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diff --git a/browser/modules/EveryWindow.sys.mjs b/browser/modules/EveryWindow.sys.mjs
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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 enables consumers to register callbacks on every
+ * current and future browser window.
+ *
+ * Usage: EveryWindow.registerCallback(id, init, uninit);
+ *        EveryWindow.unregisterCallback(id);
+ *
+ * id is expected to be a unique value that identifies the
+ * consumer, to be used for unregistration. If the id is already
+ * in use, registerCallback returns false without doing anything.
+ *
+ * Each callback will receive the window for which it is presently
+ * being called as the first argument.
+ *
+ * init is called on every existing window at the time of registration,
+ * and on all future windows at browser-delayed-startup-finished.
+ *
+ * uninit is called on every existing window if requested at the time
+ * of unregistration, and at the time of domwindowclosed.
+ * If the window is closing, a second argument is passed with value `true`.
+ */
+
+var initialized = false;
+var callbacks = new Map();
+
+function callForEveryWindow(callback) {
+  let windowList = Services.wm.getEnumerator("navigator:browser");
+  for (let win of windowList) {
+    win.delayedStartupPromise.then(() => {
+      callback(win);
+    });
+  }
+}
+
+export const EveryWindow = {
+  /**
+   * The current list of all browser windows whose delayedStartupPromise has resolved
+   */
+  get readyWindows() {
+    return Array.from(Services.wm.getEnumerator("navigator:browser")).filter(
+      win => win.gBrowserInit?.delayedStartupFinished
+    );
+  },
+
+  /**
+   * Registers init and uninit functions to be called on every window.
+   *
+   * @param {string} id A unique identifier for the consumer, to be
+   *   used for unregistration.
+   * @param {function} init The function to be called on every currently
+   *   existing window and every future window after delayed startup.
+   * @param {function} uninit The function to be called on every window
+   *   at the time of callback unregistration or after domwindowclosed.
+   * @returns {boolean} Returns false if the id was taken, else true.
+   */
+  registerCallback: function EW_registerCallback(id, init, uninit) {
+    if (callbacks.has(id)) {
+      return false;
+    }
+
+    if (!initialized) {
+      let addUnloadListener = win => {
+        function observer(subject, topic, data) {
+          if (topic == "domwindowclosed" && subject === win) {
+            Services.ww.unregisterNotification(observer);
+            for (let c of callbacks.values()) {
+              c.uninit(win, true);
+            }
+          }
+        }
+        Services.ww.registerNotification(observer);
+      };
+
+      Services.obs.addObserver(win => {
+        for (let c of callbacks.values()) {
+          c.init(win);
+        }
+        addUnloadListener(win);
+      }, "browser-delayed-startup-finished");
+
+      callForEveryWindow(addUnloadListener);
+
+      initialized = true;
+    }
+
+    callForEveryWindow(init);
+    callbacks.set(id, { id, init, uninit });
+
+    return true;
+  },
+
+  /**
+   * Unregisters a previously registered consumer.
+   *
+   * @param {string} id The id to unregister.
+   * @param {boolean} [callUninit=true] Whether to call the registered uninit
+   *   function on every window.
+   */
+  unregisterCallback: function EW_unregisterCallback(id, callUninit = true) {
+    if (!callbacks.has(id)) {
+      return;
+    }
+
+    if (callUninit) {
+      callForEveryWindow(callbacks.get(id).uninit);
+    }
+
+    callbacks.delete(id);
+  },
+};