diff --git a/browser/components/BrowserGlue.sys.mjs b/browser/components/BrowserGlue.sys.mjs
index 5fe80cf17fd7..aa511b4de16a 100644
--- a/browser/components/BrowserGlue.sys.mjs
+++ b/browser/components/BrowserGlue.sys.mjs
@@ -429,7 +429,6 @@ let JSWINDOWACTORS = {
     },
     matches: ["about:shoppingsidebar"],
     remoteTypes: ["privilegedabout"],
-    messageManagerGroups: ["shopping-sidebar", "browsers"],
   },
 
   AboutWelcome: {
@@ -880,7 +879,6 @@ let JSWINDOWACTORS = {
     },
     matches: ["about:shoppingsidebar"],
     remoteTypes: ["privilegedabout"],
-    messageManagerGroups: ["shopping-sidebar", "browsers"],
   },
 
   SpeechDispatcher: {
diff --git a/browser/components/places/content/bookmarksHistoryTooltip.inc.xhtml b/browser/components/places/content/bookmarksHistoryTooltip.inc.xhtml
index d117d3ba5f09..445f2cdcb4ff 100644
--- a/browser/components/places/content/bookmarksHistoryTooltip.inc.xhtml
+++ b/browser/components/places/content/bookmarksHistoryTooltip.inc.xhtml
@@ -5,7 +5,7 @@
 <!-- Bookmarks and history tooltip -->
 <tooltip id="bhTooltip" noautohide="true"
          class="places-tooltip"
-         onpopupshowing="return window.browsingContext.topChromeWindow.BookmarksEventHandler.fillInBHTooltip(this, event)"
+         onpopupshowing="return window.top.BookmarksEventHandler.fillInBHTooltip(this, event)"
          onpopuphiding="this.removeAttribute('position')">
   <box class="places-tooltip-box">
     <description class="tooltip-label places-tooltip-title"/>
diff --git a/browser/components/places/content/bookmarksSidebar.js b/browser/components/places/content/bookmarksSidebar.js
index 8acf7acf373b..60cc7eb06741 100644
--- a/browser/components/places/content/bookmarksSidebar.js
+++ b/browser/components/places/content/bookmarksSidebar.js
@@ -29,8 +29,7 @@ XPCOMUtils.defineLazyScriptGetter(
 var gCumulativeSearches = 0;
 
 function init() {
-  let top = window.browsingContext.topChromeWindow;
-  let uidensity = top.document.documentElement.getAttribute("uidensity");
+  let uidensity = window.top.document.documentElement.getAttribute("uidensity");
   if (uidensity) {
     document.documentElement.setAttribute("uidensity", uidensity);
   }
diff --git a/browser/components/shopping/content/shopping-sidebar.js b/browser/components/shopping/content/shopping-sidebar.js
index 820a327c32f3..09c6015bec86 100644
--- a/browser/components/shopping/content/shopping-sidebar.js
+++ b/browser/components/shopping/content/shopping-sidebar.js
@@ -27,7 +27,6 @@
           remote="true"
           src="about:shoppingsidebar"
           type="content"
-          messagemanagergroup="shopping-sidebar"
           />
       `;
     }
diff --git a/browser/components/sidebar/browser-sidebar.js b/browser/components/sidebar/browser-sidebar.js
index 4b2eed09d43d..e95aa93e912d 100644
--- a/browser/components/sidebar/browser-sidebar.js
+++ b/browser/components/sidebar/browser-sidebar.js
@@ -158,12 +158,11 @@ var SidebarController = {
         "viewReviewCheckerSidebar",
         {
           elementId: "sidebar-switcher-review-checker",
-          url: "about:shoppingsidebar",
+          url: "chrome://browser/content/shopping/shopping.html",
           menuId: "menu_reviewCheckerSidebar",
           menuL10nId: "menu-view-review-checker",
           revampL10nId: "sidebar-menu-review-checker-label",
           iconUrl: "chrome://browser/content/shopping/assets/shopping.svg",
-          remoteType: E10SUtils.PRIVILEGEDABOUT_REMOTE_TYPE,
         }
       );
     }
@@ -218,14 +217,6 @@ var SidebarController = {
     }
     return (this._browser = document.getElementById("sidebar"));
   },
-
-  set browser(browser) {
-    let currentBrowser = this.browser;
-    currentBrowser.destroy();
-    currentBrowser.replaceWith(browser);
-    this._browser = browser;
-  },
-
   POSITION_START_PREF: "sidebar.position_start",
   DEFAULT_SIDEBAR_ID: "viewBookmarksSidebar",
   TOOLS_PREF: "sidebar.main.tools",
@@ -696,8 +687,9 @@ var SidebarController = {
 
