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diff --git a/browser/modules/BrowserWindowTracker.jsm b/browser/modules/BrowserWindowTracker.jsm
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * This module tracks each browser window and informs network module
+ * the current selected tab's content outer window ID.
+ */
+
+var EXPORTED_SYMBOLS = ["BrowserWindowTracker"];
+
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+const lazy = {};
+
+// Lazy getters
+ChromeUtils.defineESModuleGetters(lazy, {
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+  PromiseUtils: "resource://gre/modules/PromiseUtils.sys.mjs",
+});
+
+// Constants
+const TAB_EVENTS = ["TabBrowserInserted", "TabSelect"];
+const WINDOW_EVENTS = ["activate", "unload"];
+const DEBUG = false;
+
+// Variables
+let _lastCurrentBrowserId = 0;
+let _trackedWindows = [];
+
+// Global methods
+function debug(s) {
+  if (DEBUG) {
+    dump("-*- UpdateBrowserIDHelper: " + s + "\n");
+  }
+}
+
+function _updateCurrentBrowserId(browser) {
+  if (
+    !browser.browserId ||
+    browser.browserId === _lastCurrentBrowserId ||
+    browser.ownerGlobal != _trackedWindows[0]
+  ) {
+    return;
+  }
+
+  // Guard on DEBUG here because materializing a long data URI into
+  // a JS string for concatenation is not free.
+  if (DEBUG) {
+    debug(
+      `Current window uri=${browser.currentURI?.spec} browser id=${browser.browserId}`
+    );
+  }
+
+  _lastCurrentBrowserId = browser.browserId;
+  let idWrapper = Cc["@mozilla.org/supports-PRUint64;1"].createInstance(
+    Ci.nsISupportsPRUint64
+  );
+  idWrapper.data = _lastCurrentBrowserId;
+  Services.obs.notifyObservers(idWrapper, "net:current-browser-id");
+}
+
+function _handleEvent(event) {
+  switch (event.type) {
+    case "TabBrowserInserted":
+      if (
+        event.target.ownerGlobal.gBrowser.selectedBrowser ===
+        event.target.linkedBrowser
+      ) {
+        _updateCurrentBrowserId(event.target.linkedBrowser);
+      }
+      break;
+    case "TabSelect":
+      _updateCurrentBrowserId(event.target.linkedBrowser);
+      break;
+    case "activate":
+      WindowHelper.onActivate(event.target);
+      break;
+    case "unload":
+      WindowHelper.removeWindow(event.currentTarget);
+      break;
+  }
+}
+
+function _trackWindowOrder(window) {
+  if (window.windowState == window.STATE_MINIMIZED) {
+    let firstMinimizedWindow = _trackedWindows.findIndex(
+      w => w.windowState == w.STATE_MINIMIZED
+    );
+    if (firstMinimizedWindow == -1) {
+      firstMinimizedWindow = _trackedWindows.length;
+    }
+    _trackedWindows.splice(firstMinimizedWindow, 0, window);
+  } else {
+    _trackedWindows.unshift(window);
+  }
+}
+
+function _untrackWindowOrder(window) {
+  let idx = _trackedWindows.indexOf(window);
+  if (idx >= 0) {
+    _trackedWindows.splice(idx, 1);
+  }
+}
+
+// Methods that impact a window. Put into single object for organization.
+var WindowHelper = {
+  addWindow(window) {
+    // Add event listeners
+    TAB_EVENTS.forEach(function (event) {
+      window.gBrowser.tabContainer.addEventListener(event, _handleEvent);
+    });
+    WINDOW_EVENTS.forEach(function (event) {
+      window.addEventListener(event, _handleEvent);
+    });
+
+    _trackWindowOrder(window);
+
+    // Update the selected tab's content outer window ID.
+    _updateCurrentBrowserId(window.gBrowser.selectedBrowser);
+  },
+
+  removeWindow(window) {
+    _untrackWindowOrder(window);
+
+    // Remove the event listeners
+    TAB_EVENTS.forEach(function (event) {
+      window.gBrowser.tabContainer.removeEventListener(event, _handleEvent);
+    });
+    WINDOW_EVENTS.forEach(function (event) {
+      window.removeEventListener(event, _handleEvent);
+    });
+  },
+
+  onActivate(window) {
+    // If this window was the last focused window, we don't need to do anything
+    if (window == _trackedWindows[0]) {
+      return;
+    }
+
+    _untrackWindowOrder(window);
+    _trackWindowOrder(window);
+
+    _updateCurrentBrowserId(window.gBrowser.selectedBrowser);
+  },
+};
+
+const BrowserWindowTracker = {
+  pendingWindows: new Map(),
+
+  /**
+   * Get the most recent browser window.
+   *
+   * @param options an object accepting the arguments for the search.
+   *        * private: true to restrict the search to private windows
+   *            only, false to restrict the search to non-private only.
+   *            Omit the property to search in both groups.
