diff --git a/browser/base/content/browser-menubar.inc b/browser/base/content/browser-menubar.inc
index e818cdfafcbe..6dc75f7547fe 100644
--- a/browser/base/content/browser-menubar.inc
+++ b/browser/base/content/browser-menubar.inc
@@ -34,6 +34,12 @@
                           accesskey="&newPrivateWindow.accesskey;"
                           command="Tools:PrivateBrowsing"
                           key="key_privatebrowsing"/>
+#ifdef E10S_TESTING_ONLY
+                <menuitem id="menu_newNonRemoteWindow"
+                          label="&newNonRemoteWindow.label;"
+                          hidden="true"
+                          command="Tools:NonRemoteWindow"/>
+#endif
 #ifdef MAC_NON_BROWSER_WINDOW
                 <menuitem id="menu_openLocation"
                           label="&openLocationCmd.label;"
diff --git a/browser/base/content/browser-sets.inc b/browser/base/content/browser-sets.inc
index b353313cd07b..49be3820199c 100644
--- a/browser/base/content/browser-sets.inc
+++ b/browser/base/content/browser-sets.inc
@@ -102,6 +102,10 @@
      oncommand="Cc['@mozilla.org/browser/browserglue;1'].getService(Ci.nsIBrowserGlue).sanitize(window);"/>
     <command id="Tools:PrivateBrowsing"
       oncommand="OpenBrowserWindow({private: true});"/>
+#ifdef E10S_TESTING_ONLY
+    <command id="Tools:NonRemoteWindow"
+      oncommand="OpenBrowserWindow({remote: false});"/>
+#endif
     <command id="History:UndoCloseTab" oncommand="undoCloseTab();"/>
     <command id="History:UndoCloseWindow" oncommand="undoCloseWindow();"/>
   </commandset>
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 12a03b01616c..8831891dc375 100755
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1466,6 +1466,9 @@ var gBrowserInit = {
     IndexedDBPromptHelper.init();
     CanvasPermissionPromptHelper.init();
 
+    if (AppConstants.E10S_TESTING_ONLY)
+      gRemoteTabsUI.init();
+
     // Initialize the full zoom setting.
     // We do this before the session restore service gets initialized so we can
     // apply full zoom settings to tabs restored by the session restore service.
@@ -1979,6 +1982,9 @@ if (AppConstants.platform == "macosx") {
     // initialize the private browsing UI
     gPrivateBrowsingUI.init();
 
+    if (AppConstants.E10S_TESTING_ONLY) {
+      gRemoteTabsUI.init();
+    }
   };
 
   gBrowserInit.nonBrowserWindowShutdown = function() {
@@ -8277,6 +8283,27 @@ var gPrivateBrowsingUI = {
   }
 };
 
+var gRemoteTabsUI = {
+  init() {
+    if (window.location.href != getBrowserURL() &&
+        // Also check hidden window for the Mac no-window case
+        window.location.href != "chrome://browser/content/hiddenWindow.xul") {
+      return;
+    }
+
+    if (AppConstants.platform == "macosx" &&
+        Services.prefs.getBoolPref("layers.acceleration.disabled")) {
+      // On OS X, "Disable Hardware Acceleration" also disables OMTC and forces
+      // a fallback to Basic Layers. This is incompatible with e10s.
+      return;
+    }
+
+    let newNonRemoteWindow = document.getElementById("menu_newNonRemoteWindow");
+    let autostart = Services.appinfo.browserTabsRemoteAutostart;
+    newNonRemoteWindow.hidden = !autostart;
+  }
+};
+
 /**
  * Switch to a tab that has a given URI, and focuses its browser window.
  * If a matching tab is in this window, it will be switched to. Otherwise, other
@@ -8534,6 +8561,16 @@ var TabContextMenu = {
     if (this.contextTab.hasAttribute("customizemode"))
       document.getElementById("context_openTabInWindow").disabled = true;
 