     this.hideSwitcherPanel();
 
-    if (!this.browser.isRemoteBrowser) {
-      this.browser.contentWindow.updatePosition?.();
+    let content = SidebarController.browser.contentWindow;
+    if (content && content.updatePosition) {
+      content.updatePosition();
     }
   },
 
@@ -876,10 +868,8 @@ var SidebarController = {
    * window, only when the user opens the sidebar.
    */
   _fireFocusedEvent() {
-    if (!this.browser.isRemoteBrowser) {
-      let event = new CustomEvent("SidebarFocused", { bubbles: true });
-      this.browser.contentWindow.dispatchEvent(event);
-    }
+    let event = new CustomEvent("SidebarFocused", { bubbles: true });
+    this.browser.contentWindow.dispatchEvent(event);
   },
 
   /**
@@ -1495,218 +1485,92 @@ var SidebarController = {
    * @param {string} commandID ID of the sidebar.
    * @returns {Promise<void>}
    */
-  async _show(commandID) {
-    if (this.sidebarRevampEnabled) {
-      this.sidebarContainer.hidden = false;
-      this._box.dispatchEvent(
-        new CustomEvent("sidebar-show", { detail: { viewId: commandID } })
-      );
-
-      // Whenever a panel is shown, the sidebar is collapsed. Upon hiding
-      // that panel afterwards, `expanded` reverts back to what it was prior
-      // to calling `show()`. Thus, we store the expanded state at this point.
-      this._previousExpandedState = this.sidebarMain.expanded;
-
-      this.toggleExpanded(false);
-    } else {
-      this.hideSwitcherPanel();
-    }
-
-    this.selectMenuItem(commandID);
-    this._box.hidden = this._splitter.hidden = false;
-
-    this._box.setAttribute("checked", "true");
-    this._box.setAttribute("sidebarcommand", commandID);
-
-    let { icon, url, title, sourceL10nEl, contextMenuId, remoteType } =
-      this.sidebars.get(commandID);
-    if (icon) {
-      this._switcherTarget.style.setProperty(
-        "--webextension-menuitem-image",
-        icon
-      );
-    } else {
-      this._switcherTarget.style.removeProperty(
-        "--webextension-menuitem-image"
-      );
-    }
-
-    if (contextMenuId) {
-      this._box.setAttribute("context", contextMenuId);
-    } else {
-      this._box.removeAttribute("context");
-    }
-
-    // use to live update <tree> elements if the locale changes
-    this.lastOpenedId = commandID;
-    // These title changes only apply to the old sidebar menu
-    if (!this.sidebarRevampEnabled) {
-      this.title = title;
-      // Keep the title element in the switcher in sync with any l10n changes.
-      this.observeTitleChanges(sourceL10nEl);
-    }
-
-    await this._loadURL(url, remoteType);
-
-    // Now that the currentId is updated, fire a show event.
-    this._fireShowEvent();
-    this._recordBrowserSize();
-  },
-
-  /**
-   * Loads the given URL into the browser.
-   *
-   * If the current sidebar browser doesn't match the remoteType
-   * of the new URL, the browser will be replaced with one that does.
-   *
-   * @param {string} url to load.