+   *        * allowPopups: true if popup windows are permissable.
+   */
+  getTopWindow(options = {}) {
+    for (let win of _trackedWindows) {
+      if (
+        !win.closed &&
+        (options.allowPopups || win.toolbar.visible) &&
+        (!("private" in options) ||
+          lazy.PrivateBrowsingUtils.permanentPrivateBrowsing ||
+          lazy.PrivateBrowsingUtils.isWindowPrivate(win) == options.private)
+      ) {
+        return win;
+      }
+    }
+    return null;
+  },
+
+  /**
+   * Get a window that is in the process of loading. Only supports windows
+   * opened via the `openWindow` function in this module or that have been
+   * registered with the `registerOpeningWindow` function.
+   *
+   * @param {Object} options
+   *   Options for the search.
+   * @param {boolean} [options.private]
+   *   true to restrict the search to private windows only, false to restrict
+   *   the search to non-private only. Omit the property to search in both
+   *   groups.
+   *
+   * @returns {Promise<Window> | null}
+   */
+  getPendingWindow(options = {}) {
+    for (let pending of this.pendingWindows.values()) {
+      if (
+        !("private" in options) ||
+        lazy.PrivateBrowsingUtils.permanentPrivateBrowsing ||
+        pending.isPrivate == options.private
+      ) {
+        return pending.deferred.promise;
+      }
+    }
+    return null;
+  },
+
+  /**
+   * Registers a browser window that is in the process of opening. Normally it
+   * would be preferable to use the standard method for opening the window from
+   * this module.
+   *
+   * @param {Window} window
+   *   The opening window.
+   * @param {boolean} isPrivate
+   *   Whether the opening window is a private browsing window.
+   */
+  registerOpeningWindow(window, isPrivate) {
+    let deferred = lazy.PromiseUtils.defer();
+
+    this.pendingWindows.set(window, {
+      isPrivate,
+      deferred,
+    });
+  },
+
+  /**
+   * A standard function for opening a new browser window.
+   *
+   * @param {Object} [options]
+   *   Options for the new window.
+   * @param {boolean} [options.private]
+   *   True to make the window a private browsing window.
+   * @param {String} [options.features]
+   *   Additional window features to give the new window.
+   * @param {nsIArray | nsISupportsString} [options.args]
+   *   Arguments to pass to the new window.
+   *
+   * @returns {Window}
+   */
+  openWindow({
+    private: isPrivate = false,
+    features = undefined,
+    args = null,
+  } = {}) {
+    let windowFeatures = "chrome,dialog=no,all";
+    if (features) {
+      windowFeatures += `,${features}`;
+    }
+    if (isPrivate) {
+      windowFeatures += ",private";
+    }
+
+    let win = Services.ww.openWindow(
+      null,
+      AppConstants.BROWSER_CHROME_URL,
+      "_blank",
+      windowFeatures,
+      args
+    );
+    this.registerOpeningWindow(win, isPrivate);
+    return win;
+  },
+
+  /**
+   * Number of currently open browser windows.
+   */
+  get windowCount() {
+    return _trackedWindows.length;
+  },
+
+  /**
+   * Array of browser windows ordered by z-index, in reverse order.
+   * This means that the top-most browser window will be the first item.
+   */
+  get orderedWindows() {
+    // Clone the windows array immediately as it may change during iteration,
+    // we'd rather have an outdated order than skip/revisit windows.
+    return [..._trackedWindows];
+  },
+
+  getAllVisibleTabs() {
+    let tabs = [];
+    for (let win of BrowserWindowTracker.orderedWindows) {
+      for (let tab of win.gBrowser.visibleTabs) {
+        // Only use tabs which are not discarded / unrestored
+        if (tab.linkedPanel) {
+          let { contentTitle, browserId } = tab.linkedBrowser;
+          tabs.push({ contentTitle, browserId });
+        }
+      }
+    }
+    return tabs;
+  },
+
+  track(window) {
+    let pending = this.pendingWindows.get(window);
+    if (pending) {
+      this.pendingWindows.delete(window);
+      // Waiting for delayed startup to complete ensures that this new window
+      // has started loading its initial urls.
+      window.delayedStartupPromise.then(() => pending.deferred.resolve(window));
+    }
+
+    return WindowHelper.addWindow(window);
+  },
+
+  getBrowserById(browserId) {
+    for (let win of BrowserWindowTracker.orderedWindows) {
+      for (let tab of win.gBrowser.visibleTabs) {
+        if (tab.linkedPanel && tab.linkedBrowser.browserId === browserId) {
+          return tab.linkedBrowser;
+        }
+      }
+    }
+    return null;
+  },
+
+  // For tests only, this function will remove this window from the list of
+  // tracked windows. Please don't forget to add it back at the end of your
+  // tests!
+  untrackForTestsOnly(window) {
+    return WindowHelper.removeWindow(window);
+  },
+};