeterministicGetWeakSetKeys(rootElsWeakSet).filter(
-          el => el.isConnected
-        ).length > 0;
+      let hasLoginForm = !!ChromeUtils.nondeterministicGetWeakSetKeys(
+        rootElsWeakSet
+      ).filter(el => el.isConnected).length;
       return (
         (hasLoginForm && !thisWindow.isSecureContext) ||
         Array.prototype.some.call(thisWindow.frames, frame =>
@@ -1117,7 +1116,7 @@ this.LoginManagerContent = {
     }
 
     // If too few or too many fields, bail out.
-    if (pwFields.length == 0) {
+    if (!pwFields.length) {
       log("(form ignored -- no password fields.)");
       return null;
     } else if (pwFields.length > 3) {
@@ -1728,7 +1727,7 @@ this.LoginManagerContent = {
       // checks) logins available, and there isn't a need to show
       // the insecure form warning.
       if (
-        foundLogins.length == 0 &&
+        !foundLogins.length &&
         (InsecurePasswordUtils.isFormSecure(form) ||
           !LoginHelper.showInsecureFieldWarning)
       ) {
@@ -1831,7 +1830,7 @@ this.LoginManagerContent = {
       // Nothing to do if we have no matching logins available.
       // Only insecure pages reach this block and logs the same
       // telemetry flag.
-      if (foundLogins.length == 0) {
+      if (!foundLogins.length) {
         // We don't log() here since this is a very common case.
         autofillResult = AUTOFILL_RESULT.NO_SAVED_LOGINS;
         return;
@@ -1874,7 +1873,7 @@ this.LoginManagerContent = {
         return fit;
       }, this);
 
-      if (logins.length == 0) {
+      if (!logins.length) {
         log("form not filled, none of the logins fit in the field");
         autofillResult = AUTOFILL_RESULT.NO_LOGINS_FIT;
         return;
@@ -1915,7 +1914,7 @@ this.LoginManagerContent = {
         let matchingLogins = logins.filter(
           l => l.username.toLowerCase() == username
         );
-        if (matchingLogins.length == 0) {
+        if (!matchingLogins.length) {
           log(
             "Password not filled. None of the stored logins match the username already present."
           );
diff --git a/toolkit/components/passwordmgr/LoginManagerParent.jsm b/toolkit/components/passwordmgr/LoginManagerParent.jsm
index 104d5db6fc62..5e241d36249e 100644
--- a/toolkit/components/passwordmgr/LoginManagerParent.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerParent.jsm
@@ -611,7 +611,7 @@ this.LoginManagerParent = {
     // If we didn't find a username field, but seem to be changing a
     // password, allow the user to select from a list of applicable
     // logins to update the password for.
-    if (!usernameField && oldPasswordField && logins.length > 0) {
+    if (!usernameField && oldPasswordField && logins.length) {
       let prompter = this._getPrompter(browser, openerTopWindowID);
 
       if (logins.length == 1) {
diff --git a/toolkit/components/passwordmgr/LoginManagerPrompter.jsm b/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
index a7c9110837e5..a1dafa76f0ff 100644
--- a/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
@@ -450,7 +450,7 @@ LoginManagerPrompter.prototype = {
 
       // XXX Like the original code, we can't deal with multiple
       // account selection. (bug 227632)
-      if (foundLogins.length > 0) {
+      if (foundLogins.length) {
         selectedLogin = foundLogins[0];
 
         // If the caller provided a username, try to use it. If they
@@ -687,7 +687,7 @@ LoginManagerPrompter.prototype = {
       this.log(foundLogins.length, "matching logins remain after deduping");
 
       // XXX Can't select from multiple accounts yet. (bug 227632)
-      if (foundLogins.length > 0) {
+      if (foundLogins.length) {
         selectedLogin = foundLogins[0];
         this._SetAuthInfo(
           aAuthInfo,
@@ -1035,7 +1035,7 @@ LoginManagerPrompter.prototype = {
     let updateButtonStatus = element => {
       let mainActionButton = element.button;
       // Disable the main button inside the menu-button if the password field is empty.
-      if (login.password.length == 0) {
+      if (!login.password.length) {
         mainActionButton.setAttribute("disabled", true);
         chromeDoc
           .getElementById("password-notification-password")
@@ -1064,7 +1064,7 @@ LoginManagerPrompter.prototype = {
         foundLogins,
         autoSavedLoginGuid
       );
-      let msgNames = logins.length == 0 ? saveMsgNames : changeMsgNames;
+      let msgNames = !logins.length ? saveMsgNames : changeMsgNames;
 
       // Update the label based on whether this will be a new login or not.
       let label = this._getLocalizedString(msgNames.buttonLabel);
diff --git a/toolkit/components/passwordmgr/LoginRecipes.jsm b/toolkit/components/passwordmgr/LoginRecipes.jsm
index 6859634573f1..8403ac119e09 100644
--- a/toolkit/components/passwordmgr/LoginRecipes.jsm
+++ b/toolkit/components/passwordmgr/LoginRecipes.jsm
@@ -151,7 +151,7 @@ LoginRecipesParent.prototype = {
     log.debug("Adding recipe:", recipe);
     let recipeKeys = Object.keys(recipe);
     let unknownKeys = recipeKeys.filter(key => !SUPPORTED_KEYS.includes(key));
-    if (unknownKeys.length > 0) {
+    if (unknownKeys.length) {
       throw new Error(
         "The following recipe keys aren't supported: " + unknownKeys.join(", ")
       );
@@ -160,7 +160,7 @@ LoginRecipesParent.prototype = {
     let missingRequiredKeys = REQUIRED_KEYS.filter(
       key => !recipeKeys.includes(key)
     );
-    if (missingRequiredKeys.length > 0) {
+    if (missingRequiredKeys.length) {
       throw new Error(
         "The following required recipe keys are missing: " +
           missingRequiredKeys.join(", ")
diff --git a/toolkit/components/passwordmgr/content/passwordManager.js b/toolkit/components/passwordmgr/content/passwordManager.js
index a5bbec97c91c..3a4751a83c2d 100644
--- a/toolkit/components/passwordmgr/content/passwordManager.js
+++ b/toolkit/components/passwordmgr/content/passwordManager.js
@@ -316,7 +316,7 @@ function LoadSignons() {
   SignonColumnSort(lastSignonSortColumn);
 
   // disable "remove all signons" button if there are no signons
-  if (signons.length == 0) {
+  if (!signons.length) {
     removeAllButton.setAttribute("disabled", "true");
     togglePasswordsButton.setAttribute("disabled", "true");
   } else {
@@ -362,7 +362,7 @@ function SignonSelected() {
 }
 
 function DeleteSignon() {
-  let syncNeeded = signonsTreeView._filterSet.length != 0;
+  let syncNeeded = !!signonsTreeView._filterSet.length;
   let tree = signonsTree;
   let view = signonsTreeView;
   let table = GetVisibleLogins();
@@ -426,7 +426,7 @@ async function DeleteAllSignons() {
     return;
   }
 
-  let syncNeeded = signonsTreeView._filterSet.length != 0;
+  let syncNeeded = !!signonsTreeView._filterSet.length;
   let view = signonsTreeView;
   let table = GetVisibleLogins();
 
diff --git a/toolkit/components/passwordmgr/test/browser/head.js b/toolkit/components/passwordmgr/test/browser/head.js
index 611859639a4d..f08c875f8626 100644
--- a/toolkit/components/passwordmgr/test/browser/head.js
+++ b/toolkit/components/passwordmgr/test/browser/head.js
@@ -301,7 +301,7 @@ async function getCaptureDoorhangerThatMayOpen(
 
 function getDoorhangerButton(aPopup, aButtonIndex) {
   let notifications = aPopup.owner.panel.children;
-  ok(notifications.length > 0, "at least one notification displayed");
+  ok(!!notifications.length, "at least one notification displayed");
   ok(true, notifications.length + " notification(s)");
   let notification = notifications[0];
 
diff --git a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
index a760fa4f38cf..4216c728e13e 100644
--- a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
+++ b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
@@ -480,7 +480,7 @@ SimpleTest.registerCleanupFunction(() => {
     let chromeWin = Services.wm.getMostRecentWindow("navigator:browser");
     if (chromeWin && chromeWin.PopupNotifications) {
       let notes = chromeWin.PopupNotifications._currentNotifications;
-      if (notes.length > 0) {
+      if (notes.length) {
         dump("Removing " + notes.length + " popup notifications.\n");
       }
       for (let note of notes) {
diff --git a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
index 000e6ae1bb74..c7d1a4aea0d3 100644
--- a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
+++ b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
@@ -197,7 +197,7 @@ addMessageListener("proxyLoginManager", msg => {
     rv = LoginHelper.loginToVanillaObject(rv);
   } else if (
     Array.isArray(rv) &&
-    rv.length > 0 &&
+    !!rv.length &&
     rv[0] instanceof Ci.nsILoginInfo
   ) {
     rv = rv.map(login => LoginHelper.loginToVanillaObject(login));
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html b/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
index 99aaa325c222..b2549b5cd45a 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_bug_627616.html
@@ -58,7 +58,7 @@
       mm.sendAsyncMessage("prepareForNextTest");
       mm.addMessageListener("prepareForNextTestDone", function prepared(msg) {
         mm.removeMessageListener("prepareForNextTestDone", prepared);
-        if (pendingTests.length > 0) {
+        if (pendingTests.length) {
           ({expectedDialogs: gExpectedDialogs,
             test: gCurrentTest} = pendingTests.shift());
           gCurrentTest.call(this);
diff --git a/toolkit/components/passwordmgr/test/unit/test_login_autocomplete_result.js b/toolkit/components/passwordmgr/test/unit/test_login_autocomplete_result.js
index c0f67783d300..1b9a489f5bcb 100644
--- a/toolkit/components/passwordmgr/test/unit/test_login_autocomplete_result.js
+++ b/toolkit/components/passwordmgr/test/unit/test_login_autocomplete_result.js
@@ -1172,7 +1172,7 @@ add_task(async function test_all_patterns() {
       }
     });
 
-    if (pattern.items.length != 0) {
+    if (pattern.items.length) {
       Assert.throws(
         () => actual.getValueAt(pattern.items.length),
         /Index out of range\./
diff --git a/toolkit/components/places/BookmarkHTMLUtils.jsm b/toolkit/components/places/BookmarkHTMLUtils.jsm
index ee6250a635aa..b10a4f6f25df 100644
--- a/toolkit/components/places/BookmarkHTMLUtils.jsm
+++ b/toolkit/components/places/BookmarkHTMLUtils.jsm
@@ -572,8 +572,7 @@ BookmarkImporter.prototype = {
         .split(",")
         .filter(
           aTag =>
-            aTag.length > 0 &&
-            aTag.length <= PlacesUtils.bookmarks.MAX_TAG_LENGTH
+            !!aTag.length && aTag.length <= PlacesUtils.bookmarks.MAX_TAG_LENGTH
         );
 
       // If we end up with none, then delete the property completely.
@@ -864,7 +863,7 @@ function BookmarkExporter(aBookmarksTree) {
 
   for (let key of ["toolbarFolder", "unfiledBookmarksFolder"]) {
     let root = rootsMap.get(key);
-    if (root.children && root.children.length > 0) {
+    if (root.children && root.children.length) {
       if (!this._root.children) {
         this._root.children = [];
       }
diff --git a/toolkit/components/places/BookmarkJSONUtils.jsm b/toolkit/components/places/BookmarkJSONUtils.jsm
index 304b3a5109e8..e27658d57a62 100644
--- a/toolkit/components/places/BookmarkJSONUtils.jsm
+++ b/toolkit/components/places/BookmarkJSONUtils.jsm
@@ -249,11 +249,7 @@ BookmarkImporter.prototype = {
       PlacesUtils.TYPE_X_MOZ_PLACE_CONTAINER
     );
 
-    if (
-      nodes.length == 0 ||
-      !nodes[0].children ||
-      nodes[0].children.length == 0
-    ) {
+    if (!nodes.length || !nodes[0].children || !nodes[0].children.length) {
       return;
     }
 
@@ -276,7 +272,7 @@ BookmarkImporter.prototype = {
     // match what we need for insertTree, and we also have mappings of folders
     // so we can repair any searches after inserting the bookmarks (see bug 824502).
     for (let node of nodes) {
-      if (!node.children || node.children.length == 0) {
+      if (!node.children || !node.children.length) {
         continue;
       } // Nothing to restore for this root
 
@@ -292,7 +288,7 @@ BookmarkImporter.prototype = {
     // Now we can add the actual nodes to the database.
     for (let node of nodes) {
       // Drop any nodes without children, we can't insert them.
-      if (!node.children || node.children.length == 0) {
+      if (!node.children || !node.children.length) {
         continue;
       }
 
@@ -478,7 +474,7 @@ function translateTreeTypes(node) {
       .split(",")
       .filter(
         aTag =>
-          aTag.length > 0 && aTag.length <= PlacesUtils.bookmarks.MAX_TAG_LENGTH
+          !!aTag.length && aTag.length <= PlacesUtils.bookmarks.MAX_TAG_LENGTH
       );
 
     // If we end up with none, then delete the property completely.
diff --git a/toolkit/components/places/History.jsm b/toolkit/components/places/History.jsm
index 9d8a9b505db6..b046709f46a0 100644
--- a/toolkit/components/places/History.jsm
+++ b/toolkit/components/places/History.jsm
@@ -355,7 +355,7 @@ var History = Object.freeze({
   remove(pages, onResult = null) {
     // Normalize and type-check arguments
     if (Array.isArray(pages)) {
-      if (pages.length == 0) {
+      if (!pages.length) {
         throw new TypeError("Expected at least one page");
       }
     } else {
@@ -847,7 +847,7 @@ function sqlBindPlaceholders(values, prefix = "", suffix = "") {
  * @returns {Promise} resolved when done
  */
 var invalidateFrecencies = async function(db, idList) {
-  if (idList.length == 0) {
+  if (!idList.length) {
     return;
   }
   await db.execute(
@@ -946,7 +946,7 @@ var cleanupPages = async function(db, pages) {
   );
 
   let pagesToRemove = pages.filter(p => !p.hasForeign && !p.hasVisits);
-  if (pagesToRemove.length == 0) {
+  if (!pagesToRemove.length) {
     return;
   }
 
@@ -1273,7 +1273,7 @@ var removeVisitsByFilter = async function(db, filter, onResult = null) {
   );
 
   try {
-    if (visitsToRemove.length == 0) {
+    if (!visitsToRemove.length) {
       // Nothing to do
       return false;
     }
@@ -1320,7 +1320,7 @@ var removeVisitsByFilter = async function(db, filter, onResult = null) {
     PlacesUtils.history.clearEmbedVisits();
   }
 
-  return visitsToRemove.length != 0;
+  return !!visitsToRemove.length;
 };
 
 // Inner implementation of History.removeByFilter
@@ -1472,7 +1472,7 @@ var remove = async function(db, { guids, urls }, onResult = null) {
   });
 
   try {
-    if (pages.length == 0) {
+    if (!pages.length) {
       // Nothing to do
       return false;
     }
@@ -1626,7 +1626,7 @@ var update = async function(db, pageInfo) {
       ? pageInfo.previewImageURL.href
       : null;
   }
-  if (updateFragments.length > 0) {
+  if (updateFragments.length) {
     // Since this data may be written at every visit and is textual, avoid
     // overwriting the existing record if it didn't change.
     await db.execute(
diff --git a/toolkit/components/places/PlacesSyncUtils.jsm b/toolkit/components/places/PlacesSyncUtils.jsm
index 561ea6275a2c..e7c2cfcd72c4 100644
--- a/toolkit/components/places/PlacesSyncUtils.jsm
+++ b/toolkit/components/places/PlacesSyncUtils.jsm
@@ -343,7 +343,7 @@ const HistorySyncUtils = (PlacesSyncUtils.history = Object.freeze({
         WHERE url_hash = hash(:page_url) AND url = :page_url`,
       { page_url: canonicalURL.href }
     );
-    if (rows.length == 0) {
+    if (!rows.length) {
       return null;
     }
     return rows[0].getResultByName("guid");
@@ -1483,7 +1483,7 @@ const BookmarkSyncUtils = (PlacesSyncUtils.bookmarks = Object.freeze({
       db,
       PlacesUtils.bookmarks.mobileGuid
     );
-    let hasMobileBookmarks = mobileChildGuids.length > 0;
+    let hasMobileBookmarks = !!mobileChildGuids.length;
 