-   * @param {string} [remoteType] of the content.
-   * @returns {Promise<void>}
-   */
-  async _loadURL(url, remoteType) {
-    let targetURI = makeURI(url);
-
-    if (
-      remoteType &&
-      (!this.browser.isRemoteBrowser || this.browser.remoteType !== remoteType)
-    ) {
-      this.browser = this._createBrowser({ remoteType });
-    } else if (!remoteType && this.browser.hasAttribute("remote")) {
-      this.browser = this._createBrowser();
-    }
-
-    let loadPromise;
-    if (this.browser.isRemoteBrowser) {
-      this.browser.loadURI(targetURI, {
-        triggeringPrincipal:
-          Services.scriptSecurityManager.getSystemPrincipal(),
-      });
-      loadPromise = this._hasProgressStopped(targetURI);
-    } else {
-      this.browser.setAttribute("src", url); // kick off async load
-      loadPromise = this._hasLoaded(url);
-    }
-
-    await loadPromise;
-  },
-
-  /**
-   * Creates a new browser, either non-remote or remote
-   * with the given remoteType.
-   *
-   * @param {object} [options]
-   * @param {string} [options.remoteType] to set for the browser.
-   * @returns {Browser}
-   */
-  _createBrowser(options = { remoteType: E10SUtils.NOT_REMOTE }) {
-    let { remoteType } = options;
-    let browser = document.createXULElement("browser");
-    browser.setAttribute("id", "sidebar");
-    browser.setAttribute("autoscroll", "false");
-    browser.setAttribute("disablehistory", "true");
-    browser.setAttribute("disablefullscreen", "true");
-    browser.setAttribute("tooltip", "aHTMLTooltip");
-    if (remoteType) {
-      browser.setAttribute("type", "content");
-      browser.setAttribute("remote", "true");
-      browser.setAttribute("remoteType", remoteType);
-      browser.setAttribute("maychangeremoteness", "true");
-      browser.setAttribute("messagemanagergroup", "sidebar-browsers");
-    }
-    return browser;
-  },
-
-  /**
-   * Wait for a URL to have loaded or unloaded in the sidebar browser.
-   *
-   * @param {string} url to check has loaded.
-   * @returns {Promise<void>}
-   */
-  async _hasLoaded(url) {
+  _show(commandID) {
     return new Promise(resolve => {
-      if (
-        this.browser.contentDocument.location.href == url &&
-        this.browser.contentDocument.readyState === "complete"
-      ) {
-        resolve();
-        return;
+      if (this.sidebarRevampEnabled) {
+        this.sidebarContainer.hidden = false;
+        this._box.dispatchEvent(
+          new CustomEvent("sidebar-show", { detail: { viewId: commandID } })
+        );
+
+        // Whenever a panel is shown, the sidebar is collapsed. Upon hiding
+        // that panel afterwards, `expanded` reverts back to what it was prior
+        // to calling `show()`. Thus, we store the expanded state at this point.
+        this._previousExpandedState = this.sidebarMain.expanded;
+
+        this.toggleExpanded(false);
+      } else {
+        this.hideSwitcherPanel();
       }
 