+    if (AppConstants.E10S_TESTING_ONLY) {
+      menuItems = aPopupMenu.getElementsByAttribute("tbattr", "tabbrowser-remote");
+      for (let menuItem of menuItems) {
+        menuItem.hidden = !gMultiProcessBrowser;
+        if (menuItem.id == "context_openNonRemoteWindow") {
+          menuItem.disabled = !!parseInt(this.contextTab.getAttribute("usercontextid"));
+        }
+      }
+    }
+
     disabled = gBrowser.visibleTabs.length == 1;
     menuItems = aPopupMenu.getElementsByAttribute("tbattr", "tabbrowser-multiple-visible");
     for (let menuItem of menuItems)
diff --git a/browser/base/content/browser.xul b/browser/base/content/browser.xul
index 3058058ab390..206575daa5c6 100644
--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -96,6 +96,12 @@
                 accesskey="&moveToNewWindow.accesskey;"
                 tbattr="tabbrowser-multiple"
                 oncommand="gBrowser.replaceTabWithWindow(TabContextMenu.contextTab);"/>
+#ifdef E10S_TESTING_ONLY
+      <menuitem id="context_openNonRemoteWindow" label="Open in new non-e10s window"
+                tbattr="tabbrowser-remote"
+                hidden="true"
+                oncommand="gBrowser.openNonRemoteWindow(TabContextMenu.contextTab);"/>
+#endif
       <menuseparator id="context_sendTabToDevice_separator"/>
       <menu id="context_sendTabToDevice" label="&sendTabToDevice.label;"
             accesskey="&sendTabToDevice.accesskey;">
diff --git a/browser/base/content/tabbrowser.xml b/browser/base/content/tabbrowser.xml
index e14f5955d253..df1e479d677b 100644
--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -3954,6 +3954,20 @@
         </body>
       </method>
 
+      <!-- Opens a given tab to a non-remote window. -->
+      <method name="openNonRemoteWindow">
+        <parameter name="aTab"/>
+        <body>
+          <![CDATA[
+            if (!AppConstants.E10S_TESTING_ONLY) {
+              throw "This method is intended only for e10s testing!";
+            }
+            let url = aTab.linkedBrowser.currentURI.spec;
+            return window.openDialog("chrome://browser/content/", "_blank", "chrome,all,dialog=no,non-remote", url);
+          ]]>
+        </body>
+      </method>
+
       <method name="moveTabTo">
         <parameter name="aTab"/>
         <parameter name="aIndex"/>
@@ -5530,11 +5544,14 @@
             }
           } else {
             label = tab._fullLabel || tab.getAttribute("label");
-            if (AppConstants.NIGHTLY_BUILD &&
+            if (AppConstants.E10S_TESTING_ONLY &&
                 tab.linkedBrowser &&
-                tab.linkedBrowser.isRemoteBrowser &&
-                tab.linkedBrowser.frameLoader) {
-              label += " (pid " + tab.linkedBrowser.frameLoader.tabParent.osPid + ")";
+                tab.linkedBrowser.isRemoteBrowser) {
+              label += " - e10s";
+              if (tab.linkedBrowser.frameLoader &&
+                  Services.appinfo.maxWebProcessCount > 1) {
+                label += " (" + tab.linkedBrowser.frameLoader.tabParent.osPid + ")";
+              }
             }
             if (tab.userContextId) {
               label = this.mStringBundle.getFormattedString("tabs.containers.tooltip", [label, ContextualIdentityService.getUserContextLabel(tab.userContextId)]);
diff --git a/browser/components/customizableui/CustomizableUI.jsm b/browser/components/customizableui/CustomizableUI.jsm
index 81dd963122b9..de20ffe7e87c 100644
--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -59,7 +59,7 @@ const kSubviewEvents = [
  * The current version. We can use this to auto-add new default widgets as necessary.
  * (would be const but isn't because of testing purposes)
  */
-var kVersion = 13;
+var kVersion = 12;
 