     Services.prefs.setBoolPref(MOBILE_BOOKMARKS_PREF, hasMobileBookmarks);
   },
@@ -2022,11 +2022,11 @@ function tagItem(item, tags) {
   // tag IDs, we temporarily tag a dummy URI, ensuring the tags exist.
   let dummyURI = PlacesUtils.toURI("about:weave#BStore_tagURI");
   let bookmarkURI = PlacesUtils.toURI(item.url.href);
-  if (newTags && newTags.length > 0) {
+  if (newTags && newTags.length) {
     PlacesUtils.tagging.tagURI(dummyURI, newTags, SOURCE_SYNC);
   }
   PlacesUtils.tagging.untagURI(bookmarkURI, null, SOURCE_SYNC);
-  if (newTags && newTags.length > 0) {
+  if (newTags && newTags.length) {
     PlacesUtils.tagging.tagURI(bookmarkURI, newTags, SOURCE_SYNC);
   }
   PlacesUtils.tagging.untagURI(dummyURI, null, SOURCE_SYNC);
diff --git a/toolkit/components/places/PlacesTransactions.jsm b/toolkit/components/places/PlacesTransactions.jsm
index d89d6ce4c7dc..103b82739f1c 100644
--- a/toolkit/components/places/PlacesTransactions.jsm
+++ b/toolkit/components/places/PlacesTransactions.jsm
@@ -265,7 +265,7 @@ class TransactionsHistoryArray extends Array {
       throw new Error("aProxifiedTransaction is not a proxified transaction");
     }
 
-    if (this.length == 0 || forceNewEntry) {
+    if (!this.length || forceNewEntry) {
       this.clearRedoEntries();
       this.unshift([proxifiedTransaction]);
     } else {
@@ -296,7 +296,7 @@ class TransactionsHistoryArray extends Array {
    * Clear all entries.
    */
   clearAllEntries() {
-    if (this.length > 0) {
+    if (this.length) {
       this.splice(0);
       this._undoPosition = 0;
     }
@@ -315,7 +315,7 @@ var PlacesTransactions = {
    */
   batch(transactionsToBatch) {
     if (Array.isArray(transactionsToBatch)) {
-      if (transactionsToBatch.length == 0) {
+      if (!transactionsToBatch.length) {
         throw new Error("Must pass a non-empty array");
       }
 
@@ -725,7 +725,7 @@ function DefineTransaction(requiredProps = [], optionalProps = []) {
       return new ctor(input);
     }
 
-    if (requiredProps.length > 0 || optionalProps.length > 0) {
+    if (requiredProps.length || optionalProps.length) {
       // Bind the input properties to the arguments of execute.
       input = DefineTransaction.verifyInput(
         input,
@@ -858,7 +858,7 @@ DefineTransaction.defineArrayInputProp = function(name, basePropertyName) {
                           ${basePropertyName} as  input properties`);
         }
         let array = this.validateValue(input[name]);
-        if (required && array.length == 0) {
+        if (required && !array.length) {
           throw new Error(`Empty array passed for required input property:
                            ${name}`);
         }
@@ -896,7 +896,7 @@ DefineTransaction.getInputObjectForSingleValue = function(
   // * a single optional property with no required properties.
   if (
     requiredProps.length > 1 ||
-    (requiredProps.length == 0 && optionalProps.length > 1)
+    (!requiredProps.length && optionalProps.length > 1)
   ) {
     throw new Error("Transaction input isn't an object");
   }
@@ -915,7 +915,7 @@ DefineTransaction.verifyInput = function(
   requiredProps = [],
   optionalProps = []
 ) {
-  if (requiredProps.length == 0 && optionalProps.length == 0) {
+  if (!requiredProps.length && !optionalProps.length) {
     return {};
   }
 
@@ -1092,7 +1092,7 @@ PT.NewBookmark.prototype = Object.seal({
   async execute({ parentGuid, url, index, title, tags }) {
     let info = { parentGuid, index, url, title };
     // Filter tags to exclude already existing ones.
-    if (tags.length > 0) {
+    if (tags.length) {
       let currentTags = PlacesUtils.tagging.getTagsForURI(
         Services.io.newURI(url.href)
       );
@@ -1101,7 +1101,7 @@ PT.NewBookmark.prototype = Object.seal({
 
     async function createItem() {
       info = await PlacesUtils.bookmarks.insert(info);
-      if (tags.length > 0) {
+      if (tags.length) {
         PlacesUtils.tagging.tagURI(Services.io.newURI(url.href), tags);
       }
     }
@@ -1111,7 +1111,7 @@ PT.NewBookmark.prototype = Object.seal({
     this.undo = async function() {
       // Pick up the removed info so we have the accurate last-modified value.
       await PlacesUtils.bookmarks.remove(info);
-      if (tags.length > 0) {
+      if (tags.length) {
         PlacesUtils.tagging.untagURI(Services.io.newURI(url.href), tags);
       }
     };
@@ -1151,7 +1151,7 @@ PT.NewFolder.prototype = Object.seal({
       guid: parentGuid,
     };
 
-    if (children && children.length > 0) {
+    if (children && children.length) {
       // Ensure to specify a guid for each child to be restored on redo.
       info.children[0].children = children.map(c => {
         c.guid = PlacesUtils.history.makeGuid();
@@ -1302,7 +1302,7 @@ PT.EditUrl.prototype = Object.seal({
     async function updateItem() {
       updatedInfo = await PlacesUtils.bookmarks.update(updatedInfo);
       // Move tags from the original URI to the new URI.
-      if (originalTags.length > 0) {
+      if (originalTags.length) {
         // Untag the original URI only if this was the only bookmark.
         if (!(await PlacesUtils.bookmarks.fetch({ url: originalInfo.url }))) {
           PlacesUtils.tagging.untagURI(originalURI, originalTags);
@@ -1311,7 +1311,7 @@ PT.EditUrl.prototype = Object.seal({
         newURIAdditionalTags = originalTags.filter(
           t => !currentNewURITags.includes(t)
         );
-        if (newURIAdditionalTags && newURIAdditionalTags.length > 0) {
+        if (newURIAdditionalTags && newURIAdditionalTags.length) {
           PlacesUtils.tagging.tagURI(uri, newURIAdditionalTags);
         }
       }
@@ -1321,11 +1321,11 @@ PT.EditUrl.prototype = Object.seal({
     this.undo = async function() {
       await PlacesUtils.bookmarks.update(originalInfo);
       // Move tags from new URI to original URI.
-      if (originalTags.length > 0) {
+      if (originalTags.length) {
         // Only untag the new URI if this is the only bookmark.
         if (
           newURIAdditionalTags &&
-          newURIAdditionalTags.length > 0 &&
+          !!newURIAdditionalTags.length &&
           !(await PlacesUtils.bookmarks.fetch({ url }))
         ) {
           PlacesUtils.tagging.untagURI(uri, newURIAdditionalTags);
@@ -1416,7 +1416,7 @@ PT.SortByName.prototype = {
       let node = root.getChild(i);
       oldOrderGuids.push(node.bookmarkGuid);
       if (PlacesUtils.nodeIsSeparator(node)) {
-        if (preSepNodes.length > 0) {
+        if (preSepNodes.length) {
           preSepNodes.sort(sortingMethod);
           newOrderGuids.push(...preSepNodes.map(n => n.bookmarkGuid));
           preSepNodes = [];
@@ -1427,7 +1427,7 @@ PT.SortByName.prototype = {
       }
     }
     root.containerOpen = false;
-    if (preSepNodes.length > 0) {
+    if (preSepNodes.length) {
       preSepNodes.sort(sortingMethod);
       newOrderGuids.push(...preSepNodes.map(n => n.bookmarkGuid));
     }
@@ -1558,21 +1558,21 @@ PT.Untag.prototype = {
       let uri = Services.io.newURI(url.href);
       let tagsToRemove;
       let tagsSet = PlacesUtils.tagging.getTagsForURI(uri);
-      if (tags.length > 0) {
+      if (tags.length) {
         tagsToRemove = tags.filter(t => tagsSet.includes(t));
       } else {
         tagsToRemove = tagsSet;
       }
-      if (tagsToRemove.length > 0) {
+      if (tagsToRemove.length) {
         PlacesUtils.tagging.untagURI(uri, tagsToRemove);
       }
       onUndo.unshift(() => {
-        if (tagsToRemove.length > 0) {
+        if (tagsToRemove.length) {
           PlacesUtils.tagging.tagURI(uri, tagsToRemove);
         }
       });
       onRedo.push(() => {
-        if (tagsToRemove.length > 0) {
+        if (tagsToRemove.length) {
           PlacesUtils.tagging.untagURI(uri, tagsToRemove);
         }
       });
diff --git a/toolkit/components/places/PlacesUtils.jsm b/toolkit/components/places/PlacesUtils.jsm
index d0ea3aa769aa..2e33923e4585 100644
--- a/toolkit/components/places/PlacesUtils.jsm
+++ b/toolkit/components/places/PlacesUtils.jsm
@@ -856,7 +856,7 @@ var PlacesUtils = {
   _shutdownFunctions: [],
   registerShutdownFunction: function PU_registerShutdownFunction(aFunc) {
     // If this is the first registered function, add the shutdown observer.
-    if (this._shutdownFunctions.length == 0) {
+    if (!this._shutdownFunctions.length) {
       Services.obs.addObserver(this, this.TOPIC_SHUTDOWN);
     }
     this._shutdownFunctions.push(aFunc);
@@ -867,7 +867,7 @@ var PlacesUtils = {
     switch (aTopic) {
       case this.TOPIC_SHUTDOWN:
         Services.obs.removeObserver(this, this.TOPIC_SHUTDOWN);
-        while (this._shutdownFunctions.length > 0) {
+        while (this._shutdownFunctions.length) {
           this._shutdownFunctions.shift().apply(this);
         }
         break;
@@ -2824,7 +2824,7 @@ var GuidHelper = {
           "SELECT b.id, b.guid from moz_bookmarks b WHERE b.guid = :guid LIMIT 1",
           { guid: aGuid }
         );
-        if (rows.length == 0) {
+        if (!rows.length) {
           throw new Error("no item found for the given GUID");
         }
 
@@ -2879,7 +2879,7 @@ var GuidHelper = {
           "SELECT b.id, b.guid from moz_bookmarks b WHERE b.id = :id LIMIT 1",
           { id: aItemId }
         );
-        if (rows.length == 0) {
+        if (!rows.length) {
           throw new Error("no item found for the given itemId");
         }
 
diff --git a/toolkit/components/places/TaggingService.jsm b/toolkit/components/places/TaggingService.jsm
index a481ff52344d..87eabd072350 100644
--- a/toolkit/components/places/TaggingService.jsm
+++ b/toolkit/components/places/TaggingService.jsm
@@ -131,7 +131,7 @@ TaggingService.prototype = {
           tag.__defineGetter__("name", () => this._tagFolders[tag.id]);
         } else if (
           typeof idOrName == "string" &&
-          idOrName.length > 0 &&
+          !!idOrName.length &&
           idOrName.length <= PlacesUtils.bookmarks.MAX_TAG_LENGTH
         ) {
           // This is a tag name.
@@ -151,7 +151,7 @@ TaggingService.prototype = {
 
   // nsITaggingService
   tagURI: function TS_tagURI(aURI, aTags, aSource) {
-    if (!aURI || !aTags || !Array.isArray(aTags) || aTags.length == 0) {
+    if (!aURI || !aTags || !Array.isArray(aTags) || !aTags.length) {
       throw Components.Exception(
         "Invalid value for tags",
         Cr.NS_ERROR_INVALID_ARG
@@ -230,7 +230,7 @@ TaggingService.prototype = {
 
   // nsITaggingService
   untagURI: function TS_untagURI(aURI, aTags, aSource) {
-    if (!aURI || (aTags && (!Array.isArray(aTags) || aTags.length == 0))) {
+    if (!aURI || (aTags && (!Array.isArray(aTags) || !aTags.length))) {
       throw Components.Exception(
         "Invalid value for tags",
         Cr.NS_ERROR_INVALID_ARG
diff --git a/toolkit/components/places/UnifiedComplete.jsm b/toolkit/components/places/UnifiedComplete.jsm
index bfa37975456b..545476e10085 100644
--- a/toolkit/components/places/UnifiedComplete.jsm
+++ b/toolkit/components/places/UnifiedComplete.jsm
@@ -647,7 +647,7 @@ function Search(
 
   // This allows to handle leading or trailing restriction characters specially.
   this._leadingRestrictionToken = null;
-  if (tokens.length > 0) {
+  if (tokens.length) {
     if (
       UrlbarTokenizer.isRestrictionToken(tokens[0]) &&
       (tokens.length > 1 ||
@@ -672,7 +672,7 @@ function Search(
   // keyword, a search engine alias, an extension keyword, or simply a URL or
   // part of the search string the user has typed.  We won't know until we
   // create the heuristic result.
-  let firstToken = this._searchTokens.length > 0 && this._searchTokens[0].value;
+  let firstToken = !!this._searchTokens.length && this._searchTokens[0].value;
   this._heuristicToken =
     firstToken && this._trimmedOriginalSearchString.startsWith(firstToken)
       ? firstToken
@@ -1339,7 +1339,7 @@ Search.prototype = {
     // We always try to make the first result a special "heuristic" result.  The
     // heuristics below determine what type of result it will be, if any.
 
-    let hasSearchTerms = this._searchTokens.length > 0;
+    let hasSearchTerms = !!this._searchTokens.length;
 
     if (hasSearchTerms) {
       // It may be a keyword registered by an extension.
@@ -2042,7 +2042,7 @@ Search.prototype = {
     // when searching for "Firefox".
     let terms = parseResult.terms.toLowerCase();
     if (
-      this._searchTokens.length > 0 &&
+      this._searchTokens.length &&
       this._searchTokens.every(token => !terms.includes(token.value))
     ) {
       return;
@@ -2209,7 +2209,7 @@ Search.prototype = {
       // This requires walking the previous matches and marking their existence
       // into the current buckets, so that, when we'll add the new matches to
       // the buckets, we can either append or replace a match.
-      if (this._previousSearchMatchTypes.length > 0) {
+      if (this._previousSearchMatchTypes.length) {
         for (let type of this._previousSearchMatchTypes) {
           for (let bucket of this._buckets) {
             if (type == bucket.type && bucket.count < bucket.available) {
@@ -2254,7 +2254,7 @@ Search.prototype = {
    *        Whether to notify a result change.
    */
   _cleanUpNonCurrentMatches(type, notify = true) {
-    if (this._previousSearchMatchTypes.length == 0 || !this.pending) {
+    if (!this._previousSearchMatchTypes.length || !this.pending) {
       return;
     }
 
@@ -2295,10 +2295,7 @@ Search.prototype = {
    * If in restrict mode, cleanups non current matches for all the empty types.
    */
   cleanUpRestrictNonCurrentMatches() {
-    if (
-      this.hasBehavior("restrict") &&
-      this._previousSearchMatchTypes.length > 0
-    ) {
+    if (this.hasBehavior("restrict") && this._previousSearchMatchTypes.length) {
       for (let type of new Set(this._previousSearchMatchTypes)) {
         if (this._counts[type] == 0) {
           // Don't notify, since we are about to notify completion.
@@ -2613,7 +2610,7 @@ Search.prototype = {
       return false;
     }
 
-    if (this._searchString.length == 0) {
+    if (!this._searchString.length) {
       return false;
     }
 
@@ -2903,8 +2900,8 @@ UnifiedComplete.prototype = {
       // that may leave exposed unrelated matches for a longer time.
       let previousSearchString = result.searchString;
       let stringsRelated =
-        previousSearchString.length > 0 &&
-        searchString.length > 0 &&
+        !!previousSearchString.length &&
+        !!searchString.length &&
         (previousSearchString.includes(searchString) ||
           searchString.includes(previousSearchString));
       if (insertMethod == UrlbarUtils.INSERTMETHOD.MERGE || stringsRelated) {
diff --git a/toolkit/components/places/tests/PlacesTestUtils.jsm b/toolkit/components/places/tests/PlacesTestUtils.jsm
index a595d6744e82..408a5ed47f97 100644
--- a/toolkit/components/places/tests/PlacesTestUtils.jsm
+++ b/toolkit/components/places/tests/PlacesTestUtils.jsm
@@ -198,7 +198,7 @@ var PlacesTestUtils = Object.freeze({
       "SELECT id FROM moz_places WHERE url_hash = hash(:url) AND url = :url",
       { url }
     );
-    return rows.length > 0;
+    return !!rows.length;
   },
 