-      // make sure to clear the timeout if the load is aborted
-      this.browser.addEventListener("unload", () => {
-        if (this.browser.loadingTimerID) {
-          clearTimeout(this.browser.loadingTimerID);
-          delete this.browser.loadingTimerID;
-          resolve();
-        }
-      });
-      this.browser.addEventListener(
-        "load",
-        () => {
-          // We're handling the 'load' event before it bubbles up to the usual
-          // (non-capturing) event handlers. Let it bubble up before resolving.
-          this.browser.loadingTimerID = setTimeout(() => {
+      this.selectMenuItem(commandID);
+      this._box.hidden = this._splitter.hidden = false;
+
+      this._box.setAttribute("checked", "true");
+      this._box.setAttribute("sidebarcommand", commandID);
+
+      let { icon, url, title, sourceL10nEl, contextMenuId } =
+        this.sidebars.get(commandID);
+      if (icon) {
+        this._switcherTarget.style.setProperty(
+          "--webextension-menuitem-image",
+          icon
+        );
+      } else {
+        this._switcherTarget.style.removeProperty(
+          "--webextension-menuitem-image"
+        );
+      }
+
+      if (contextMenuId) {
+        this._box.setAttribute("context", contextMenuId);
+      } else {
+        this._box.removeAttribute("context");
+      }
+
+      // use to live update <tree> elements if the locale changes
+      this.lastOpenedId = commandID;
+      // These title changes only apply to the old sidebar menu
+      if (!this.sidebarRevampEnabled) {
+        this.title = title;
+        // Keep the title element in the switcher in sync with any l10n changes.
+        this.observeTitleChanges(sourceL10nEl);
+      }
+
+      this.browser.setAttribute("src", url); // kick off async load
+
+      if (this.browser.contentDocument.location.href != url) {
+        // make sure to clear the timeout if the load is aborted
+        this.browser.addEventListener("unload", () => {
+          if (this.browser.loadingTimerID) {
+            clearTimeout(this.browser.loadingTimerID);
             delete this.browser.loadingTimerID;
             resolve();
-          }, 0);
-        },
-        { capture: true, once: true }
-      );
-    });
-  },
-
-  /**
-   * Wait for a URI's progress to have stopped after loading
-   * in a remote sidebar browser.
-   *
-   * @param {URI} targetURI to check has loaded.
-   * @returns {Promise<void>}
-   */
-  async _hasProgressStopped(targetURI) {
-    return new Promise(resolve => {
-      let b = this.browser;
-
-      if (
-        b.currentURI?.equalsExceptRef(targetURI) &&
-        !b.webProgress.isLoadingDocument
-      ) {
-        resolve();
-        return;
-      }
-
-      let referenceKeeper = new Set();
-      const { STATE_IS_WINDOW, STATE_STOP } = Ci.nsIWebProgressListener;
-      let progListener = {
-        onStateChange: (webProgress, request, flags, _status) => {
-          if (
-            flags & STATE_IS_WINDOW &&
-            flags & STATE_STOP &&
-            webProgress.isTopLevel &&
-            (request?.originalURI?.equalsExceptRef(targetURI) ||
-              b.currentURI?.equalsExceptRef(targetURI))
-          ) {
-            resolve();
-            referenceKeeper.delete(filter);
-            b.removeProgressListener(filter);
           }
-        },
-        QueryInterface: ChromeUtils.generateQI([
-          "nsIWebProgressListener",
-          "nsISupportsWeakReference",
-        ]),
-      };
-      const filter = Cc[
-        "@mozilla.org/appshell/component/browser-status-filter;1"
-      ].createInstance(Ci.nsIWebProgress);
-      filter.addProgressListener(
-        progListener,
-        Ci.nsIWebProgress.NOTIFY_STATE_WINDOW
-      );
-      referenceKeeper.add(filter);
-      this.browser.addProgressListener(
-        filter,
-        Ci.nsIWebProgress.NOTIFY_STATE_WINDOW
-      );
+        });
+        this.browser.addEventListener(
+          "load",
+          () => {
+            // We're handling the 'load' event before it bubbles up to the usual
+            // (non-capturing) event handlers. Let it bubble up before resolving.
+            this.browser.loadingTimerID = setTimeout(() => {
+              delete this.browser.loadingTimerID;
+              resolve();
+
+              // Now that the currentId is updated, fire a show event.
+              this._fireShowEvent();
+              this._recordBrowserSize();
+            }, 0);
+          },
+          { capture: true, once: true }
+        );
+      } else {
+        resolve();
+
+        // Now that the currentId is updated, fire a show event.
+        this._fireShowEvent();
+        this._recordBrowserSize();
+      }
     });
   },
 