 /**
  * Buttons removed from built-ins by version they were removed. kVersion must be
@@ -345,6 +345,7 @@ var CustomizableUIInternal = {
         "preferences-button",
         "add-ons-button",
         "sync-button",
+        "e10s-button",
       ];
 
       if (!AppConstants.MOZ_DEV_EDITION) {
@@ -451,17 +452,6 @@ var CustomizableUIInternal = {
         }
       }
     }
-
-    // Remove the old placements from the now-gone Nightly-only
-    // "New non-e10s window" button.
-    if (currentVersion < 13 && gSavedState.placements) {
-      for (let placements of Object.values(gSavedState.placements)) {
-        let buttonIndex = placements.indexOf("e10s-button");
-        if (buttonIndex != -1) {
-          placements.spice(buttonIndex, 1);
-        }
-      }
-    }
   },
 
   /**
diff --git a/browser/components/customizableui/CustomizableWidgets.jsm b/browser/components/customizableui/CustomizableWidgets.jsm
index 936cf374b3aa..4ce4f7baf077 100644
--- a/browser/components/customizableui/CustomizableWidgets.jsm
+++ b/browser/components/customizableui/CustomizableWidgets.jsm
@@ -997,3 +997,21 @@ if (Services.prefs.getBoolPref("privacy.panicButton.enabled")) {
     },
   });
 }
+
+if (AppConstants.E10S_TESTING_ONLY) {
+  if (Services.appinfo.browserTabsRemoteAutostart) {
+    CustomizableWidgets.push({
+      id: "e10s-button",
+      defaultArea: CustomizableUI.AREA_PANEL,
+      onBuild(aDocument) {
+        let node = aDocument.createElementNS(kNSXUL, "toolbarbutton");
+        node.setAttribute("label", CustomizableUI.getLocalizedProperty(this, "label"));
+        node.setAttribute("tooltiptext", CustomizableUI.getLocalizedProperty(this, "tooltiptext"));
+      },
+      onCommand(aEvent) {
+        let win = aEvent.view;
+        win.OpenBrowserWindow({remote: false});
+      },
+    });
+  }
+}
diff --git a/browser/components/customizableui/test/head.js b/browser/components/customizableui/test/head.js
index c4faea4e02c0..5267dcf2da21 100644
--- a/browser/components/customizableui/test/head.js
+++ b/browser/components/customizableui/test/head.js
@@ -17,6 +17,11 @@ Services.scriptloader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/
 Services.prefs.setBoolPref("browser.uiCustomization.skipSourceNodeCheck", true);
 registerCleanupFunction(() => Services.prefs.clearUserPref("browser.uiCustomization.skipSourceNodeCheck"));
 
+// Remove temporary e10s related new window options in customize ui,
+// they break a lot of tests.
+CustomizableUI.destroyWidget("e10s-button");
+CustomizableUI.removeWidgetFromArea("e10s-button");
+
 var {synthesizeDragStart, synthesizeDrop} = EventUtils;
 
 const kNSXUL = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
diff --git a/browser/components/preferences/in-content/main.js b/browser/components/preferences/in-content/main.js
index 92f3f649056c..696ee6b69e92 100644
--- a/browser/components/preferences/in-content/main.js
+++ b/browser/components/preferences/in-content/main.js
@@ -97,6 +97,11 @@ const APP_ICON_ATTR_NAME = "appHandlerIcon";
 XPCOMUtils.defineLazyModuleGetter(this, "OS",
   "resource://gre/modules/osfile.jsm");
 
+if (AppConstants.E10S_TESTING_ONLY) {
+  XPCOMUtils.defineLazyModuleGetter(this, "UpdateUtils",
+    "resource://gre/modules/UpdateUtils.jsm");
+}
+
 if (AppConstants.MOZ_DEV_EDITION) {
   XPCOMUtils.defineLazyModuleGetter(this, "fxAccounts",
     "resource://gre/modules/FxAccounts.jsm");
@@ -314,6 +319,26 @@ var gMainPane = {
       }
     }
 