   /**
diff --git a/toolkit/components/places/tests/favicons/test_root_icons.js b/toolkit/components/places/tests/favicons/test_root_icons.js
index 45846b2acd54..43cc2cf43973 100644
--- a/toolkit/components/places/tests/favicons/test_root_icons.js
+++ b/toolkit/components/places/tests/favicons/test_root_icons.js
@@ -131,7 +131,7 @@ add_task(async function test_removePagesByTimeframe() {
   await PlacesTestUtils.promiseAsyncUpdates();
   rows = await db.execute("SELECT * FROM moz_icons");
   // Debug logging for possible intermittent failure (bug 1358368).
-  if (rows.length != 0) {
+  if (rows.length) {
     dump_table("moz_icons");
   }
   Assert.equal(rows.length, 0, "There should be no icon entry");
diff --git a/toolkit/components/places/tests/head_common.js b/toolkit/components/places/tests/head_common.js
index 156668b9af6f..e706084852e9 100644
--- a/toolkit/components/places/tests/head_common.js
+++ b/toolkit/components/places/tests/head_common.js
@@ -707,8 +707,8 @@ function do_compare_arrays(a1, a2, sorted) {
     return a1.every((e, i) => e == a2[i]);
   }
   return (
-    a1.filter(e => !a2.includes(e)).length == 0 &&
-    a2.filter(e => !a1.includes(e)).length == 0
+    !a1.filter(e => !a2.includes(e)).length &&
+    !a2.filter(e => !a1.includes(e)).length
   );
 }
 
@@ -804,7 +804,7 @@ async function foreign_count(url) {
     `,
     { url }
   );
-  return rows.length == 0 ? 0 : rows[0].getResultByName("foreign_count");
+  return !rows.length ? 0 : rows[0].getResultByName("foreign_count");
 }
 
 function compareAscending(a, b) {
@@ -906,9 +906,9 @@ async function compareFavicons(icon1, icon2, msg) {
   }
 
   let data1 = await getIconData(icon1);
-  Assert.ok(data1.length > 0, "Should fetch icon data");
+  Assert.ok(!!data1.length, "Should fetch icon data");
   let data2 = await getIconData(icon2);
-  Assert.ok(data2.length > 0, "Should fetch icon data");
+  Assert.ok(!!data2.length, "Should fetch icon data");
   Assert.deepEqual(data1, data2, msg);
 }
 
diff --git a/toolkit/components/places/tests/history/test_async_history_api.js b/toolkit/components/places/tests/history/test_async_history_api.js
index 557860751856..bc94b2baed5b 100644
--- a/toolkit/components/places/tests/history/test_async_history_api.js
+++ b/toolkit/components/places/tests/history/test_async_history_api.js
@@ -376,7 +376,7 @@ add_task(async function test_non_addable_uri_errors() {
   });
 
   let placesResult = await promiseUpdatePlaces(places);
-  if (placesResult.results.length > 0) {
+  if (placesResult.results.length) {
     do_throw("Unexpected success.");
   }
   for (let place of placesResult.errors) {
@@ -397,7 +397,7 @@ add_task(async function test_duplicate_guid_errors() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -411,7 +411,7 @@ add_task(async function test_duplicate_guid_errors() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(badPlace.uri));
   placesResult = await promiseUpdatePlaces(badPlace);
-  if (placesResult.results.length > 0) {
+  if (placesResult.results.length) {
     do_throw("Unexpected success.");
   }
   let badPlaceInfo = placesResult.errors[0];
@@ -439,7 +439,7 @@ add_task(async function test_invalid_referrerURI_ignored() {
   );
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -474,7 +474,7 @@ add_task(async function test_nonnsIURI_referrerURI_ignored() {
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -508,7 +508,7 @@ add_task(async function test_old_referrer_ignored() {
   // as being invalid.
   Assert.equal(false, await PlacesUtils.history.hasVisits(referrerPlace.uri));
   let placesResult = await promiseUpdatePlaces(referrerPlace);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
 
@@ -526,7 +526,7 @@ add_task(async function test_old_referrer_ignored() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -558,7 +558,7 @@ add_task(async function test_place_id_ignored() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -575,7 +575,7 @@ add_task(async function test_place_id_ignored() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(badPlace.uri));
   placesResult = await promiseUpdatePlaces(badPlace);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   placeInfo = placesResult.results[0];
@@ -631,7 +631,7 @@ add_task(async function test_add_visit() {
 
   let callbackCount = 0;
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   for (let placeInfo of placesResult.results) {
@@ -698,7 +698,7 @@ add_task(async function test_properties_saved() {
 
   let callbackCount = 0;
   let placesResult = await promiseUpdatePlaces(places);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   for (let placeInfo of placesResult.results) {
@@ -774,7 +774,7 @@ add_task(async function test_guid_saved() {
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -802,7 +802,7 @@ add_task(async function test_referrer_saved() {
 
   let resultCount = 0;
   let placesResult = await promiseUpdatePlaces(places);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   for (let placeInfo of placesResult.results) {
@@ -846,14 +846,14 @@ add_task(async function test_guid_change_saved() {
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   // Then, change the guid with visits.
   place.guid = "_GUIDCHANGE_";
   place.visits = [new VisitInfo()];
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   do_check_guid_for_uri(place.uri, place.guid);
@@ -871,7 +871,7 @@ add_task(async function test_title_change_saved() {
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
 
@@ -879,7 +879,7 @@ add_task(async function test_title_change_saved() {
   place.title = "";
   place.visits = [new VisitInfo()];
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   do_check_title_for_uri(place.uri, null);
@@ -888,7 +888,7 @@ add_task(async function test_title_change_saved() {
   place.title = "title change";
   place.visits = [new VisitInfo()];
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   do_check_title_for_uri(place.uri, place.title);
@@ -897,7 +897,7 @@ add_task(async function test_title_change_saved() {
   place.title = null;
   place.visits = [new VisitInfo()];
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   do_check_title_for_uri(place.uri, place.title);
@@ -916,14 +916,14 @@ add_task(async function test_no_title_does_not_clear_title() {
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
 
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   // Now, make sure that not specifying a title does not clear it.
   delete place.title;
   place.visits = [new VisitInfo()];
   placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   do_check_title_for_uri(place.uri, TITLE);
@@ -950,7 +950,7 @@ add_task(async function test_title_change_notifies() {
 
   PlacesUtils.history.addObserver(silentObserver);
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
 
@@ -1167,7 +1167,7 @@ add_task(async function test_ignore_errors() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
@@ -1181,7 +1181,7 @@ add_task(async function test_ignore_errors() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(badPlace.uri));
   placesResult = await promiseUpdatePlaces(badPlace, { ignoreErrors: true });
-  if (placesResult.results.length > 0) {
+  if (placesResult.results.length) {
     do_throw("Unexpected success.");
   }
   Assert.equal(
@@ -1239,7 +1239,7 @@ add_task(async function test_ignore_results_and_errors() {
 
   Assert.equal(false, await PlacesUtils.history.hasVisits(place.uri));
   let placesResult = await promiseUpdatePlaces(place);
-  if (placesResult.errors.length > 0) {
+  if (placesResult.errors.length) {
     do_throw("Unexpected error.");
   }
   let placeInfo = placesResult.results[0];
diff --git a/toolkit/components/places/tests/history/test_update.js b/toolkit/components/places/tests/history/test_update.js
index 484856d7ce32..406a8190590e 100644
--- a/toolkit/components/places/tests/history/test_update.js
+++ b/toolkit/components/places/tests/history/test_update.js
@@ -165,7 +165,7 @@ add_task(async function test_description_change_saved() {
   await PlacesUtils.history.update({ url: TEST_URL, description });
   descriptionInDB = await PlacesTestUtils.fieldInDB(TEST_URL, "description");
   Assert.ok(
-    0 < descriptionInDB.length < description.length,
+    !!descriptionInDB.length < description.length,
     "a long description should be truncated"
   );
 
diff --git a/toolkit/components/places/tests/queries/test_redirects.js b/toolkit/components/places/tests/queries/test_redirects.js
index 2ed2cb8de26a..f087239ae3cc 100644
--- a/toolkit/components/places/tests/queries/test_redirects.js
+++ b/toolkit/components/places/tests/queries/test_redirects.js
@@ -50,9 +50,9 @@ function check_results_callback(aSequence) {
     if (!includeHidden && isHidden(aVisit)) {
       // If the page has any non-hidden visit, then it's visible.
       if (
-        visits.filter(function(refVisit) {
+        !visits.filter(function(refVisit) {
           return refVisit.uri == aVisit.uri && !isHidden(refVisit);
-        }).length == 0
+        }).length
       ) {
         return false;
       }
diff --git a/toolkit/components/places/tests/queries/test_results-as-visit.js b/toolkit/components/places/tests/queries/test_results-as-visit.js
index cad40143894b..b10fb00bab1a 100644
--- a/toolkit/components/places/tests/queries/test_results-as-visit.js
+++ b/toolkit/components/places/tests/queries/test_results-as-visit.js
@@ -20,7 +20,7 @@ function createTestData() {
         title: aPage.title,
         uri: aPage.uri,
         lastVisit: newTimeInMicroseconds(),
-        isTag: aPage.tags && aPage.tags.length > 0,
+        isTag: aPage.tags && !!aPage.tags.length,
         tagArray: aPage.tags,
       });
     }
diff --git a/toolkit/components/places/tests/unit/test_async_transactions.js b/toolkit/components/places/tests/unit/test_async_transactions.js
index 3660d6d85e14..38ce6c76d629 100644
--- a/toolkit/components/places/tests/unit/test_async_transactions.js
+++ b/toolkit/components/places/tests/unit/test_async_transactions.js
@@ -1733,10 +1733,9 @@ add_task(async function test_untag_uri() {
     }
     function ensureTagsUnset() {
       for (let url of urls) {
-        let expectedTags =
-          tagsRemoved.length == 0
-            ? []
-            : preRemovalTags.get(url).filter(tag => !tagsRemoved.includes(tag));
+        let expectedTags = !tagsRemoved.length
+          ? []
+          : preRemovalTags.get(url).filter(tag => !tagsRemoved.includes(tag));
         ensureTagsForURI(url, expectedTags);
       }
     }
diff --git a/toolkit/components/places/tests/unit/test_bookmarks_html.js b/toolkit/components/places/tests/unit/test_bookmarks_html.js
index da0580f9b7be..b6bcb39ae569 100644
--- a/toolkit/components/places/tests/unit/test_bookmarks_html.js
+++ b/toolkit/components/places/tests/unit/test_bookmarks_html.js
@@ -370,7 +370,7 @@ function checkItem(aExpected, aNode) {
           let folder = aNode.QueryInterface(
             Ci.nsINavHistoryContainerResultNode
           );
-          Assert.equal(folder.hasChildren, aExpected.children.length > 0);
+          Assert.equal(folder.hasChildren, !!aExpected.children.length);
           folder.containerOpen = true;
           Assert.equal(folder.childCount, aExpected.children.length);
 
diff --git a/toolkit/components/places/tests/unit/test_bookmarks_json.js b/toolkit/components/places/tests/unit/test_bookmarks_json.js
index b217f0f676bd..ba4999bcf876 100644
--- a/toolkit/components/places/tests/unit/test_bookmarks_json.js
+++ b/toolkit/components/places/tests/unit/test_bookmarks_json.js
@@ -270,7 +270,7 @@ async function checkItem(aExpected, aNode) {
         break;
       case "children":
         let folder = aNode.QueryInterface(Ci.nsINavHistoryContainerResultNode);
-        Assert.equal(folder.hasChildren, aExpected.children.length > 0);
+        Assert.equal(folder.hasChildren, !!aExpected.children.length);
         folder.containerOpen = true;
         Assert.equal(folder.childCount, aExpected.children.length);
 
diff --git a/toolkit/components/places/tests/unit/test_null_interfaces.js b/toolkit/components/places/tests/unit/test_null_interfaces.js
index e59b413946f4..30861bc7fadd 100644
--- a/toolkit/components/places/tests/unit/test_null_interfaces.js
+++ b/toolkit/components/places/tests/unit/test_null_interfaces.js
@@ -50,7 +50,7 @@ function run_test() {
       let mesg = "";
       if (typeof func != "function") {
         mesg = "Not a function!";
-      } else if (func.length == 0) {
+      } else if (!func.length) {
         mesg = "No args needed!";
       } else if (name == "QueryInterface") {
         mesg = "Ignore QI!";
diff --git a/toolkit/components/places/tests/unit/test_promiseBookmarksTree.js b/toolkit/components/places/tests/unit/test_promiseBookmarksTree.js
index 1f2bb138ed31..94a3739ca920 100644
--- a/toolkit/components/places/tests/unit/test_promiseBookmarksTree.js
+++ b/toolkit/components/places/tests/unit/test_promiseBookmarksTree.js
@@ -76,7 +76,7 @@ async function compareToNode(aItem, aNode, aIsRootItem, aExcludedGuids = []) {
         expectedChildrenNodes.push(childNode);
       }
 
-      if (expectedChildrenNodes.length > 0) {
+      if (expectedChildrenNodes.length) {
         Assert.ok(Array.isArray(aItem.children));
         Assert.equal(aItem.children.length, expectedChildrenNodes.length);
         for (let i = 0; i < aItem.children.length; i++) {
@@ -187,7 +187,7 @@ async function test_promiseBookmarksTreeForEachNode(
   aOptions,
   aExcludedGuids
 ) {
-  Assert.ok(aNode.bookmarkGuid && aNode.bookmarkGuid.length > 0);
+  Assert.ok(aNode.bookmarkGuid && !!aNode.bookmarkGuid.length);
   let item = await PlacesUtils.promiseBookmarksTree(
     aNode.bookmarkGuid,
     aOptions
diff --git a/toolkit/components/promiseworker/PromiseWorker.jsm b/toolkit/components/promiseworker/PromiseWorker.jsm
index ec77a991a2c7..61ad1c09fdb7 100644
--- a/toolkit/components/promiseworker/PromiseWorker.jsm
+++ b/toolkit/components/promiseworker/PromiseWorker.jsm
@@ -42,7 +42,7 @@ Queue.prototype = {
     return this._array.push(x);
   },
   isEmpty: function isEmpty() {
-    return this._array.length == 0;
+    return !this._array.length;
   },
 };
 
diff --git a/toolkit/components/satchel/FormAutoComplete.jsm b/toolkit/components/satchel/FormAutoComplete.jsm
index 528a5752dbd3..e492b5eee6de 100644
--- a/toolkit/components/satchel/FormAutoComplete.jsm
+++ b/toolkit/components/satchel/FormAutoComplete.jsm
@@ -627,13 +627,13 @@ FormAutoCompleteResult.prototype = {
   searchString: "",
   errorDescription: "",
   get defaultIndex() {
-    if (this.entries.length == 0) {
+    if (!this.entries.length) {
       return -1;
     }
     return 0;
   },
   get searchResult() {
-    if (this.entries.length == 0) {
+    if (!this.entries.length) {
       return Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
     }
     return Ci.nsIAutoCompleteResult.RESULT_SUCCESS;
diff --git a/toolkit/components/satchel/InputListAutoComplete.jsm b/toolkit/components/satchel/InputListAutoComplete.jsm
index 0dcf31e846a9..07f185a36d2d 100644
--- a/toolkit/components/satchel/InputListAutoComplete.jsm
+++ b/toolkit/components/satchel/InputListAutoComplete.jsm
@@ -14,11 +14,10 @@ InputListAutoComplete.prototype = {
 
   autoCompleteSearch(aUntrimmedSearchString, aField) {
     let [values, labels] = this.getListSuggestions(aField);
-    let searchResult =
-      values.length > 0
-        ? Ci.nsIAutoCompleteResult.RESULT_SUCCESS
-        : Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
-    let defaultIndex = values.length > 0 ? 0 : -1;
+    let searchResult = values.length
+      ? Ci.nsIAutoCompleteResult.RESULT_SUCCESS
+      : Ci.nsIAutoCompleteResult.RESULT_NOMATCH;
+    let defaultIndex = values.length ? 0 : -1;
 
     return new FormAutoCompleteResult(
       aUntrimmedSearchString,
diff --git a/toolkit/components/satchel/test/satchel_common.js b/toolkit/components/satchel/test/satchel_common.js
index 4b9f5c46fc6b..012fe2cae860 100644
--- a/toolkit/components/satchel/test/satchel_common.js
+++ b/toolkit/components/satchel/test/satchel_common.js
@@ -90,7 +90,7 @@ var checkObserver = {
   },
 
   waitForChecks(callback) {
-    if (this.verifyStack.length == 0) {
+    if (!this.verifyStack.length) {
       callback();
     } else {
       this.callback = callback;
@@ -101,7 +101,7 @@ var checkObserver = {
     if (data != "formhistory-add" && data != "formhistory-update") {
       return;
     }
-    ok(this.verifyStack.length > 0, "checking if saved form data was expected");
+    ok(!!this.verifyStack.length, "checking if saved form data was expected");
 