@@ -1736,22 +1600,12 @@ var SidebarController = {
     this.selectMenuItem("");
 
     // Replace the document currently displayed in the sidebar with about:blank
-    // so that we can free memory by unloading the page.
-    if (this.browser.isRemoteBrowser) {
-      let nullPrincipal = Services.scriptSecurityManager.createNullPrincipal(
-        {}
-      );
-      this.browser.loadURI(Services.io.newURI("about:blank"), {
-        triggeringPrincipal: nullPrincipal,
-      });
-      this.browser.createAboutBlankDocumentViewer(nullPrincipal, nullPrincipal);
-    } else {
-      // We need to explicitly create a new content viewer because the old one
-      // doesn't get destroyed until about:blank has loaded (which does not happen
-      // as long as the element is hidden).
-      this.browser.setAttribute("src", "about:blank");
-      this.browser.docShell?.createAboutBlankDocumentViewer(null, null);
-    }
+    // so that we can free memory by unloading the page. We need to explicitly
+    // create a new content viewer because the old one doesn't get destroyed
+    // until about:blank has loaded (which does not happen as long as the
+    // element is hidden).
+    this.browser.setAttribute("src", "about:blank");
+    this.browser.docShell?.createAboutBlankDocumentViewer(null, null);
 
     this._box.removeAttribute("checked");
     this._box.removeAttribute("context");
diff --git a/browser/components/sidebar/sidebar-page.mjs b/browser/components/sidebar/sidebar-page.mjs
index 5b421fcb7dc5..c79c4784fa97 100644
--- a/browser/components/sidebar/sidebar-page.mjs
+++ b/browser/components/sidebar/sidebar-page.mjs
@@ -39,7 +39,7 @@ export class SidebarPage extends MozLitElement {
   }
 
   get topWindow() {
-    return this.ownerGlobal.browsingContext.topChromeWindow;
+    return this.ownerGlobal.top;
   }
 
   get sidebarController() {
diff --git a/browser/components/sidebar/tests/browser/browser.toml b/browser/components/sidebar/tests/browser/browser.toml
index 5d45b4642499..bd032a674458 100644
--- a/browser/components/sidebar/tests/browser/browser.toml
+++ b/browser/components/sidebar/tests/browser/browser.toml
@@ -41,8 +41,6 @@ run-if = ["os == 'mac'"] # Mac only feature
 
 ["browser_sidebar_prefs.js"]
 
-["browser_sidebar_remote.js"]
-
 ["browser_syncedtabs_sidebar.js"]
 
 ["browser_toolbar_sidebar_button.js"]
diff --git a/browser/components/sidebar/tests/browser/browser_customize_sidebar.js b/browser/components/sidebar/tests/browser/browser_customize_sidebar.js
index 676de5f7a176..c63ffa907a34 100644
--- a/browser/components/sidebar/tests/browser/browser_customize_sidebar.js
+++ b/browser/components/sidebar/tests/browser/browser_customize_sidebar.js
@@ -10,8 +10,8 @@ const TAB_DIRECTION_PREF = "sidebar.verticalTabs";
 
 async function showCustomizePanel(win) {
   await win.SidebarController.show("viewCustomizeSidebar");
-  return TestUtils.waitForCondition(() => {
-    const document = win.SidebarController.browser.contentDocument;
+  const document = win.SidebarController.browser.contentDocument;
+  return TestUtils.waitForCondition(async () => {
     const component = document.querySelector("sidebar-customize");
     if (!component?.positionInputs || !component?.visibilityInputs) {
       return false;
@@ -122,13 +122,12 @@ add_task(async function test_customize_not_added_in_menubar() {
 