+    if (AppConstants.E10S_TESTING_ONLY) {
+      setEventListener("e10sAutoStart", "command",
+        gMainPane.enableE10SChange);
+      let e10sCheckbox = document.getElementById("e10sAutoStart");
+
+      let e10sPref = document.getElementById("browser.tabs.remote.autostart");
+      let e10sTempPref = document.getElementById("e10sTempPref");
+      let e10sForceEnable = document.getElementById("e10sForceEnable");
+
+      let preffedOn = e10sPref.value || e10sTempPref.value || e10sForceEnable.value;
+
+      if (preffedOn) {
+        // The checkbox is checked if e10s is preffed on and enabled.
+        e10sCheckbox.checked = Services.appinfo.browserTabsRemoteAutostart;
+
+        // but if it's force disabled, then the checkbox is disabled.
+        e10sCheckbox.disabled = !Services.appinfo.browserTabsRemoteAutostart;
+      }
+    }
+
     if (AppConstants.MOZ_DEV_EDITION) {
       let uAppData = OS.Constants.Path.userApplicationDataDir;
       let ignoreSeparateProfile = OS.Path.join(uAppData, "ignore-dev-edition-profile");
@@ -521,6 +546,54 @@ var gMainPane = {
     this.readBrowserContainersCheckbox();
   },
 
+  isE10SEnabled() {
+    let e10sEnabled;
+    try {
+      let e10sStatus = Components.classes["@mozilla.org/supports-PRUint64;1"]
+        .createInstance(Ci.nsISupportsPRUint64);
+      let appinfo = Services.appinfo.QueryInterface(Ci.nsIObserver);
+      appinfo.observe(e10sStatus, "getE10SBlocked", "");
+      e10sEnabled = e10sStatus.data < 2;
+    } catch (e) {
+      e10sEnabled = false;
+    }
+
+    return e10sEnabled;
+  },
+
+  enableE10SChange() {
+    if (AppConstants.E10S_TESTING_ONLY) {
+      let e10sCheckbox = document.getElementById("e10sAutoStart");
+      let e10sPref = document.getElementById("browser.tabs.remote.autostart");
+      let e10sTempPref = document.getElementById("e10sTempPref");
+
+      let prefsToChange;
+      if (e10sCheckbox.checked) {
+        // Enabling e10s autostart
+        prefsToChange = [e10sPref];
+      } else {
+        // Disabling e10s autostart
+        prefsToChange = [e10sPref];
+        if (e10sTempPref.value) {
+          prefsToChange.push(e10sTempPref);
+        }
+      }
+
+      let buttonIndex = confirmRestartPrompt(e10sCheckbox.checked, 0,
+        true, false);
+      if (buttonIndex == CONFIRM_RESTART_PROMPT_RESTART_NOW) {
+        for (let prefToChange of prefsToChange) {
+          prefToChange.value = e10sCheckbox.checked;
+        }
+
+        Services.startup.quit(Ci.nsIAppStartup.eAttemptQuit | Ci.nsIAppStartup.eRestart);
+      }
+
+      // Revert the checkbox in case we didn't quit
+      e10sCheckbox.checked = e10sPref.value || e10sTempPref.value;
+    }
+  },
+
   separateProfileModeChange() {
     if (AppConstants.MOZ_DEV_EDITION) {
       function quitApp() {
@@ -1212,7 +1285,7 @@ var gMainPane = {
   },
 
   buildContentProcessCountMenuList() {
-    if (Services.appinfo.browserTabsRemoteAutostart) {
+    if (gMainPane.isE10SEnabled()) {
       let processCountPref = document.getElementById("dom.ipc.processCount");
       let e10sRolloutProcessCountPref =
         document.getElementById("dom.ipc.processCount.web");
diff --git a/browser/components/preferences/in-content/main.xul b/browser/components/preferences/in-content/main.xul
index 64fa7366814f..92c896b458fc 100644
--- a/browser/components/preferences/in-content/main.xul
+++ b/browser/components/preferences/in-content/main.xul
@@ -17,6 +17,19 @@
 <stringbundle id="bundlePreferences" src="chrome://browser/locale/preferences.properties"/>
 
 <preferences id="mainPreferences" hidden="true" data-category="paneGeneral">
+
+#ifdef E10S_TESTING_ONLY
+    <preference id="browser.tabs.remote.autostart"
+                name="browser.tabs.remote.autostart"
+                type="bool"/>
+    <preference id="e10sTempPref"
+                name="browser.tabs.remote.autostart.2"
+                type="bool"/>
+    <preference id="e10sForceEnable"
+                name="browser.tabs.remote.force-enable"
+                type="bool"/>
+#endif
+
     <!-- Startup -->
     <preference id="browser.startup.page"
                 name="browser.startup.page"
@@ -285,6 +298,11 @@
   </vbox>
 #endif
 