     // Make sure that every piece of data we expect to be saved is saved, and no
     // more. Here it is assumed that for every entry satchel saves or modifies, a
@@ -116,7 +116,7 @@ var checkObserver = {
 
     countEntries(expected.name, expected.value, function(num) {
       ok(num > 0, expected.message);
-      if (checkObserver.verifyStack.length == 0) {
+      if (!checkObserver.verifyStack.length) {
         let callback = checkObserver.callback;
         checkObserver.callback = null;
         callback();
diff --git a/toolkit/components/satchel/test/unit/test_history_api.js b/toolkit/components/satchel/test/unit/test_history_api.js
index 773d6a8dff4f..001f248734af 100644
--- a/toolkit/components/satchel/test/unit/test_history_api.js
+++ b/toolkit/components/satchel/test/unit/test_history_api.js
@@ -248,7 +248,7 @@ add_task(async function() {
 
     let processFirstResult = function processResults(results) {
       // Only handle the first result
-      if (results.length > 0) {
+      if (results.length) {
         let result = results[0];
         return [
           result.timesUsed,
diff --git a/toolkit/components/search/SearchEngine.jsm b/toolkit/components/search/SearchEngine.jsm
index 810771f002e4..c5f80de63034 100644
--- a/toolkit/components/search/SearchEngine.jsm
+++ b/toolkit/components/search/SearchEngine.jsm
@@ -1639,7 +1639,7 @@ SearchEngine.prototype = {
           break;
       }
     }
-    if (!this.name || this._urls.length == 0) {
+    if (!this.name || !this._urls.length) {
       SearchUtils.fail("_parse: No name, or missing URL!", Cr.NS_ERROR_FAILURE);
     }
     if (!this.supportsResponseType(SearchUtils.URL_TYPE.SEARCH)) {
diff --git a/toolkit/components/search/SearchService.jsm b/toolkit/components/search/SearchService.jsm
index 9130672270d4..2b79446aacf1 100644
--- a/toolkit/components/search/SearchService.jsm
+++ b/toolkit/components/search/SearchService.jsm
@@ -1814,7 +1814,7 @@ SearchService.prototype = {
         e => !ignoredJAREngines.includes(e)
       );
       // Don't allow all engines to be hidden
-      if (filteredEngineNames.length > 0) {
+      if (filteredEngineNames.length) {
         engineNames = filteredEngineNames;
       }
     }
diff --git a/toolkit/components/search/tests/xpcshell/test_SearchStaticData.js b/toolkit/components/search/tests/xpcshell/test_SearchStaticData.js
index f9a105c0e3e6..b99ea4d3e6c4 100644
--- a/toolkit/components/search/tests/xpcshell/test_SearchStaticData.js
+++ b/toolkit/components/search/tests/xpcshell/test_SearchStaticData.js
@@ -25,7 +25,7 @@ function run_test() {
       d.startsWith("www.google.")
     )
   );
-  Assert.ok(SearchStaticData.getAlternateDomains("google.com").length == 0);
+  Assert.ok(!SearchStaticData.getAlternateDomains("google.com").length);
 
   // Test that methods from SearchStaticData module can be overwritten,
   // needed for hotfixing.
diff --git a/toolkit/components/search/tests/xpcshell/test_notifications.js b/toolkit/components/search/tests/xpcshell/test_notifications.js
index 8320534ccdbb..93af8f2f7653 100644
--- a/toolkit/components/search/tests/xpcshell/test_notifications.js
+++ b/toolkit/components/search/tests/xpcshell/test_notifications.js
@@ -41,7 +41,7 @@ class SearchObserver {
 
     this.expectedNotifications.shift();
 
-    if (this.expectedNotifications.length == 0) {
+    if (!this.expectedNotifications.length) {
       this.deferred.resolve(this.addedEngine);
       Services.obs.removeObserver(
         this.observer,
diff --git a/toolkit/components/search/tests/xpcshell/test_searchSuggest.js b/toolkit/components/search/tests/xpcshell/test_searchSuggest.js
index 1e0bfe6d504a..d4aee9456f7f 100644
--- a/toolkit/components/search/tests/xpcshell/test_searchSuggest.js
+++ b/toolkit/components/search/tests/xpcshell/test_searchSuggest.js
@@ -482,7 +482,7 @@ add_task(async function empty_searchTerm() {
   let controller = new SearchSuggestionController();
   let result = await controller.fetch("", false, getEngine);
   Assert.equal(result.term, "");
-  Assert.ok(result.local.length > 0);
+  Assert.ok(!!result.local.length);
   Assert.equal(result.remote.length, 0);
 });
 
diff --git a/toolkit/components/search/tests/xpcshell/test_validate_manifests.js b/toolkit/components/search/tests/xpcshell/test_validate_manifests.js
index f9be94000f70..2e838a752abd 100644
--- a/toolkit/components/search/tests/xpcshell/test_validate_manifests.js
+++ b/toolkit/components/search/tests/xpcshell/test_validate_manifests.js
@@ -43,7 +43,7 @@ async function getSearchExtensions() {
 add_task(async function test_validate_manifest() {
   let searchExtensions = await getSearchExtensions();
   ok(
-    searchExtensions.length > 0,
+    !!searchExtensions.length,
     `Found ${searchExtensions.length} search extensions`
   );
   for (const xpi of searchExtensions) {
diff --git a/toolkit/components/sessionstore/SessionStoreFunctions.jsm b/toolkit/components/sessionstore/SessionStoreFunctions.jsm
index 18bfed4a91a0..c887af745600 100644
--- a/toolkit/components/sessionstore/SessionStoreFunctions.jsm
+++ b/toolkit/components/sessionstore/SessionStoreFunctions.jsm
@@ -370,7 +370,7 @@ var SessionStoreFuncInternal = {
         data[aOrigins[i]] = hostData;
       }
     }
-    if (aOrigins.length > 0) {
+    if (aOrigins.length) {
       return data;
     }
 
diff --git a/toolkit/components/telemetry/app/TelemetrySend.jsm b/toolkit/components/telemetry/app/TelemetrySend.jsm
index 9d5131058f2d..381af4130282 100644
--- a/toolkit/components/telemetry/app/TelemetrySend.jsm
+++ b/toolkit/components/telemetry/app/TelemetrySend.jsm
@@ -511,7 +511,7 @@ var SendScheduler = {
       );
 
       // Bail out if there is nothing to send.
-      if (pending.length == 0 && current.length == 0) {
+      if (!pending.length && !current.length) {
         this._log.trace("_doSendTask - no pending pings, bailing out");
         this._sendTaskState = "bail out - no pings to send";
         return;
@@ -808,7 +808,7 @@ var TelemetrySendImpl = {
     // Scan the pending pings - that gives us a list sorted by last modified, descending.
     let infos = await TelemetryStorage.loadPendingPingList();
     this._log.info("_checkPendingPings - pending ping count: " + infos.length);
-    if (infos.length == 0) {
+    if (!infos.length) {
       this._log.trace("_checkPendingPings - no pending pings");
       return;
     }
@@ -1105,7 +1105,7 @@ var TelemetrySendImpl = {
       pingSends.push(p);
     }
 
-    if (persistedPingIds.length > 0) {
+    if (persistedPingIds.length) {
       pingSends.push(
         this._sendPersistedPings(persistedPingIds).catch(ex => {
           this._log.info("sendPings - persisted pings not sent", ex);
diff --git a/toolkit/components/telemetry/pings/ModulesPing.jsm b/toolkit/components/telemetry/pings/ModulesPing.jsm
index 50dabe885da4..108cbf361363 100644
--- a/toolkit/components/telemetry/pings/ModulesPing.jsm
+++ b/toolkit/components/telemetry/pings/ModulesPing.jsm
@@ -80,7 +80,7 @@ var TelemetryModules = Object.freeze({
     try {
       Telemetry.getLoadedModules().then(
         modules => {
-          modules = modules.filter(module => module.name.length > 0);
+          modules = modules.filter(module => !!module.name.length);
 
           // Cut the list of modules to MAX_MODULES_NUM entries.
           if (modules.length > MAX_MODULES_NUM) {
diff --git a/toolkit/components/telemetry/pings/TelemetrySession.jsm b/toolkit/components/telemetry/pings/TelemetrySession.jsm
index 99ed609f9cc9..1cff9a1db842 100644
--- a/toolkit/components/telemetry/pings/TelemetrySession.jsm
+++ b/toolkit/components/telemetry/pings/TelemetrySession.jsm
@@ -721,7 +721,7 @@ var Impl = {
 
       if (
         this._slowSQLStartup &&
-        Object.keys(this._slowSQLStartup).length != 0 &&
+        !!Object.keys(this._slowSQLStartup).length &&
         (Object.keys(this._slowSQLStartup.mainThread).length ||
           Object.keys(this._slowSQLStartup.otherThreads).length)
       ) {
@@ -740,7 +740,7 @@ var Impl = {
       // Adding captured stacks to the payload only if any exist and clearing
       // captures for this sub-session.
       let stacks = protect(() => Telemetry.snapshotCapturedStacks(true));
-      if (stacks && "captures" in stacks && stacks.captures.length > 0) {
+      if (stacks && "captures" in stacks && stacks.captures.length) {
         payloadObj.processes.parent.capturedStacks = stacks;
       }
     }
@@ -1107,7 +1107,7 @@ var Impl = {
     reason = reason || REASON_GATHER_PAYLOAD;
     // This function returns the current Telemetry payload to the caller.
     // We only gather startup info once.
-    if (Object.keys(this._slowSQLStartup).length == 0) {
+    if (!Object.keys(this._slowSQLStartup).length) {
       this._slowSQLStartup = Telemetry.slowSQL;
     }
     Services.telemetry.gatherMemory();
diff --git a/toolkit/components/telemetry/tests/browser/browser_HybridContentTelemetry.js b/toolkit/components/telemetry/tests/browser/browser_HybridContentTelemetry.js
index fe211279959a..fcbddede48ad 100644
--- a/toolkit/components/telemetry/tests/browser/browser_HybridContentTelemetry.js
+++ b/toolkit/components/telemetry/tests/browser/browser_HybridContentTelemetry.js
@@ -44,7 +44,7 @@ async function waitForProcessesEvents(
 async function waitForEvent(aProcess, aEventData) {
   await waitForProcessesEvents([aProcess], events => {
     let processEvents = events[aProcess].map(e => e.slice(1));
-    if (processEvents.length == 0) {
+    if (!processEvents.length) {
       return false;
     }
 
diff --git a/toolkit/components/telemetry/tests/browser/browser_TelemetryGC.js b/toolkit/components/telemetry/tests/browser/browser_TelemetryGC.js
index 439284a53da7..f8718f144f8d 100644
--- a/toolkit/components/telemetry/tests/browser/browser_TelemetryGC.js
+++ b/toolkit/components/telemetry/tests/browser/browser_TelemetryGC.js
@@ -51,7 +51,7 @@ function check(entries) {
 
       ok("slices_list" in gc, "slices_list field present");
       ok(Array.isArray(gc.slices_list), "slices_list is an array");
-      ok(gc.slices_list.length > 0, "slices_list array non-empty");
+      ok(!!gc.slices_list.length, "slices_list array non-empty");
       ok(gc.slices_list.length <= 4, "slices_list array is not too long");
 
       ok("totals" in gc, "totals field present");
diff --git a/toolkit/components/telemetry/tests/unit/test_ChildEvents.js b/toolkit/components/telemetry/tests/unit/test_ChildEvents.js
index 0aada9c22319..ff3da1954ccd 100644
--- a/toolkit/components/telemetry/tests/unit/test_ChildEvents.js
+++ b/toolkit/components/telemetry/tests/unit/test_ChildEvents.js
@@ -128,17 +128,17 @@ add_task(async function() {
 
   Assert.ok("parent" in snapshot, "Should have main process section");
   Assert.ok(
-    snapshot.parent.length > 0,
+    !!snapshot.parent.length,
     "Main process section should have events."
   );
   Assert.ok("content" in snapshot, "Should have child process section");
   Assert.ok(
-    snapshot.content.length > 0,
+    !!snapshot.content.length,
     "Child process section should have events."
   );
   Assert.ok("dynamic" in snapshot, "Should have dynamic process section");
   Assert.ok(
-    snapshot.dynamic.length > 0,
+    !!snapshot.dynamic.length,
     "Dynamic process section should have events."
   );
 
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
index 3d296c2f1d59..8e1364ec003f 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
@@ -739,7 +739,7 @@ function checkSystemSection(data) {
 
   Assert.ok("adapters" in gfxData);
   Assert.ok(
-    gfxData.adapters.length > 0,
+    !!gfxData.adapters.length,
     "There must be at least one GFX adapter."
   );
   for (let adapter of gfxData.adapters) {
@@ -982,7 +982,7 @@ function checkAddonsSection(data, expectBrokenAddons, partialAddonsRecords) {
 function checkExperimentsSection(data) {
   // We don't expect the experiments section to be always available.
   let experiments = data.experiments || {};
-  if (Object.keys(experiments).length == 0) {
+  if (!Object.keys(experiments).length) {
     return;
   }
 
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetrySession.js b/toolkit/components/telemetry/tests/unit/test_TelemetrySession.js
index 30a2ae3095e3..f13a2940bcdf 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetrySession.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetrySession.js
@@ -732,7 +732,7 @@ add_task(async function test_checkSubsessionScalars() {
   }
   // No scalar must be reported in classic pings (e.g. saved-session).
   Assert.ok(
-    Object.keys(classic.processes.parent.scalars).length == 0,
+    !Object.keys(classic.processes.parent.scalars).length,
     "Scalars must not be reported in a classic ping."
   );
 
diff --git a/toolkit/components/thumbnails/test/browser_thumbnails_bg_queueing.js b/toolkit/components/thumbnails/test/browser_thumbnails_bg_queueing.js
index 696b11b90de1..da68a688d636 100644
--- a/toolkit/components/thumbnails/test/browser_thumbnails_bg_queueing.js
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_bg_queueing.js
@@ -16,7 +16,7 @@ function* runTests() {
     BackgroundPageThumbs.capture(url, {
       timeout: isTimeoutTest ? 100 : 30000,
       onDone: function onDone(capturedURL) {
-        ok(urls.length > 0, "onDone called, so URLs should still remain");
+        ok(!!urls.length, "onDone called, so URLs should still remain");
         is(
           capturedURL,
           urls.shift(),
diff --git a/toolkit/components/tooltiptext/TooltipTextProvider.jsm b/toolkit/components/tooltiptext/TooltipTextProvider.jsm
index 63667e7871c5..30bb183c9123 100644
--- a/toolkit/components/tooltiptext/TooltipTextProvider.jsm
+++ b/toolkit/components/tooltiptext/TooltipTextProvider.jsm
@@ -65,7 +65,7 @@ TooltipTextProvider.prototype = {
         var bundle = Services.strings.createBundle(
           "chrome://global/locale/layout/HtmlForm.properties"
         );
-        if (files.length == 0) {
+        if (!files.length) {
           if (tipElement.multiple) {
             titleText = bundle.GetStringFromName("NoFilesSelected");
           } else {
diff --git a/toolkit/components/url-classifier/UrlClassifierHashCompleter.jsm b/toolkit/components/url-classifier/UrlClassifierHashCompleter.jsm
index f653fbd384bc..96028334fa55 100644
--- a/toolkit/components/url-classifier/UrlClassifierHashCompleter.jsm
+++ b/toolkit/components/url-classifier/UrlClassifierHashCompleter.jsm
@@ -292,7 +292,7 @@ HashCompleter.prototype = {
 
     // If we don't have an in-flight request, make one
     let pendingUrls = Object.keys(this._pendingRequests);
-    if (!this._currentRequest && pendingUrls.length > 0) {
+    if (!this._currentRequest && pendingUrls.length) {
       let nextUrl = pendingUrls[0];
       this._currentRequest = this._pendingRequests[nextUrl];
       delete this._pendingRequests[nextUrl];
diff --git a/toolkit/components/url-classifier/UrlClassifierListManager.jsm b/toolkit/components/url-classifier/UrlClassifierListManager.jsm
index 27bdfece85cc..c52a1abbb7e0 100644
--- a/toolkit/components/url-classifier/UrlClassifierListManager.jsm
+++ b/toolkit/components/url-classifier/UrlClassifierListManager.jsm
@@ -564,7 +564,7 @@ PROT_ListManager.prototype.makeUpdateRequest_ = function(updateUrl, tableData) {
   log("update request: " + JSON.stringify(streamerMap, undefined, 2) + "\n");
 