 add_task(async function test_manage_preferences_navigation() {
   const win = await BrowserTestUtils.openNewBrowserWindow();
-  await showCustomizePanel(win);
-  const sidebarBox = win.document.getElementById("sidebar-box");
-  await BrowserTestUtils.waitForCondition(
-    () => BrowserTestUtils.isVisible(sidebarBox),
-    "Sidebar panel is visible"
-  );
-
+  const { SidebarController } = win;
+  const { contentWindow } = SidebarController.browser;
+  const sidebar = document.querySelector("sidebar-main");
+  ok(sidebar, "Sidebar is shown.");
+  await sidebar.updateComplete;
+  await toggleSidebarPanel(win, "viewCustomizeSidebar");
   let customizeDocument = win.SidebarController.browser.contentDocument;
   const customizeComponent =
     customizeDocument.querySelector("sidebar-customize");
@@ -139,7 +138,7 @@ add_task(async function test_manage_preferences_navigation() {
   EventUtils.synthesizeMouseAtCenter(
     manageSettings.querySelector("a"),
     {},
-    customizeDocument.ownerGlobal
+    contentWindow
   );
   await BrowserTestUtils.waitForCondition(
     () =>
diff --git a/browser/components/sidebar/tests/browser/browser_sidebar_remote.js b/browser/components/sidebar/tests/browser/browser_sidebar_remote.js
deleted file mode 100644
index dd2971c8c770..000000000000
--- a/browser/components/sidebar/tests/browser/browser_sidebar_remote.js
+++ /dev/null
@@ -1,47 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
-   https://creativecommons.org/publicdomain/zero/1.0/ */
-
-"use strict";
-
-add_task(async function test_remote_sidebar_browser() {
-  await SpecialPowers.pushPrefEnv({
-    set: [
-      ["browser.shopping.experience2023.integratedSidebar", true],
-      ["sidebar.main.tools", "reviewchecker,syncedtabs,history"],
-    ],
-  });
-  const sidebar = document.querySelector("sidebar-main");
-  ok(sidebar, "Sidebar is shown.");
-
-  // Non-remote sidebar
-  await SidebarController.show("viewHistorySidebar");
-  ok(SidebarController.browser, "Sidebar browser is shown.");
-  ok(
-    !SidebarController.browser.hasAttribute("remote"),
-    "Sidebar browser is not remote."
-  );
-
-  // Remote content sidebar
-  await SidebarController.show("viewReviewCheckerSidebar");
-  ok(SidebarController.browser, "Sidebar browser is shown.");
-  Assert.equal(
-    SidebarController.browser.getAttribute("remote"),
-    "true",
-    "Sidebar browser is remote."
-  );
-  Assert.equal(
-    SidebarController.browser.getAttribute("type"),
-    "content",
-    "Sidebar browser is remote."
-  );
-
-  // Another non-remote sidebar
-  await SidebarController.show("viewTabsSidebar");
-  ok(SidebarController.browser, "Sidebar browser is shown.");
-  ok(
-    !SidebarController.browser.hasAttribute("remote"),
-    "Sidebar browser is not remote."
-  );
-
-  await SpecialPowers.popPrefEnv();
-});
diff --git a/toolkit/modules/LightweightThemeConsumer.sys.mjs b/toolkit/modules/LightweightThemeConsumer.sys.mjs
index f188c682d59b..7bfa61c8183b 100644
--- a/toolkit/modules/LightweightThemeConsumer.sys.mjs
+++ b/toolkit/modules/LightweightThemeConsumer.sys.mjs
@@ -274,7 +274,7 @@ LightweightThemeConsumer.prototype = {
       // If enabled, apply the dark theme variant to private browsing windows.
       if (
         !Services.prefs.getBoolPref("browser.theme.dark-private-windows") ||
-        !this._win.browsingContext.usePrivateBrowsing ||
+        !lazy.PrivateBrowsingUtils.isWindowPrivate(this._win) ||
         lazy.PrivateBrowsingUtils.permanentPrivateBrowsing
       ) {
         return false;