+#ifdef E10S_TESTING_ONLY
+  <checkbox id="e10sAutoStart"
+            label="&e10sEnabled.label;"/>
+#endif
+
 #ifdef HAVE_SHELL_SERVICE
   <vbox id="defaultBrowserBox">
     <checkbox id="alwaysCheckDefault" preference="browser.shell.checkDefaultBrowser"
diff --git a/browser/locales/en-US/chrome/browser/browser.dtd b/browser/locales/en-US/chrome/browser/browser.dtd
index 1103f042a06d..dfbaeaba44cd 100644
--- a/browser/locales/en-US/chrome/browser/browser.dtd
+++ b/browser/locales/en-US/chrome/browser/browser.dtd
@@ -294,6 +294,7 @@ These should match what Safari and other Apple applications use on OS X Lion. --
 <!ENTITY newNavigatorCmd.accesskey      "N">
 <!ENTITY newPrivateWindow.label     "New Private Window">
 <!ENTITY newPrivateWindow.accesskey "W">
+<!ENTITY newNonRemoteWindow.label   "New Non-e10s Window">
 
 <!ENTITY editMenu.label         "Edit">
 <!ENTITY editMenu.accesskey       "E">
diff --git a/browser/locales/en-US/chrome/browser/customizableui/customizableWidgets.properties b/browser/locales/en-US/chrome/browser/customizableui/customizableWidgets.properties
index 762068a1688b..84403c3ef53a 100644
--- a/browser/locales/en-US/chrome/browser/customizableui/customizableWidgets.properties
+++ b/browser/locales/en-US/chrome/browser/customizableui/customizableWidgets.properties
@@ -101,6 +101,9 @@ panic-button.tooltiptext = Forget about some browsing history
 devtools-webide-button2.label = WebIDE
 devtools-webide-button2.tooltiptext = Open WebIDE (%S)
 
+e10s-button.label = New Non-e10s Window
+e10s-button.tooltiptext = Open a new Non-e10s Window
+
 toolbarspring.label = Flexible Space
 toolbarseparator.label = Separator
 toolbarspacer.label = Space
diff --git a/browser/locales/en-US/chrome/browser/preferences/main.dtd b/browser/locales/en-US/chrome/browser/preferences/main.dtd
index cf5c569ed6ea..924f6f2fd14e 100644
--- a/browser/locales/en-US/chrome/browser/preferences/main.dtd
+++ b/browser/locales/en-US/chrome/browser/preferences/main.dtd
@@ -44,3 +44,5 @@
 <!ENTITY useFirefoxSync.label             "Tip: This uses separate profiles. Use Sync to share data between them.">
 <!ENTITY getStarted.notloggedin.label     "Sign in to &syncBrand.shortName.label;…">
 <!ENTITY getStarted.configured.label      "Open &syncBrand.shortName.label; preferences">
+
+<!ENTITY e10sEnabled.label                "Enable multi-process &brandShortName;">
diff --git a/browser/themes/shared/toolbarbutton-icons.inc.css b/browser/themes/shared/toolbarbutton-icons.inc.css
index d1e672db7f46..9ee2107c3397 100644
--- a/browser/themes/shared/toolbarbutton-icons.inc.css
+++ b/browser/themes/shared/toolbarbutton-icons.inc.css
@@ -229,6 +229,14 @@ toolbar[brighttext] {
   list-style-image: url("chrome://browser/skin/new-window.svg");
 }
 