   // Don't send an empty request.
-  if (streamerMap.requestPayload.length > 0) {
+  if (streamerMap.requestPayload.length) {
     this.makeUpdateRequestForEntry_(
       updateUrl,
       streamerMap.tableList,
diff --git a/toolkit/components/url-classifier/tests/mochitest/test_classifier.html b/toolkit/components/url-classifier/tests/mochitest/test_classifier.html
index 6d5eba3dc291..13b74ebac56d 100644
--- a/toolkit/components/url-classifier/tests/mochitest/test_classifier.html
+++ b/toolkit/components/url-classifier/tests/mochitest/test_classifier.html
@@ -99,7 +99,7 @@ function testService() {
         is(errorCode, test.result,
            `Successful asynchronous classification of ${test.url}`);
         SpecialPowers.doUrlClassifyLocal(uri, tables, function(results) {
-          if (results.length == 0) {
+          if (!results.length) {
             is(test.table, "",
                `Successful asynchronous local classification of ${test.url}`);
           } else {
diff --git a/toolkit/components/url-classifier/tests/unit/head_urlclassifier.js b/toolkit/components/url-classifier/tests/unit/head_urlclassifier.js
index d2a6fa913725..98f429669064 100644
--- a/toolkit/components/url-classifier/tests/unit/head_urlclassifier.js
+++ b/toolkit/components/url-classifier/tests/unit/head_urlclassifier.js
@@ -268,7 +268,7 @@ var gAssertions = {
     // work with a copy of the list.
     urls = urls.slice(0);
     var doLookup = function() {
-      if (urls.length > 0) {
+      if (urls.length) {
         var tables = useMoz ? mozTables : allTables;
         var fragment = urls.shift();
         var principal = Services.scriptSecurityManager.createContentPrincipal(
@@ -382,7 +382,7 @@ function doUpdateTest(updates, assertions, successCallback, errorCallback) {
   };
 
   var runUpdate = function() {
-    if (updates.length > 0) {
+    if (updates.length) {
       var update = updates.shift();
       doStreamUpdate(update, runUpdate, errorUpdate, null);
     } else {
diff --git a/toolkit/components/xulstore/old/XULStore.jsm b/toolkit/components/xulstore/old/XULStore.jsm
index 62b8e3d91ece..7bdb63a0b662 100644
--- a/toolkit/components/xulstore/old/XULStore.jsm
+++ b/toolkit/components/xulstore/old/XULStore.jsm
@@ -254,10 +254,10 @@ XULStore.prototype = {
       if (attrs && attr in attrs) {
         delete attrs[attr];
 
-        if (Object.getOwnPropertyNames(attrs).length == 0) {
+        if (!Object.getOwnPropertyNames(attrs).length) {
           delete ids[id];
 
-          if (Object.getOwnPropertyNames(ids).length == 0) {
+          if (!Object.getOwnPropertyNames(ids).length) {
             delete this._data[docURI];
           }
         }
diff --git a/toolkit/content/aboutNetworking.js b/toolkit/content/aboutNetworking.js
index d5c1e25848e8..6b1feef9217b 100644
--- a/toolkit/content/aboutNetworking.js
+++ b/toolkit/content/aboutNetworking.js
@@ -302,7 +302,7 @@ function updateLogFile() {
 
   // If the log file was set from an env var, we disable the ability to set it
   // at runtime.
-  if (logPath.length > 0) {
+  if (logPath.length) {
     currentLogFile.innerText = logPath;
     setLogFileButton.disabled = true;
   } else {
@@ -319,7 +319,7 @@ function updateLogModules() {
     gEnv.get("NSPR_LOG_MODULES");
   let currentLogModules = document.getElementById("current-log-modules");
   let setLogModulesButton = document.getElementById("set-log-modules-button");
-  if (logModules.length > 0) {
+  if (logModules.length) {
     currentLogModules.innerText = logModules;
     // If the log modules are set by an environment variable at startup, do not
     // allow changing them throught a pref. It would be difficult to figure out
diff --git a/toolkit/content/aboutSupport.js b/toolkit/content/aboutSupport.js
index 0b93bfc46959..cb0f8bf708d2 100644
--- a/toolkit/content/aboutSupport.js
+++ b/toolkit/content/aboutSupport.js
@@ -642,7 +642,7 @@ var snapshotFormatters = {
         trs.push(buildRow(key, value));
       }
 
-      if (trs.length == 0) {
+      if (!trs.length) {
         $("graphics-" + id + "-tbody").style.display = "none";
         return;
       }
@@ -686,7 +686,7 @@ var snapshotFormatters = {
           }
 
           let contents;
-          if (entry.message.length > 0 && entry.message[0] == "#") {
+          if (entry.message.length && entry.message[0] == "#") {
             // This is a failure ID. See nsIGfxInfo.idl.
             let m = /#BLOCKLIST_FEATURE_FAILURE_BUG_(\d+)/.exec(entry.message);
             if (m) {
@@ -1327,7 +1327,7 @@ Serializer.prototype = {
         colHeadings[i] = this._nodeText(col).trim();
       }
     }
-    let hasColHeadings = Object.keys(colHeadings).length > 0;
+    let hasColHeadings = !!Object.keys(colHeadings).length;
     if (!hasColHeadings) {
       tableHeadingElem = null;
     }
diff --git a/toolkit/content/aboutTelemetry.js b/toolkit/content/aboutTelemetry.js
index 584972696ba7..177543682cf3 100644
--- a/toolkit/content/aboutTelemetry.js
+++ b/toolkit/content/aboutTelemetry.js
@@ -367,7 +367,7 @@ var PingPicker = {
     let sourceArchivedContainer = document.getElementById(
       "ping-source-archive-container"
     );
-    let archivedDisabled = archivedPingList.length == 0;
+    let archivedDisabled = !archivedPingList.length;
     sourceArchived.disabled = archivedDisabled;
     sourceArchivedContainer.classList.toggle("disabled", archivedDisabled);
 
@@ -753,7 +753,7 @@ var EnvironmentData = {
     );
     this.renderPersona(addons, addonSection, "persona");
 
-    let hasAddonData = Object.keys(ping.environment.addons).length > 0;
+    let hasAddonData = !!Object.keys(ping.environment.addons).length;
     let s = GenericSubsection.renderSubsectionHeader(
       "addons",
       hasAddonData,
@@ -981,7 +981,7 @@ var StackRenderer = {
       hideE.hidden = true;
     }
 
-    if (aStacks.length == 0) {
+    if (!aStacks.length) {
       return;
     }
 
@@ -1124,9 +1124,7 @@ var CapturedStacks = {
         ? payload.processes.parent.capturedStacks
         : false;
     let hasData =
-      capturedStacks &&
-      capturedStacks.stacks &&
-      capturedStacks.stacks.length > 0;
+      capturedStacks && capturedStacks.stacks && !!capturedStacks.stacks.length;
     setHasData("captured-stacks-section", hasData);
     if (!hasData) {
       return;
@@ -1584,7 +1582,7 @@ var Search = {
 function RenderObject(aObject) {
   let output = "";
   if (Array.isArray(aObject)) {
-    if (aObject.length == 0) {
+    if (!aObject.length) {
       return "[]";
     }
     output = "[" + JSON.stringify(aObject[0]);
@@ -1594,7 +1592,7 @@ function RenderObject(aObject) {
     return output + "]";
   }
   let keys = Object.keys(aObject);
-  if (keys.length == 0) {
+  if (!keys.length) {
     return "{}";
   }
   output = '{"' + keys[0] + '":\u00A0' + JSON.stringify(aObject[keys[0]]);
@@ -1763,7 +1761,7 @@ var AddonDetails = {
     let addonSection = document.getElementById("addon-details");
     removeAllChildNodes(addonSection);
     let addonDetails = aPing.payload.addonDetails;
-    const hasData = addonDetails && Object.keys(addonDetails).length > 0;
+    const hasData = addonDetails && !!Object.keys(addonDetails).length;
     setHasData("addon-details-section", hasData);
     if (!hasData) {
       return;
@@ -1792,7 +1790,7 @@ var AddonDetails = {
 
 class Section {
   static renderContent(data, process, div, section) {
-    if (data && Object.keys(data).length > 0) {
+    if (data && Object.keys(data).length) {
       let s = GenericSubsection.renderSubsectionHeader(process, true, section);
       let heading = document.createElement("h2");
       document.l10n.setAttributes(heading, "about-telemetry-process", {
@@ -1968,7 +1966,7 @@ var Events = {
       for (const process of Object.keys(aPayload.processes)) {
         let data = aPayload.processes[process].events;
         hasData = hasData || data !== {};
-        if (data && Object.keys(data).length > 0) {
+        if (data && Object.keys(data).length) {
           let s = GenericSubsection.renderSubsectionHeader(
             process,
             true,
@@ -1990,7 +1988,7 @@ var Events = {
       for (const process of Object.keys(aPayload.events)) {
         let data = process;
         hasData = hasData || data !== {};
-        if (data && Object.keys(data).length > 0) {
+        if (data && Object.keys(data).length) {
           let s = GenericSubsection.renderSubsectionHeader(
             process,
             true,
@@ -2498,7 +2496,7 @@ var SessionInformation = {
     let infoSection = document.getElementById("session-info");
     removeAllChildNodes(infoSection);
 
-    let hasData = Object.keys(aPayload.info).length > 0;
+    let hasData = !!Object.keys(aPayload.info).length;
     setHasData("session-info-section", hasData);
 
     if (hasData) {
@@ -2516,7 +2514,7 @@ var SimpleMeasurements = {
     let simpleMeasurements = this.sortStartupMilestones(
       aPayload.simpleMeasurements
     );
-    let hasData = Object.keys(simpleMeasurements).length > 0;
+    let hasData = !!Object.keys(simpleMeasurements).length;
     setHasData("simple-measurements-section", hasData);
 
     if (hasData) {
diff --git a/toolkit/content/aboutUrlClassifier.js b/toolkit/content/aboutUrlClassifier.js
index 133422edf2db..5996631dcd5f 100644
--- a/toolkit/content/aboutUrlClassifier.js
+++ b/toolkit/content/aboutUrlClassifier.js
@@ -656,7 +656,7 @@ var Debug = {
       env.get("MOZ_LOG_MODULES") ||
       env.get("NSPR_LOG_MODULES");
 
-    if (logModules.length > 0) {
+    if (logModules.length) {
       document.getElementById("set-log-modules").disabled = true;
       for (let module of this.modules) {
         document.getElementById("chk-" + module).disabled = true;
@@ -669,7 +669,7 @@ var Debug = {
     // Disable set log file if log file is already set
     // by environment variable.
     let logFile = env.get("MOZ_LOG_FILE") || env.get("NSPR_LOG_FILE");
-    if (logFile.length > 0) {
+    if (logFile.length) {
       document.getElementById("set-log-file").disabled = true;
       document.getElementById("log-file").value = logFile;
     }
diff --git a/toolkit/content/plugins.js b/toolkit/content/plugins.js
index 73c31f151b17..f48fb51fbdb1 100644
--- a/toolkit/content/plugins.js
+++ b/toolkit/content/plugins.js
@@ -22,7 +22,7 @@ RPMAddMessageListener("PluginList", function({ data: aPlugins }) {
 
   // "Installed plugins"
   var id, label;
-  if (aPlugins.length > 0) {
+  if (aPlugins.length) {
     id = "plugs";
     label = "installed-plugins-label";
   } else {
diff --git a/toolkit/content/tests/widgets/test_videocontrols.html b/toolkit/content/tests/widgets/test_videocontrols.html
index 043cced89973..48f3ac2df4f2 100644
--- a/toolkit/content/tests/widgets/test_videocontrols.html
+++ b/toolkit/content/tests/widgets/test_videocontrols.html
@@ -92,7 +92,7 @@ function verifyExpectedEvent(event) {
     expectingEventPromise.reject(new Error(`Got event: ${event.type}, expected: ${checkingEvent}`));
   }
 
-  if (expectingEvents.length == 0) {
+  if (!expectingEvents.length) {
     SimpleTest.executeSoon(expectingEventPromise.resolve);
   }
 }
diff --git a/toolkit/content/tests/widgets/tree_shared.js b/toolkit/content/tests/widgets/tree_shared.js
index 3c99bf143506..1ddd622a1fef 100644
--- a/toolkit/content/tests/widgets/tree_shared.js
+++ b/toolkit/content/tests/widgets/tree_shared.js
@@ -1312,7 +1312,7 @@ function testtag_tree_TreeView_rows(tree, testid, rowInfo, startRow) {
       return false;
     },
     isContainerEmpty(row) {
-      return row.children != null && row.children.rows.length == 0;
+      return row.children != null && !row.children.rows.length;
     },
     isSeparator(row) {
       return row.separator;
diff --git a/toolkit/content/treeUtils.js b/toolkit/content/treeUtils.js
index 5d45fa38e124..98673ee845bc 100644
--- a/toolkit/content/treeUtils.js
+++ b/toolkit/content/treeUtils.js
@@ -62,7 +62,7 @@ var gTreeUtils = {
     aLastSortAscending
   ) {
     var ascending = aColumn == aLastSortColumn ? !aLastSortAscending : true;
-    if (aDataSet.length == 0) {
+    if (!aDataSet.length) {
       return ascending;
     }
 
diff --git a/toolkit/content/widgets/marquee.js b/toolkit/content/widgets/marquee.js
index aee381301838..d5e2c806b7b3 100644
--- a/toolkit/content/widgets/marquee.js
+++ b/toolkit/content/widgets/marquee.js
@@ -379,7 +379,7 @@ this.MarqueeBaseImplWidget = class {
   }
 
   _mutationActor(aMutations) {
-    while (aMutations.length > 0) {
+    while (aMutations.length) {
       var mutation = aMutations.shift();
       var attrName = mutation.attributeName.toLowerCase();
       var oldValue = mutation.oldValue;
diff --git a/toolkit/content/widgets/richlistbox.js b/toolkit/content/widgets/richlistbox.js
index 22f6780e7441..994a58521948 100644
--- a/toolkit/content/widgets/richlistbox.js
+++ b/toolkit/content/widgets/richlistbox.js
@@ -176,7 +176,7 @@
       this.selectItem(val);
     }
     get selectedItem() {
-      return this.selectedItems.length > 0 ? this.selectedItems[0] : null;
+      return this.selectedItems.length ? this.selectedItems[0] : null;
     }
 
     // nsIDOMXULSelectControlElement
@@ -197,7 +197,7 @@
       }
     }
     get selectedIndex() {
-      if (this.selectedItems.length > 0) {
+      if (this.selectedItems.length) {
         return this.getIndexOfItem(this.selectedItems[0]);
       }
       return -1;
@@ -212,7 +212,7 @@
       return val;
     }
     get value() {
-      if (this.selectedItems.length > 0) {
+      if (this.selectedItems.length) {
         return this.selectedItem.value;
       }
       return null;
@@ -573,7 +573,7 @@
     // nsIDOMXULMultiSelectControlElement
     clearSelection() {
       if (this.selectedItems) {
-        while (this.selectedItems.length > 0) {
+        while (this.selectedItems.length) {
           let item = this.selectedItems[0];
           item.selected = false;
           this.selectedItems.remove(item);
@@ -642,7 +642,7 @@
     scrollOnePage(aDirection) {
       var children = this.itemChildren;
 
-      if (children.length == 0) {
+      if (!children.length) {
         return 0;
       }
 
diff --git a/toolkit/content/widgets/toolbarbutton.js b/toolkit/content/widgets/toolbarbutton.js
index 653d28547a86..cc7ddab2fd8a 100644
--- a/toolkit/content/widgets/toolbarbutton.js
+++ b/toolkit/content/widgets/toolbarbutton.js
@@ -109,7 +109,7 @@
 
         this.appendChild(this.constructor.badgedFragment.cloneNode(true));
 
-        if (moveChildren.length > 0) {
+        if (moveChildren.length) {
           let { badgeStack, icon } = this;
           for (let child of moveChildren) {
             badgeStack.insertBefore(child, icon);
diff --git a/toolkit/crashreporter/CrashSubmit.jsm b/toolkit/crashreporter/CrashSubmit.jsm
index 8d424938cb01..dcdb8c104d42 100644
--- a/toolkit/crashreporter/CrashSubmit.jsm
+++ b/toolkit/crashreporter/CrashSubmit.jsm
@@ -272,7 +272,7 @@ Submitter.prototype = {
       );
     }
 
-    if (this.additionalDumps.length > 0) {
+    if (this.additionalDumps.length) {
       let names = [];
       for (let i of this.additionalDumps) {
         names.push(i.name);
diff --git a/toolkit/modules/ActorManagerChild.jsm b/toolkit/modules/ActorManagerChild.jsm
index 649e2daeb2b1..ebe9d09c8aef 100644
--- a/toolkit/modules/ActorManagerChild.jsm
+++ b/toolkit/modules/ActorManagerChild.jsm
@@ -110,7 +110,7 @@ class Dispatcher {
       this.messages.set(msg, actors);
     }
 