+#e10s-button {
+  list-style-image: url("chrome://browser/skin/new-window.svg");
+}
+
+#e10s-button > .toolbarbutton-icon {
+  transform: scaleY(-1);
+}
+
 #new-tab-button {
   list-style-image: url("chrome://browser/skin/new-tab.svg");
 }
diff --git a/devtools/client/locales/en-US/performance.dtd b/devtools/client/locales/en-US/performance.dtd
index 5b5c1eda045a..4009ae10450f 100644
--- a/devtools/client/locales/en-US/performance.dtd
+++ b/devtools/client/locales/en-US/performance.dtd
@@ -15,6 +15,11 @@
   -  is displayed as the tooltip for the buffer capacity during a recording. -->
 <!ENTITY performanceUI.bufferStatusTooltip "The profiler stores samples in a circular buffer, and once the buffer reaches the limit for a recording, newer samples begin to overwrite samples at the beginning of the recording.">
 
+<!-- LOCALIZATION NOTE (performanceUI.disabledRealTime.nonE10SBuild): This string
+  -  is displayed as a message for why the real time overview graph is disabled
+  -  when running on a non-multiprocess build. -->
+<!ENTITY performanceUI.disabledRealTime.nonE10SBuild "Realtime recording data disabled on non-multiprocess Firefox.">
+
 <!-- LOCALIZATION NOTE (performanceUI.disabledRealTime.disabledE10S): This string
   -  is displayed as a message for why the real time overview graph is disabled
   -  when running on a build that can run multiprocess Firefox, but just is not enabled. -->
diff --git a/devtools/client/performance/performance-controller.js b/devtools/client/performance/performance-controller.js
index bf53c248b024..5d2724acec8a 100644
--- a/devtools/client/performance/performance-controller.js
+++ b/devtools/client/performance/performance-controller.js
@@ -517,17 +517,18 @@ var PerformanceController = {
    * @return {object}
    */
   getMultiprocessStatus: function () {
-    // If testing, set enabled to true so we have realtime rendering tests
-    // in non-e10s. This function is overridden wholesale in tests
-    // when we want to test multiprocess support
+    // If testing, set both supported and enabled to true so we
+    // have realtime rendering tests in non-e10s. This function is
+    // overridden wholesale in tests when we want to test multiprocess support
     // specifically.
     if (flags.testing) {
-      return { enabled: true };
+      return { supported: true, enabled: true };
     }
+    let supported = system.constants.E10S_TESTING_ONLY;
     // This is only checked on tool startup -- requires a restart if
     // e10s subsequently enabled.
     let enabled = this._e10s;
-    return { enabled };
+    return { supported, enabled };
   },
 
   /**
@@ -555,9 +556,12 @@ var PerformanceController = {
    * if e10s is not possible on the platform. If e10s is on, no attribute is set.
    */
   _setMultiprocessAttributes: function () {
-    let { enabled } = this.getMultiprocessStatus();
-    if (!enabled) {
+    let { enabled, supported } = this.getMultiprocessStatus();
+    if (!enabled && supported) {
       $("#performance-view").setAttribute("e10s", "disabled");
+    } else if (!enabled && !supported) {
+      // Could be a chance where the directive goes away yet e10s is still on
+      $("#performance-view").setAttribute("e10s", "unsupported");
     }
   },
 
diff --git a/devtools/client/performance/performance.xul b/devtools/client/performance/performance.xul
index effbc8f8b8d0..7ceb41c84851 100644
--- a/devtools/client/performance/performance.xul
+++ b/devtools/client/performance/performance.xul
@@ -211,6 +211,8 @@
               <hbox pack="center">
                 <html:div class='recording-button-mount'/>
               </hbox>
+              <label class="realtime-disabled-message"
+                     value="&performanceUI.disabledRealTime.nonE10SBuild;"/>
               <label class="realtime-disabled-on-e10s-message"
                      value="&performanceUI.disabledRealTime.disabledE10S;"/>
               <label class="buffer-status-message"
@@ -235,6 +237,8 @@
                 <label class="console-profile-command" />
                 <label value="&performanceUI.console.stopCommandEnd;" />
               </hbox>
+              <label class="realtime-disabled-message"
+                     value="&performanceUI.disabledRealTime.nonE10SBuild;"/>
               <label class="realtime-disabled-on-e10s-message"
                      value="&performanceUI.disabledRealTime.disabledE10S;"/>
               <label class="buffer-status-message"
diff --git a/devtools/client/performance/test/browser_perf-recording-notices-05.js b/devtools/client/performance/test/browser_perf-recording-notices-05.js
index 5107f81a6bad..b6267470d19f 100644
--- a/devtools/client/performance/test/browser_perf-recording-notices-05.js
+++ b/devtools/client/performance/test/browser_perf-recording-notices-05.js
@@ -20,21 +20,35 @@ add_task(function* () {
   // Set a fast profiler-status update interval
   yield gFront.setProfilerStatusInterval(10);
 