-    if (actors.length == 0) {
+    if (!actors.length) {
       this.mm.addMessageListener(msg, this);
     }
 
diff --git a/toolkit/modules/AppMenuNotifications.jsm b/toolkit/modules/AppMenuNotifications.jsm
index f18963e87baa..7b67ab49251c 100644
--- a/toolkit/modules/AppMenuNotifications.jsm
+++ b/toolkit/modules/AppMenuNotifications.jsm
@@ -42,7 +42,7 @@ var AppMenuNotifications = {
         this.uninit();
         break;
       case "appMenu-notifications-request":
-        if (this._notifications.length != 0) {
+        if (this._notifications.length) {
           Services.obs.notifyObservers(null, "appMenu-notifications", "init");
         }
         break;
@@ -50,7 +50,7 @@ var AppMenuNotifications = {
   },
 
   get activeNotification() {
-    if (this._notifications.length > 0) {
+    if (this._notifications.length) {
       const doorhanger = this._notifications.find(
         n => !n.dismissed && !n.options.badgeOnly
       );
diff --git a/toolkit/modules/CertUtils.jsm b/toolkit/modules/CertUtils.jsm
index bf76fbd930e7..69f6cbf3a52b 100644
--- a/toolkit/modules/CertUtils.jsm
+++ b/toolkit/modules/CertUtils.jsm
@@ -27,7 +27,7 @@ const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
  *         expected certificate's attribute names / values.
  */
 function readCertPrefs(aPrefBranch) {
-  if (Services.prefs.getBranch(aPrefBranch).getChildList("").length == 0) {
+  if (!Services.prefs.getBranch(aPrefBranch).getChildList("").length) {
     return null;
   }
 
@@ -36,7 +36,7 @@ function readCertPrefs(aPrefBranch) {
   while (true) {
     let prefBranchCert = Services.prefs.getBranch(aPrefBranch + counter + ".");
     let prefCertAttrs = prefBranchCert.getChildList("");
-    if (prefCertAttrs.length == 0) {
+    if (!prefCertAttrs.length) {
       break;
     }
 
@@ -70,7 +70,7 @@ function readCertPrefs(aPrefBranch) {
  */
 function validateCert(aCertificate, aCerts) {
   // If there are no certificate requirements then just exit
-  if (!aCerts || aCerts.length == 0) {
+  if (!aCerts || !aCerts.length) {
     return;
   }
 
diff --git a/toolkit/modules/Console.jsm b/toolkit/modules/Console.jsm
index 0c9a1cbbdfcc..220009464541 100644
--- a/toolkit/modules/Console.jsm
+++ b/toolkit/modules/Console.jsm
@@ -233,7 +233,7 @@ function log(aThing) {
       reply += "  " + debugElement(aThing) + "\n";
     } else {
       let keys = Object.getOwnPropertyNames(aThing);
-      if (keys.length > 0) {
+      if (keys.length) {
         reply += type + "\n";
         keys.forEach(function(aProp) {
           reply += logProperty(aProp, aThing[aProp]);
diff --git a/toolkit/modules/InlineSpellChecker.jsm b/toolkit/modules/InlineSpellChecker.jsm
index 19e18669af78..23dff64056a7 100644
--- a/toolkit/modules/InlineSpellChecker.jsm
+++ b/toolkit/modules/InlineSpellChecker.jsm
@@ -338,7 +338,7 @@ InlineSpellChecker.prototype = {
   },
   // callback for removing the last added word to the dictionary LIFO fashion
   undoAddToDictionary() {
-    if (this.mAddedWordStack.length > 0) {
+    if (this.mAddedWordStack.length) {
       var word = this.mAddedWordStack.pop();
       if (this.mRemote) {
         this.mRemote.undoAddToDictionary(word);
@@ -349,7 +349,7 @@ InlineSpellChecker.prototype = {
   },
   canUndo() {
     // Return true if we have words on the stack
-    return this.mAddedWordStack.length > 0;
+    return !!this.mAddedWordStack.length;
   },
   ignoreWord() {
     if (this.mRemote) {
diff --git a/toolkit/modules/NewTabUtils.jsm b/toolkit/modules/NewTabUtils.jsm
index 749a6ee3411a..63802928b6a0 100644
--- a/toolkit/modules/NewTabUtils.jsm
+++ b/toolkit/modules/NewTabUtils.jsm
@@ -542,7 +542,7 @@ var BlockedLinks = {
    * @return Whether the list is empty.
    */
   isEmpty: function BlockedLinks_isEmpty() {
-    return Object.keys(this.links).length == 0;
+    return !Object.keys(this.links).length;
   },
 
   /**
diff --git a/toolkit/modules/PopupNotifications.jsm b/toolkit/modules/PopupNotifications.jsm
index fa1f599eb817..59ddfdeb929b 100644
--- a/toolkit/modules/PopupNotifications.jsm
+++ b/toolkit/modules/PopupNotifications.jsm
@@ -1074,7 +1074,7 @@ PopupNotifications.prototype = {
       popupnotification.notification = n;
       let menuitems = [];
 
-      if (n.mainAction && n.secondaryActions && n.secondaryActions.length > 0) {
+      if (n.mainAction && n.secondaryActions && n.secondaryActions.length) {
         let telemetryStatId = TELEMETRY_STAT_ACTION_2;
 
         let secondaryAction = n.secondaryActions[0];
@@ -1334,7 +1334,7 @@ PopupNotifications.prototype = {
       notifications = this._currentNotifications;
     }
 
-    let haveNotifications = notifications.length > 0;
+    let haveNotifications = !!notifications.length;
     if (!anchors.size && haveNotifications) {
       anchors = this._getAnchorsForNotifications(notifications);
     }
@@ -1381,7 +1381,7 @@ PopupNotifications.prototype = {
       }
     }
 
-    if (notificationsToShow.length > 0) {
+    if (notificationsToShow.length) {
       let anchorElement = anchors.values().next().value;
       if (anchorElement) {
         this._showPanel(notificationsToShow, anchorElement);
@@ -1443,7 +1443,7 @@ PopupNotifications.prototype = {
           /([-\w]+-notification-icon\s?)/g,
           ""
         );
-        if (notifications.length > 0) {
+        if (notifications.length) {
           // Find the first notification this anchor used for.
           let notification = notifications[0];
           for (let n of notifications) {
@@ -1555,7 +1555,7 @@ PopupNotifications.prototype = {
       return;
     }
 
-    if (this._currentNotifications.length == 0) {
+    if (!this._currentNotifications.length) {
       return;
     }
 
@@ -1627,7 +1627,7 @@ PopupNotifications.prototype = {
     let ourNotifications = this._getNotificationsForBrowser(ourBrowser);
     let other = otherBrowser.ownerGlobal.PopupNotifications;
     if (!other) {
-      if (ourNotifications.length > 0) {
+      if (ourNotifications.length) {
         Cu.reportError(
           "unable to swap notifications: otherBrowser doesn't support notifications"
         );
@@ -1667,10 +1667,10 @@ PopupNotifications.prototype = {
     this._setNotificationsForBrowser(otherBrowser, ourNotifications);
     other._setNotificationsForBrowser(ourBrowser, otherNotifications);
 
-    if (otherNotifications.length > 0) {
+    if (otherNotifications.length) {
       this._update(otherNotifications);
     }
-    if (ourNotifications.length > 0) {
+    if (ourNotifications.length) {
       other._update(ourNotifications);
     }
   },
diff --git a/toolkit/modules/Promise-backend.js b/toolkit/modules/Promise-backend.js
index 1bbb66139103..1bcfd68f5491 100644
--- a/toolkit/modules/Promise-backend.js
+++ b/toolkit/modules/Promise-backend.js
@@ -737,7 +737,7 @@ this.PromiseWalker = {
     // Change the promise status and schedule our handlers for processing.
     aPromise[N_INTERNALS].status = aStatus;
     aPromise[N_INTERNALS].value = aValue;
-    if (aPromise[N_INTERNALS].handlers.length > 0) {
+    if (aPromise[N_INTERNALS].handlers.length) {
       this.schedulePromise(aPromise);
     } else if (Cu && aStatus == STATUS_REJECTED) {
       // This is a rejection and the promise is the last in the chain.
@@ -841,7 +841,7 @@ this.PromiseWalker = {
     }
 
     // Process all the known handlers eagerly.
-    while (this.handlers.length > 0) {
+    while (this.handlers.length) {
       this.handlers.shift().process();
     }
   },
diff --git a/toolkit/modules/ShortcutUtils.jsm b/toolkit/modules/ShortcutUtils.jsm
index a769aa5cf2b8..549075be0149 100644
--- a/toolkit/modules/ShortcutUtils.jsm
+++ b/toolkit/modules/ShortcutUtils.jsm
@@ -292,7 +292,7 @@ var ShortcutUtils = {
     let keycode = this.getKeycodeAttribute(chromeKey);
 
     let baseSelector = "key";
-    if (modifiers.length > 0) {
+    if (modifiers.length) {
       baseSelector += `[modifiers="${modifiersString}"]`;
     }
 
diff --git a/toolkit/modules/Sqlite.jsm b/toolkit/modules/Sqlite.jsm
index ab311e1e9d6c..a659cd4da76b 100644
--- a/toolkit/modules/Sqlite.jsm
+++ b/toolkit/modules/Sqlite.jsm
@@ -1584,7 +1584,7 @@ OpenedConnection.prototype = Object.freeze({
         "WHERE type = 'table' AND name=?",
       [name]
     ).then(function onResult(rows) {
-      return Promise.resolve(rows.length > 0);
+      return Promise.resolve(!!rows.length);
     });
   },
 
@@ -1603,7 +1603,7 @@ OpenedConnection.prototype = Object.freeze({
         "WHERE type = 'index' AND name=?",
       [name]
     ).then(function onResult(rows) {
-      return Promise.resolve(rows.length > 0);
+      return Promise.resolve(!!rows.length);
     });
   },
 
diff --git a/toolkit/modules/Troubleshoot.jsm b/toolkit/modules/Troubleshoot.jsm
index 3e855e9d4845..0dffa0c138e9 100644
--- a/toolkit/modules/Troubleshoot.jsm
+++ b/toolkit/modules/Troubleshoot.jsm
@@ -247,20 +247,20 @@ var dataProviders = {
       .trim();
     data.keyLocationServiceGoogleFound =
       keyLocationServiceGoogle != "no-google-location-service-api-key" &&
-      keyLocationServiceGoogle.length > 0;
+      !!keyLocationServiceGoogle.length;
 
     const keySafebrowsingGoogle = Services.urlFormatter
       .formatURL("%GOOGLE_SAFEBROWSING_API_KEY%")
       .trim();
     data.keySafebrowsingGoogleFound =
       keySafebrowsingGoogle != "no-google-safebrowsing-api-key" &&
-      keySafebrowsingGoogle.length > 0;
+      !!keySafebrowsingGoogle.length;
 
     const keyMozilla = Services.urlFormatter
       .formatURL("%MOZILLA_API_KEY%")
       .trim();
     data.keyMozillaFound =
-      keyMozilla != "no-mozilla-api-key" && keyMozilla.length > 0;
+      keyMozilla != "no-mozilla-api-key" && !!keyMozilla.length;
 
     done(data);
   },
diff --git a/toolkit/modules/sessionstore/SessionHistory.jsm b/toolkit/modules/sessionstore/SessionHistory.jsm
index 27300d37cbae..2e06fb07bd86 100644
--- a/toolkit/modules/sessionstore/SessionHistory.jsm
+++ b/toolkit/modules/sessionstore/SessionHistory.jsm
@@ -115,10 +115,7 @@ var SessionHistoryInternal = {
     // If either the session history isn't available yet or doesn't have any
     // valid entries, make sure we at least include the current page,
     // unless of course we just skipped all entries because aFromIdx was big enough.
-    if (
-      data.entries.length == 0 &&
-      (skippedCount != entryCount || aFromIdx < 0)
-    ) {
+    if (!data.entries.length && (skippedCount != entryCount || aFromIdx < 0)) {
       let uri = webNavigation.currentURI.displaySpec;
       let body = webNavigation.document.body;
       // We landed here because the history is inaccessible or there are no
@@ -233,7 +230,7 @@ var SessionHistoryInternal = {
               Object.getOwnPropertyNames(presState).length > 1
           );
 
-        if (presStates.length > 0) {
+        if (presStates.length) {
           entry.presState = presStates;
         }
       }
diff --git a/toolkit/modules/subprocess/subprocess_worker_common.js b/toolkit/modules/subprocess/subprocess_worker_common.js
index 7e5bde793d9e..3dbc420c7c33 100644
--- a/toolkit/modules/subprocess/subprocess_worker_common.js
+++ b/toolkit/modules/subprocess/subprocess_worker_common.js
@@ -30,7 +30,7 @@ class BasePipe {
   shiftPending() {
     let result = this.pending.shift();
 
-    if (this.closing && this.pending.length == 0) {
+    if (this.closing && !this.pending.length) {
       this.close();
     }
 
diff --git a/toolkit/modules/subprocess/subprocess_worker_unix.js b/toolkit/modules/subprocess/subprocess_worker_unix.js
index f22fdd4944d6..946655917745 100644
--- a/toolkit/modules/subprocess/subprocess_worker_unix.js
+++ b/toolkit/modules/subprocess/subprocess_worker_unix.js
@@ -165,7 +165,7 @@ class InputPipe extends Pipe {
       }
     }
 
-    if (reads.length == 0) {
+    if (!reads.length) {
       io.updatePollFds();
     }
     return result;
@@ -250,7 +250,7 @@ class OutputPipe extends Pipe {
       }
     }
 
-    if (writes.length == 0) {
+    if (!writes.length) {
       io.updatePollFds();
     }
   }
diff --git a/toolkit/modules/tests/chrome/test_findCssSelector.html b/toolkit/modules/tests/chrome/test_findCssSelector.html
index 2ad8a6fd2c57..93b66601edf8 100644
--- a/toolkit/modules/tests/chrome/test_findCssSelector.html
+++ b/toolkit/modules/tests/chrome/test_findCssSelector.html
@@ -20,7 +20,7 @@ function addTest(test) {
 }
 
 function runNextTest() {
-  if (_tests.length == 0) {
+  if (!_tests.length) {
     SimpleTest.finish();
     return;
   }
diff --git a/toolkit/modules/tests/modules/PromiseTestUtils.jsm b/toolkit/modules/tests/modules/PromiseTestUtils.jsm
index 556b09fea343..be8e00816efc 100644
--- a/toolkit/modules/tests/modules/PromiseTestUtils.jsm
+++ b/toolkit/modules/tests/modules/PromiseTestUtils.jsm
@@ -238,7 +238,7 @@ var PromiseTestUtils = {
     JSMPromise.Debugging.flushUncaughtErrors();
 
     // If there is any uncaught rejection left at this point, the test fails.
-    while (this._rejections.length > 0) {
+    while (this._rejections.length) {
       let rejection = this._rejections.shift();
 
       // If one of the ignore functions matches, ignore the rejection, then
@@ -277,7 +277,7 @@ var PromiseTestUtils = {
    */
   assertNoMoreExpectedRejections() {
     // Only log this condition is there is a failure.
-    if (this._rejectionIgnoreFns.length > 0) {
+    if (this._rejectionIgnoreFns.length) {
       Assert.equal(
         this._rejectionIgnoreFns.length,
         0,
diff --git a/toolkit/modules/tests/xpcshell/test_Promise.js b/toolkit/modules/tests/xpcshell/test_Promise.js
index cd56531bddb9..8d5b00df3939 100644
--- a/toolkit/modules/tests/xpcshell/test_Promise.js
+++ b/toolkit/modules/tests/xpcshell/test_Promise.js
@@ -722,7 +722,7 @@ tests.push(
     }
 
     let p1 = Promise.all([]).then(function onResolve(val) {
-      Assert.ok(Array.isArray(val) && val.length == 0);
+      Assert.ok(Array.isArray(val) && !val.length);
     });
 
     let p2 = Promise.all([1, 2, 3]).then(function onResolve([
diff --git a/toolkit/mozapps/downloads/DownloadUtils.jsm b/toolkit/mozapps/downloads/DownloadUtils.jsm
index 9b0e3d65fa06..1ad264062bb2 100644
--- a/toolkit/mozapps/downloads/DownloadUtils.jsm
+++ b/toolkit/mozapps/downloads/DownloadUtils.jsm
@@ -453,7 +453,7 @@ var DownloadUtils = {
       // Display special text for file protocol
       displayHost = gBundle.GetStringFromName(gStr.doneFileScheme);
       fullHost = displayHost;
-    } else if (displayHost.length == 0) {
+    } else if (!displayHost.length) {
       // Got nothing; show the scheme (data: about: moz-icon:)
       displayHost = gBundle.formatStringFromName(gStr.doneScheme, [uri.scheme]);
       fullHost = displayHost;
diff --git a/toolkit/mozapps/extensions/AddonManager.jsm b/toolkit/mozapps/extensions/AddonManager.jsm
index 37535e84a35d..2d963421aa31 100644
--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -987,7 +987,7 @@ var AddonManagerInternal = {
       this.types[type].providers = this.types[type].providers.filter(
         p => p != aProvider
       );
-      if (this.types[type].providers.length == 0) {
+      if (!this.types[type].providers.length) {
         let oldType = this.types[type].type;
         delete this.types[type];
 
@@ -1366,7 +1366,7 @@ var AddonManagerInternal = {
     let difference = Extension.comparePermissions(oldPerms, newPerms);
 
     // If there are no new permissions, just go ahead with the update
-    if (difference.origins.length == 0 && difference.permissions.length == 0) {
+    if (!difference.origins.length && !difference.permissions.length) {
       return Promise.resolve();
     }
 
@@ -4614,7 +4614,7 @@ AMTelemetry = {
 
     extra = { ...extraVars, ...extra };
 
-    let hasExtraVars = Object.keys(extra).length > 0;
+    let hasExtraVars = !!Object.keys(extra).length;
     extra = this.formatExtraVars(extra);
 
     this.recordEvent({
diff --git a/toolkit/mozapps/extensions/Blocklist.jsm b/toolkit/mozapps/extensions/Blocklist.jsm
index 2c3eb6db44a0..64f8096d9135 100644
--- a/toolkit/mozapps/extensions/Blocklist.jsm
+++ b/toolkit/mozapps/extensions/Blocklist.jsm
@@ -466,12 +466,12 @@ async function targetAppFilter(entry, environment) {
 
   // Iterate the targeted applications, at least one of them must match.
   // If no target application, keep the entry.
-  if (versionRange.length == 0) {
+  if (!versionRange.length) {
     return entry;
   }
   for (const vr of versionRange) {
     const { targetApplication = [] } = vr;
-    if (targetApplication.length == 0) {
+    if (!targetApplication.length) {
       return entry;
     }
     for (const ta of targetApplication) {
@@ -1525,7 +1525,7 @@ function matchesOSABI(blocklistElement) {
   let os = blocklistElement.getAttribute("os");
   if (os) {
     let choices = os.split(",");
-    if (choices.length > 0 && !choices.includes(gApp.OS)) {
+    if (choices.length && !choices.includes(gApp.OS)) {
       return false;
     }
   }
@@ -1533,7 +1533,7 @@ function matchesOSABI(blocklistElement) {
   let xpcomabi = blocklistElement.getAttribute("xpcomabi");
   if (xpcomabi) {
     let choices = xpcomabi.split(",");
-    if (choices.length > 0 && !choices.includes(gApp.XPCOMABI)) {
+    if (choices.length && !choices.includes(gApp.XPCOMABI)) {
       return false;
     }
   }
@@ -2257,7 +2257,7 @@ var BlocklistXML = {
             );
         }
       }
-      if (this._gfxEntries.length > 0) {
+      if (this._gfxEntries.length) {
         this._notifyObserversBlocklistGFX();
       }
     } catch (e) {
@@ -2337,7 +2337,7 @@ var BlocklistXML = {
     }
     // if only the extension ID is specified block all versions of the
     // extension for the current application.
-    if (blockEntry.versions.length == 0) {
+    if (!blockEntry.versions.length) {
       blockEntry.versions.push(new BlocklistItemData(null));
     }
 
@@ -2384,7 +2384,7 @@ var BlocklistXML = {
       return;
     }
     // Add a default versionRange if there wasn't one specified
-    if (blockEntry.versions.length == 0) {
+    if (!blockEntry.versions.length) {
       blockEntry.versions.push(new BlocklistItemData(null));
     }
 
@@ -2796,7 +2796,7 @@ var BlocklistXML = {
       }
     }
 
-    if (addonList.length == 0) {
+    if (!addonList.length) {
       this._notifyObserversBlocklistUpdated();
       return;
     }
@@ -3026,7 +3026,7 @@ BlocklistItemData.prototype = {
     }
     // return minVersion = null and maxVersion = null if no specific versionRange
     // elements were found
-    if (appVersions.length == 0) {
+    if (!appVersions.length) {
       appVersions.push({ minVersion: null, maxVersion: null });
     }
     return appVersions;
diff --git a/toolkit/mozapps/extensions/content/aboutaddons.js b/toolkit/mozapps/extensions/content/aboutaddons.js
index 652c2664ded8..1265bc2e19c2 100644
--- a/toolkit/mozapps/extensions/content/aboutaddons.js
+++ b/toolkit/mozapps/extensions/content/aboutaddons.js
@@ -852,7 +852,7 @@ customElements.define("panel-item", PanelItem);
 
 class AddonOptions extends HTMLElement {
   connectedCallback() {
-    if (this.children.length == 0) {
+    if (!this.children.length) {
       this.render();
     }
   }
@@ -1035,7 +1035,7 @@ customElements.define("five-star-rating", FiveStarRating);
 
 class ContentSelectDropdown extends HTMLElement {
   connectedCallback() {
-    if (this.children.length > 0) {
+    if (this.children.length) {
       return;
     }
     // This creates the menulist and menupopup elements needed for the inline
@@ -1332,7 +1332,7 @@ class AddonPermissionsList extends HTMLElement {
 
     this.textContent = "";
 
-    if (msgs.length > 0) {
+    if (msgs.length) {
       // Add a row for each permission message.
       for (let msg of msgs) {
         let row = document.createElement("div");
@@ -1363,7 +1363,7 @@ customElements.define("addon-permissions-list", AddonPermissionsList);
 
 class AddonDetails extends HTMLElement {
   connectedCallback() {
-    if (this.children.length == 0) {
+    if (!this.children.length) {
       this.render();
     }
     this.deck.addEventListener("view-changed", this);
@@ -1628,7 +1628,7 @@ customElements.define("addon-details", AddonDetails);
 class AddonCard extends HTMLElement {
   connectedCallback() {
     // If we've already rendered we can just update, otherwise render.
-    if (this.children.length > 0) {
+    if (this.children.length) {
       this.update();
     } else {
       this.render();
@@ -1658,7 +1658,7 @@ class AddonCard extends HTMLElement {
 
   set updateInstall(install) {
     this._updateInstall = install;
-    if (this.children.length > 0) {
+    if (this.children.length) {
       this.update();
     }
   }
@@ -1683,7 +1683,7 @@ class AddonCard extends HTMLElement {
     if (install && install.state == AddonManager.STATE_AVAILABLE) {
       this.updateInstall = install;
     }
-    if (this.children.length > 0) {
+    if (this.children.length) {
       this.render();
     }
   }
@@ -2067,7 +2067,7 @@ class AddonCard extends HTMLElement {
   }
 
   expand() {
-    if (this.children.length == 0) {
+    if (!this.children.length) {
       this.expanded = true;
     } else {
       throw new Error("expand() is only supported before render()");
@@ -2379,7 +2379,7 @@ class AddonList extends HTMLElement {
     // happpen as close to each other as possible.
     this.registerListener();
     // Don't render again if we were rendered prior to being inserted.
-    if (this.children.length == 0) {
+    if (!this.children.length) {
       // Render the initial view.
       this.render();
     }
@@ -2545,7 +2545,7 @@ class AddonList extends HTMLElement {
     let sectionCards = this.getCards(section);
 
     // If this is the first card in the section, create the heading.
-    if (sectionCards.length == 0) {
+    if (!sectionCards.length) {
       section.appendChild(this.createSectionHeading(sectionIndex));
     }
 
@@ -2621,7 +2621,7 @@ class AddonList extends HTMLElement {
     section.setAttribute("section", index);
 
     // Render the heading and add-ons if there are any.
-    if (addons.length > 0) {
+    if (addons.length) {
       section.appendChild(this.createSectionHeading(index));
 
       for (let addon of addons) {
diff --git a/toolkit/mozapps/extensions/content/aboutaddonsCommon.js b/toolkit/mozapps/extensions/content/aboutaddonsCommon.js
index 0c1e9cf838d8..7263cf74536d 100644
--- a/toolkit/mozapps/extensions/content/aboutaddonsCommon.js
+++ b/toolkit/mozapps/extensions/content/aboutaddonsCommon.js
@@ -59,7 +59,7 @@ function attachUpdateHandler(install) {
     let difference = Extension.comparePermissions(oldPerms, newPerms);
 
     // If there are no new permissions, just proceed
-    if (difference.origins.length == 0 && difference.permissions.length == 0) {
+    if (!difference.origins.length && !difference.permissions.length) {
       return Promise.resolve();
     }
 
@@ -128,7 +128,7 @@ function shouldShowPermissionsPrompt(addon) {
   }
 
   const { origins, permissions } = addon.userPermissions;
-  return origins.length > 0 || permissions.length > 0;
+  return !!origins.length || !!permissions.length;
 }
 
 function showPermissionsPrompt(addon) {
diff --git a/toolkit/mozapps/extensions/content/extensions.js b/toolkit/mozapps/extensions/content/extensions.js
index a95138c26bcd..5f5af45ef270 100644
--- a/toolkit/mozapps/extensions/content/extensions.js
+++ b/toolkit/mozapps/extensions/content/extensions.js
@@ -189,7 +189,7 @@ function initialize(event) {
   // Allow passing in a view through the window arguments
   if (
     "arguments" in window &&
-    window.arguments.length > 0 &&
+    !!window.arguments.length &&
     window.arguments[0] !== null &&
     "view" in window.arguments[0]
   ) {
@@ -1466,7 +1466,7 @@ var gCategories = {
 
       gPendingInitializations++;
       getAddonsAndInstalls(aType.id, (aAddonsList, aInstallsList) => {
-        var hidden = aAddonsList.length == 0 && aInstallsList.length == 0;
+        var hidden = !aAddonsList.length && !aInstallsList.length;
         var item = this.get(aViewId);
 
         // Don't load view that is becoming hidden
@@ -1477,7 +1477,7 @@ var gCategories = {
         item.hidden = hidden;
         Services.prefs.setBoolPref(prefName, hidden);
 
-        if (aAddonsList.length > 0 || aInstallsList.length > 0) {
+        if (aAddonsList.length || aInstallsList.length) {
           notifyInitialized();
           return;
         }
@@ -1586,7 +1586,7 @@ var gHeader = {
 
     this._search.addEventListener("command", function(aEvent) {
       var query = aEvent.target.value;
-      if (query.length == 0) {
+      if (!query.length) {
         return;
       }
 
diff --git a/toolkit/mozapps/extensions/content/shortcuts.js b/toolkit/mozapps/extensions/content/shortcuts.js
index b58b9e513f6c..3c75a4283f73 100644
--- a/toolkit/mozapps/extensions/content/shortcuts.js
+++ b/toolkit/mozapps/extensions/content/shortcuts.js
@@ -201,7 +201,7 @@ XPCOMUtils.defineLazyModuleGetters(this, {
     let modifiers = shortcut.split("+");
     let key = modifiers.pop();
 
-    if (modifiers.length > 0) {
+    if (modifiers.length) {
       let modifiersAttribute = ShortcutUtils.getModifiersAttribute(modifiers);
       let displayString =
         ShortcutUtils.getModifierString(modifiersAttribute) + key;
@@ -404,7 +404,7 @@ XPCOMUtils.defineLazyModuleGetters(this, {
     let shortcutString = getShortcutForEvent(e);
     input.value = getShortcutValue(shortcutString);
 
-    if (e.type == "keyup" || shortcutString.length == 0) {
+    if (e.type == "keyup" || !shortcutString.length) {
       return;
     }
 
@@ -608,7 +608,7 @@ XPCOMUtils.defineLazyModuleGetters(this, {
       }
     }
 
-    if (noShortcutAddons.length > 0) {
+    if (noShortcutAddons.length) {
       frag.appendChild(renderNoShortcutAddons(noShortcutAddons));
     }
 
@@ -632,7 +632,7 @@ XPCOMUtils.defineLazyModuleGetters(this, {
         .sort((a, b) => a.name.localeCompare(b.name));
       let frag;
 
-      if (addons.length > 0) {
+      if (addons.length) {
         frag = await renderAddons(addons);
       } else {
         frag = document.importNode(templates.noAddons.content, true);
diff --git a/toolkit/mozapps/extensions/internal/AddonRepository.jsm b/toolkit/mozapps/extensions/internal/AddonRepository.jsm
index 56e5e2766b4f..14c30d110e0c 100644
--- a/toolkit/mozapps/extensions/internal/AddonRepository.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonRepository.jsm
@@ -576,7 +576,7 @@ var AddonRepository = {
     let ids = await getAddonsToCache(aIds);
 
     // If there are no add-ons to cache, act as if caching is disabled
-    if (ids.length == 0) {
+    if (!ids.length) {
       return [];
     }
 
@@ -607,7 +607,7 @@ var AddonRepository = {
     let addonsToCache = await getAddonsToCache(ids);
 
     // Completely remove cache if there are no add-ons to cache
-    if (addonsToCache.length == 0) {
+    if (!addonsToCache.length) {
       logger.debug("Clearing cache because 0 add-ons were requested");
       await this._clearCache();
       return;
@@ -672,7 +672,7 @@ var AddonRepository = {
       let authors = aEntry.authors.map(
         author => new AddonManagerPrivate.AddonAuthor(author.name, author.url)
       );
-      if (authors.length > 0) {
+      if (authors.length) {
         addon.creator = authors[0];
         addon.developers = authors.slice(1);
       }
diff --git a/toolkit/mozapps/extensions/internal/GMPProvider.jsm b/toolkit/mozapps/extensions/internal/GMPProvider.jsm
index 8eed92a8bad8..7a7fecbbf2f3 100644
--- a/toolkit/mozapps/extensions/internal/GMPProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/GMPProvider.jsm
@@ -447,7 +447,7 @@ GMPWrapper.prototype = {
   },
 
   get isInstalled() {
-    return this.version && this.version.length > 0;
+    return this.version && !!this.version.length;
   },
 
   _handleEnabledChanged() {
diff --git a/toolkit/mozapps/extensions/internal/XPIDatabase.jsm b/toolkit/mozapps/extensions/internal/XPIDatabase.jsm
index c1f9b04d1312..0a6172eaafab 100644
--- a/toolkit/mozapps/extensions/internal/XPIDatabase.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIDatabase.jsm
@@ -458,7 +458,7 @@ class AddonInternal {
   }
 
   get isPlatformCompatible() {
-    if (this.targetPlatforms.length == 0) {
+    if (!this.targetPlatforms.length) {
       return true;
     }
 
@@ -901,7 +901,7 @@ AddonWrapper = class {
     let repositoryAddon = addon._repositoryAddon;
     if (repositoryAddon && "screenshots" in repositoryAddon) {
       let repositoryScreenshots = repositoryAddon.screenshots;
-      if (repositoryScreenshots && repositoryScreenshots.length > 0) {
+      if (repositoryScreenshots && repositoryScreenshots.length) {
         return repositoryScreenshots;
       }
     }
diff --git a/toolkit/mozapps/extensions/internal/XPIInstall.jsm b/toolkit/mozapps/extensions/internal/XPIInstall.jsm
index a19a362f91d9..69921a92bcb2 100644
--- a/toolkit/mozapps/extensions/internal/XPIInstall.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIInstall.jsm
@@ -456,10 +456,11 @@ async function loadManifestFromWebManifest(aPackage) {
 
   // Read the list of available locales, and pre-load messages for
   // all locales.
-  let locales =
-    extension.errors.length == 0 ? await extension.initAllLocales() : null;
+  let locales = !extension.errors.length
+    ? await extension.initAllLocales()
+    : null;
 
-  if (extension.errors.length > 0) {
+  if (extension.errors.length) {
     let error = new Error("Extension is invalid");
     // Add detailed errors on the error object so that the front end can display them
     // if needed (eg in about:debugging).
@@ -1170,7 +1171,7 @@ SafeInstallOperation.prototype = {
    * state
    */
   rollback() {
-    while (this._installedFiles.length > 0) {
+    while (this._installedFiles.length) {
       let move = this._installedFiles.pop();
       if (move.isMoveTo) {
         move.newFile.moveTo(move.oldDir.parent, move.oldDir.leafName);
@@ -1185,7 +1186,7 @@ SafeInstallOperation.prototype = {
       }
     }
 
-    while (this._createdDirs.length > 0) {
+    while (this._createdDirs.length) {
       recursiveRemove(this._createdDirs.pop());
     }
   },
@@ -3576,7 +3577,7 @@ class SystemAddonInstaller extends DirectoryInstaller {
         AddonManagerPrivate.hasUpgradeListener(addon.id)
       );
 