+  let supported = false;
   let enabled = false;
 
   PerformanceController.getMultiprocessStatus = () => {
-    return { enabled };
+    return { supported, enabled };
   };
 
+  PerformanceController._setMultiprocessAttributes();
+  ok($("#performance-view").getAttribute("e10s"), "unsupported",
+    "When e10s is disabled and no option to turn on, container has [e10s=unsupported].");
+
+  supported = true;
+  enabled = false;
   PerformanceController._setMultiprocessAttributes();
   ok($("#performance-view").getAttribute("e10s"), "disabled",
-    "When e10s is disabled, container has [e10s=disabled].");
+    "When e10s is disabled and but is supported, container has [e10s=disabled].");
 
+  supported = false;
   enabled = true;
-
   PerformanceController._setMultiprocessAttributes();
   ok($("#performance-view").getAttribute("e10s"), "",
-    "When e10s is enabled, there should be no e10s attribute.");
+    "When e10s is enabled, but not supported, this probably means we no longer have " +
+    "E10S_TESTING_ONLY, and we have no e10s attribute.");
+
+  supported = true;
+  enabled = true;
+  PerformanceController._setMultiprocessAttributes();
+  ok($("#performance-view").getAttribute("e10s"), "",
+    "When e10s is enabled and supported, there should be no e10s attribute.");
 
   yield teardownToolboxAndRemoveTab(panel);
 });
diff --git a/devtools/client/themes/performance.css b/devtools/client/themes/performance.css
index 794a1a267d9d..28887dc25f98 100644
--- a/devtools/client/themes/performance.css
+++ b/devtools/client/themes/performance.css
@@ -247,11 +247,13 @@
   margin: 3px 2px;
 }
 
+.realtime-disabled-message,
 .realtime-disabled-on-e10s-message {
   display: none;
 }
 
-#performance-view[e10s="disabled"] .realtime-disabled-on-e10s-message {
+#performance-view[e10s="disabled"] .realtime-disabled-on-e10s-message,
+#performance-view[e10s="unsupported"] .realtime-disabled-message {
   display: initial;
   opacity: 0.5;
 }
diff --git a/moz.configure b/moz.configure
index da81abffb532..4ea93b7a3d15 100755
--- a/moz.configure
+++ b/moz.configure
@@ -14,6 +14,17 @@ include('build/moz.configure/init.configure')
 # - Spidermonkey-specific options and rules should go in js/moz.configure.
 # - etc.
 
+# Multiprocess Firefox Testing UI - Nightly and Aurora
+# To be removed in Bug 1003313
+@depends(milestone)
+def e10s_testing_only(milestone):
+    if not milestone.is_release_or_beta:
+        return True
+
+set_config('E10S_TESTING_ONLY', e10s_testing_only)
+set_define('E10S_TESTING_ONLY', e10s_testing_only)
+
+
 option('--enable-artifact-builds', env='MOZ_ARTIFACT_BUILDS',
        help='Download and use prebuilt binary artifacts.')
 
diff --git a/toolkit/modules/AppConstants.jsm b/toolkit/modules/AppConstants.jsm
index 327f5e796958..db6176d95b47 100644
--- a/toolkit/modules/AppConstants.jsm
+++ b/toolkit/modules/AppConstants.jsm
@@ -180,6 +180,13 @@ this.AppConstants = Object.freeze({
   false,
 #endif
 
+  E10S_TESTING_ONLY:
+#ifdef E10S_TESTING_ONLY
+  true,
+#else
+  false,
+#endif
+
   DEBUG:
 #ifdef DEBUG
   true,