-      if (blockers.length > 0) {
+      if (blockers.length) {
         await waitForAllPromises(installs.map(postponeAddon));
       } else {
         await waitForAllPromises(installs.map(installAddon));
@@ -3730,7 +3731,7 @@ var XPIInstall = {
 
   cancelAll() {
     // Cancelling one may alter _inProgress, so don't use a simple iterator
-    while (this._inProgress.length > 0) {
+    while (this._inProgress.length) {
       let c = this._inProgress.shift();
       try {
         c.cancel();
diff --git a/toolkit/mozapps/extensions/internal/XPIProvider.jsm b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
index 9bd90866449e..7dd87d79a552 100644
--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -2914,7 +2914,7 @@ var XPIProvider = {
       let extensionListChanged = false;
       // If the database needs to be updated then open it and then update it
       // from the filesystem
-      if (updateReasons.length > 0) {
+      if (updateReasons.length) {
         AddonManagerPrivate.recordSimpleMeasure(
           "XPIDB_startup_load_reasons",
           updateReasons
diff --git a/toolkit/mozapps/extensions/test/browser/browser_html_abuse_report.js b/toolkit/mozapps/extensions/test/browser/browser_html_abuse_report.js
index 0a419dc0b620..3ff1186a5999 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_html_abuse_report.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_html_abuse_report.js
@@ -599,7 +599,7 @@ add_task(async function test_abusereport_panel_refresh() {
   const allButtons = Array.from(reportPanel.querySelectorAll("button")).filter(
     el => el !== reportPanel._iconClose
   );
-  ok(allButtons.length > 0, "panel buttons should have been found");
+  ok(!!allButtons.length, "panel buttons should have been found");
   ok(
     allButtons.every(el => el.hasAttribute("data-l10n-id")),
     "All the panel buttons have a data-l10n-id"
@@ -917,7 +917,7 @@ async function test_abuse_report_suggestions(addonId) {
     );
     if (reasonInfo.hasSuggestions) {
       ok(
-        localizedSuggestionsContent.length > 0,
+        !!localizedSuggestionsContent.length,
         `Category suggestions should not be empty for "${reason}"`
       );
     } else {
@@ -943,7 +943,7 @@ async function test_abuse_report_suggestions(addonId) {
       learnMoreLinks.push(..._suggestions.querySelectorAll(linkClass));
     }
 
-    if (learnMoreLinks.length > 0) {
+    if (learnMoreLinks.length) {
       ok(
         learnMoreLinks.every(el => el.getAttribute("target") === "_blank"),
         "All the learn more links have target _blank"
diff --git a/toolkit/mozapps/extensions/test/browser/browser_html_list_view.js b/toolkit/mozapps/extensions/test/browser/browser_html_list_view.js
index 8a3d00edd8ea..43fe01fba229 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_html_list_view.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_html_list_view.js
@@ -25,7 +25,7 @@ function getCardByAddonId(root, id) {
 }
 
 function isEmpty(el) {
-  return el.children.length == 0;
+  return !el.children.length;
 }
 
 function waitForThemeChange(list) {
@@ -775,7 +775,7 @@ add_task(async function testPluginIcons() {
 
   // Check that the icons are set to the plugin icon.
   let icons = doc.querySelectorAll(".card-heading-icon");
-  ok(icons.length > 0, "There are some plugins listed");
+  ok(!!icons.length, "There are some plugins listed");
 
   for (let icon of icons) {
     is(icon.src, pluginIconUrl, "Plugins use the plugin icon");
diff --git a/toolkit/mozapps/extensions/test/browser/browser_html_list_view_recommendations.js b/toolkit/mozapps/extensions/test/browser/browser_html_list_view_recommendations.js
index 1009ed8e415b..d0b5d606caf0 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_html_list_view_recommendations.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_html_list_view_recommendations.js
@@ -140,7 +140,7 @@ async function testListRecommendations({ type, manifestExtra = {} }) {
 
   // Check that the cards are all for the right type.
   let cards = doc.querySelectorAll("recommended-addon-card");
-  ok(cards.length > 0, "There were some cards found");
+  ok(!!cards.length, "There were some cards found");
   for (let card of cards) {
     is(card.discoAddon.type, type, `The card is for a ${type}`);
     is_element_visible(card, "The card is visible");
@@ -183,7 +183,7 @@ async function testListRecommendations({ type, manifestExtra = {} }) {
   is(hiddenCard.addonId, addonId, "The expected card was found");
   is_element_hidden(hiddenCard, "The card is still hidden");
 
-  ok(cards.length > 0, "There are still some visible cards");
+  ok(!!cards.length, "There are still some visible cards");
   for (let card of cards) {
     is(card.discoAddon.type, type, `The card is for a ${type}`);
     is_element_visible(card, "The card is visible");
diff --git a/toolkit/mozapps/extensions/test/browser/browser_installssl.js b/toolkit/mozapps/extensions/test/browser/browser_installssl.js
index 956f1a5c7bd6..8dc22ce85ca1 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_installssl.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_installssl.js
@@ -67,7 +67,7 @@ function add_install_test(mainURL, redirectURL, expectedStatus) {
 
 function run_install_tests(callback) {
   async function run_next_install_test() {
-    if (gTests.length == 0) {
+    if (!gTests.length) {
       callback();
       return;
     }
diff --git a/toolkit/mozapps/extensions/test/browser/browser_updatessl.js b/toolkit/mozapps/extensions/test/browser/browser_updatessl.js
index 35dc6d029253..886726a218ee 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_updatessl.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_updatessl.js
@@ -88,7 +88,7 @@ function add_update_test(mainURL, redirectURL, expectedStatus) {
 
 function run_update_tests(callback) {
   function run_next_update_test() {
-    if (gTests.length == 0) {
+    if (!gTests.length) {
       callback();
       return;
     }
diff --git a/toolkit/mozapps/extensions/test/browser/browser_webapi_install.js b/toolkit/mozapps/extensions/test/browser/browser_webapi_install.js
index 679cc077f3d3..cec3fb6fcb9e 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_webapi_install.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_webapi_install.js
@@ -140,7 +140,7 @@ async function testInstall(browser, args, steps, description) {
         });
       }
 
-      while (steps.length > 0) {
+      while (steps.length) {
         let nextStep = steps.shift();
         if (nextStep.action) {
           if (nextStep.action == "install") {
diff --git a/toolkit/mozapps/extensions/test/browser/head.js b/toolkit/mozapps/extensions/test/browser/head.js
index a634e8942079..0a25c811e11a 100644
--- a/toolkit/mozapps/extensions/test/browser/head.js
+++ b/toolkit/mozapps/extensions/test/browser/head.js
@@ -230,7 +230,7 @@ function run_next_test() {
     info("Test " + gTestsRun + " took " + (Date.now() - gTestStart) + "ms");
   }
 
-  if (gPendingTests.length == 0) {
+  if (!gPendingTests.length) {
     executeSoon(end_test);
     return;
   }
@@ -503,7 +503,7 @@ function get_string(aName, ...aArgs) {
   var bundle = Services.strings.createBundle(
     "chrome://mozapps/locale/extensions/extensions.properties"
   );
-  if (aArgs.length == 0) {
+  if (!aArgs.length) {
     return bundle.GetStringFromName(aName);
   }
   return bundle.formatStringFromName(aName, aArgs);
@@ -799,7 +799,7 @@ MockProvider.prototype = {
    */
   addAddon: function MP_addAddon(aAddon) {
     var oldAddons = this.addons.filter(aOldAddon => aOldAddon.id == aAddon.id);
-    var oldAddon = oldAddons.length > 0 ? oldAddons[0] : null;
+    var oldAddon = oldAddons.length ? oldAddons[0] : null;
 
     this.addons = this.addons.filter(aOldAddon => aOldAddon.id != aAddon.id);
 
@@ -973,7 +973,7 @@ MockProvider.prototype = {
         "MockProvider: pending callbacks at shutdown(): calling immediately"
       );
     }
-    while (this.callbackTimers.length > 0) {
+    while (this.callbackTimers.length) {
       // When we notify the callback timer, it removes itself from our array
       let timer = this.callbackTimers[0];
       try {
@@ -1015,7 +1015,7 @@ MockProvider.prototype = {
    */
   async getAddonsByTypes(aTypes) {
     var addons = this.addons.filter(function(aAddon) {
-      if (aTypes && aTypes.length > 0 && !aTypes.includes(aAddon.type)) {
+      if (aTypes && !!aTypes.length && !aTypes.includes(aAddon.type)) {
         return false;
       }
       return true;
@@ -1036,7 +1036,7 @@ MockProvider.prototype = {
         return false;
       }
 
-      if (aTypes && aTypes.length > 0 && !aTypes.includes(aInstall.type)) {
+      if (aTypes && !!aTypes.length && !aTypes.includes(aInstall.type)) {
         return false;
       }
 
diff --git a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
index 7d263d0cdd64..3941bcd53688 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
@@ -874,7 +874,7 @@ class EventChecker {
     }
 
     let events = this.expectedEvents[aId];
-    Assert.ok(events.length > 0, `Should be expecting events for ${aId}`);
+    Assert.ok(!!events.length, `Should be expecting events for ${aId}`);
 
     return events.shift();
   }
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_addon_manager_telemetry_events.js b/toolkit/mozapps/extensions/test/xpcshell/test_addon_manager_telemetry_events.js
index 5858e3143359..863e10b995a3 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_addon_manager_telemetry_events.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_addon_manager_telemetry_events.js
@@ -26,7 +26,7 @@ function getTelemetryEvents(includeMethods = EVENT_METHODS) {
   );
 
   ok(
-    snapshot.parent && snapshot.parent.length > 0,
+    snapshot.parent && !!snapshot.parent.length,
     "Got parent telemetry events in the snapshot"
   );
 
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js b/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js
index fd601e0fa49a..e980e99dc129 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js
@@ -81,7 +81,7 @@ async function run_test_1() {
   Assert.notEqual(testPlugin, null);
 
   let addons = await AddonManager.getAddonsByTypes(["plugin"]);
-  Assert.ok(addons.length > 0);
+  Assert.ok(!!addons.length);
 
   addons.forEach(function(p) {
     if (p.description == TEST_PLUGIN_DESCRIPTION) {
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_pref_properties.js b/toolkit/mozapps/extensions/test/xpcshell/test_pref_properties.js
index 99beedde98c2..6a0597372fa8 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_pref_properties.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_pref_properties.js
@@ -30,7 +30,7 @@ var gManagerEventsListener = {
   },
   checkExpected() {
     info("Checking expected events...");
-    while (this.expectedEvents.length > 0) {
+    while (this.expectedEvents.length) {
       let event = this.expectedEvents.pop();
       info("Looking for expected event " + event);
       let matchingEvents = this.seenEvents.filter(function(aSeenEvent) {
diff --git a/toolkit/mozapps/preferences/fontbuilder.js b/toolkit/mozapps/preferences/fontbuilder.js
index f387627eb581..5e264bfa531a 100644
--- a/toolkit/mozapps/preferences/fontbuilder.js
+++ b/toolkit/mozapps/preferences/fontbuilder.js
@@ -28,11 +28,11 @@ var FontBuilder = {
     let defaultFont = null;
     // Load Font Lists
     let fonts = await this.enumerator.EnumerateFontsAsync(aLanguage, aFontType);
-    if (fonts.length > 0) {
+    if (fonts.length) {
       defaultFont = this.enumerator.getDefaultFont(aLanguage, aFontType);
     } else {
       fonts = await this.enumerator.EnumerateFontsAsync(aLanguage, "");
-      if (fonts.length > 0) {
+      if (fonts.length) {
         defaultFont = this.enumerator.getDefaultFont(aLanguage, "");
       }
     }
@@ -44,7 +44,7 @@ var FontBuilder = {
     // Build the UI for the Default Font and Fonts for this CSS type.
     const popup = document.createXULElement("menupopup");
     let separator;
-    if (fonts.length > 0) {
+    if (fonts.length) {
       let menuitem = document.createXULElement("menuitem");
       if (defaultFont) {
         document.l10n.setAttributes(menuitem, "fonts-label-default", {
diff --git a/toolkit/mozapps/update/UpdateService.jsm b/toolkit/mozapps/update/UpdateService.jsm
index 33b0b6c4639e..442d3e5185ae 100644
--- a/toolkit/mozapps/update/UpdateService.jsm
+++ b/toolkit/mozapps/update/UpdateService.jsm
@@ -1313,7 +1313,7 @@ function pingStateAndStatusCodes(aUpdate, aStartup, aStatus) {
   let suffix = patchType + "_" + (aStartup ? AUSTLMY.STARTUP : AUSTLMY.STAGE);
   let stateCode = 0;
   let parts = aStatus.split(":");
-  if (parts.length > 0) {
+  if (parts.length) {
     switch (parts[0]) {
       case STATE_NONE:
         stateCode = 2;
@@ -1732,7 +1732,7 @@ function Update(update) {
     this._patches.push(patch);
   }
 
-  if (this._patches.length == 0 && !update.hasAttribute("unsupported")) {
+  if (!this._patches.length && !update.hasAttribute("unsupported")) {
     throw Cr.NS_ERROR_ILLEGAL_VALUE;
   }
 
@@ -2838,7 +2838,7 @@ UpdateService.prototype = {
    * @return  The nsIUpdate to offer.
    */
   selectUpdate: function AUS_selectUpdate(updates) {
-    if (updates.length == 0) {
+    if (!updates.length) {
       AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_NO_UPDATE_FOUND);
       return null;
     }
@@ -3341,7 +3341,7 @@ UpdateService.prototype = {
 function UpdateManager() {
   // Load the active-update.xml file to see if there is an active update.
   let activeUpdates = this._loadXMLFileIntoArray(FILE_ACTIVE_UPDATE_XML);
-  if (activeUpdates.length > 0) {
+  if (activeUpdates.length) {
     // Set the active update directly on the var used to cache the value.
     this._activeUpdate = activeUpdates[0];
     // This check is performed here since UpdateService:_postUpdateProcessing
@@ -3396,7 +3396,7 @@ UpdateManager.prototype = {
       this._activeUpdate = null;
       if (data != "skip-files") {
         let activeUpdates = this._loadXMLFileIntoArray(FILE_ACTIVE_UPDATE_XML);
-        if (activeUpdates.length > 0) {
+        if (activeUpdates.length) {
           this._activeUpdate = activeUpdates[0];
         }
         updates = this._loadXMLFileIntoArray(FILE_UPDATES_XML);
@@ -3488,7 +3488,7 @@ UpdateManager.prototype = {
       );
     }
     fileStream.close();
-    if (updates.length == 0) {
+    if (!updates.length) {
       LOG(
         "UpdateManager:_loadXMLFileIntoArray - update xml file " +
           fileName +
@@ -3582,7 +3582,7 @@ UpdateManager.prototype = {
       );
       return false;
     }
-    if (updates.length == 0) {
+    if (!updates.length) {
       LOG(
         "UpdateManager:_writeUpdatesToXMLFile - no updates to write. " +
           "removing file: " +
@@ -4758,7 +4758,7 @@ Downloader.prototype = {
         this._listeners.splice(i, 1);
 
         // Decrease the status update frequency when no one is listening
-        if (this._listeners.length == 0) {
+        if (!this._listeners.length) {
           this._maybeStopActiveNotifications();
         }
         return;
@@ -4770,7 +4770,7 @@ Downloader.prototype = {
    * Returns a boolean indicating whether there are any download listeners
    */
   get hasDownloadListeners() {
-    return this._listeners.length > 0;
+    return !!this._listeners.length;
   },
 
   /**
diff --git a/toolkit/mozapps/update/tests/data/sharedUpdateXML.js b/toolkit/mozapps/update/tests/data/sharedUpdateXML.js
index f691d060dc0e..34fb69e358ef 100644
--- a/toolkit/mozapps/update/tests/data/sharedUpdateXML.js
+++ b/toolkit/mozapps/update/tests/data/sharedUpdateXML.js
@@ -395,7 +395,7 @@ function readFileBytes(aFile) {
     let bytes = bis.readByteArray(Math.min(65535, count));
     data.push(String.fromCharCode.apply(null, bytes));
     count -= bytes.length;
-    if (bytes.length == 0) {
+    if (!bytes.length) {
       throw new Error("Nothing read from input stream!");
     }
   }
diff --git a/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js b/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js
index 35b0b9d01343..6e4c34114c63 100644
--- a/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js
+++ b/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js
@@ -854,7 +854,7 @@ function setupTestCommon(aAppUpdateAutoEnabled = false, aAllowBits = false) {
     .split(".")[0];
 
   if (gDebugTestLog && !gIsServiceTest) {
-    if (gTestsToLog.length == 0 || gTestsToLog.includes(gTestID)) {
+    if (!gTestsToLog.length || gTestsToLog.includes(gTestID)) {
       let logFile = do_get_file(gTestID + ".log", true);
       if (!logFile.exists()) {
         logFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);
diff --git a/toolkit/xre/test/test_install_hash.js b/toolkit/xre/test/test_install_hash.js
index 7189606403c3..3b97640e4cf0 100644
--- a/toolkit/xre/test/test_install_hash.js
+++ b/toolkit/xre/test/test_install_hash.js
@@ -50,7 +50,7 @@ function mutatePath(path, mutator) {
     path = path.parent;
   }
 
-  while (parts.length > 0) {
+  while (parts.length) {
     path.append(parts.pop());
   }