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diff --git a/remote/cdp/observers/ContextObserver.sys.mjs b/remote/cdp/observers/ContextObserver.sys.mjs
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index 0000000000..d0d6fd1f93
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+++ b/remote/cdp/observers/ContextObserver.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/. */
+
+/**
+ * Helper class to coordinate Runtime and Page events.
+ * Events have to be sent in the following order:
+ *  - Runtime.executionContextDestroyed
+ *  - Page.frameNavigated
+ *  - Runtime.executionContextCreated
+ *
+ * This class also handles the special case of Pages going from/to the BF cache.
+ * When you navigate to a new URL, the previous document may be stored in the BF Cache.
+ * All its asynchronous operations are frozen (XHR, timeouts, ...) and a `pagehide` event
+ * is fired for this document. We then navigate to the new URL.
+ * If the user navigates back to the previous page, the page is resurected from the
+ * cache. A `pageshow` event is fired and its asynchronous operations are resumed.
+ *
+ * When a page is in the BF Cache, we should consider it as frozen and shouldn't try
+ * to execute any javascript. So that the ExecutionContext should be considered as
+ * being destroyed and the document navigated.
+ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  EventEmitter: "resource://gre/modules/EventEmitter.sys.mjs",
+
+  executeSoon: "chrome://remote/content/shared/Sync.sys.mjs",
+});
+
+export class ContextObserver {
+  constructor(chromeEventHandler) {
+    this.chromeEventHandler = chromeEventHandler;
+    lazy.EventEmitter.decorate(this);
+
+    this._fissionEnabled = Services.appinfo.fissionAutostart;
+
+    this.chromeEventHandler.addEventListener("DOMWindowCreated", this, {
+      mozSystemGroup: true,
+    });
+
+    // Listen for pageshow and pagehide to track pages going in/out to/from the BF Cache
+    this.chromeEventHandler.addEventListener("pageshow", this, {
+      mozSystemGroup: true,
+    });
+    this.chromeEventHandler.addEventListener("pagehide", this, {
+      mozSystemGroup: true,
+    });
+
+    Services.obs.addObserver(this, "document-element-inserted");
+    Services.obs.addObserver(this, "inner-window-destroyed");
+
+    // With Fission disabled the `DOMWindowCreated` event is fired too late.
+    // Use the `webnavigation-create` notification instead.
+    if (!this._fissionEnabled) {
+      Services.obs.addObserver(this, "webnavigation-create");
+    }
+    Services.obs.addObserver(this, "webnavigation-destroy");
+  }
+
+  destructor() {
+    this.chromeEventHandler.removeEventListener("DOMWindowCreated", this, {
+      mozSystemGroup: true,
+    });
+    this.chromeEventHandler.removeEventListener("pageshow", this, {
+      mozSystemGroup: true,
+    });
+    this.chromeEventHandler.removeEventListener("pagehide", this, {
+      mozSystemGroup: true,
+    });
+
+    Services.obs.removeObserver(this, "document-element-inserted");
+    Services.obs.removeObserver(this, "inner-window-destroyed");
+
+    if (!this._fissionEnabled) {
+      Services.obs.removeObserver(this, "webnavigation-create");
+    }
+    Services.obs.removeObserver(this, "webnavigation-destroy");
+  }
+
+  handleEvent({ type, target, persisted }) {
+    const window = target.defaultView;
+    const frameId = window.browsingContext.id;
+    const id = window.windowGlobalChild.innerWindowId;
+
+    switch (type) {
+      case "DOMWindowCreated":
+        // Do not pass `id` here as that's the new document ID instead of the old one
+        // that is destroyed. Instead, pass the frameId and let the listener figure out
+        // what ExecutionContext(s) to destroy.
+        this.emit("context-destroyed", { frameId });
+
+        // With Fission enabled the frame is attached early enough so that
+        // expected network requests and responses are handles afterward.
+        // Otherwise send the event when `webnavigation-create` is received.
+        if (this._fissionEnabled) {
+          this.emit("frame-attached", { frameId, window });
+        }
+
+        break;
+
+      case "pageshow":
+        // `persisted` is true when this is about a page being resurected from BF Cache
+        if (!persisted) {
+          return;
+        }
+        // XXX(ochameau) we might have to emit FrameNavigate here to properly handle BF Cache
+        // scenario in Page domain events
+        this.emit("context-created", { windowId: id, window });
+        this.emit("script-loaded", { windowId: id, window });
+        break;
+
+      case "pagehide":
+        // `persisted` is true when this is about a page being frozen into BF Cache
+        if (!persisted) {
+          return;
+        }
+        this.emit("context-destroyed", { windowId: id });
+        break;
+    }
+  }
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "document-element-inserted":
+        const window = subject.defaultView;
+
+        // Ignore events without a window and those from other tabs
+        if (
+          !window ||
+          window.docShell.chromeEventHandler !== this.chromeEventHandler
+        ) {
+          return;
+        }
+
+        // Send when the document gets attached to the window, and its location
+        // is available.
+        this.emit("frame-navigated", {
+          frameId: window.browsingContext.id,
+          window,
+        });
+
+        const id = window.windowGlobalChild.innerWindowId;
+        this.emit("context-created", { windowId: id, window });
+        // Delay script-loaded to allow context cleanup to happen first
+        lazy.executeSoon(() => {
+          this.emit("script-loaded", { windowId: id, window });
+        });
+        break;
+      case "inner-window-destroyed":
+        const windowId = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
+        this.emit("context-destroyed", { windowId });
+        break;
+      case "webnavigation-create":
+        subject.QueryInterface(Ci.nsIDocShell);
+        this.onDocShellCreated(subject);
+        break;
+      case "webnavigation-destroy":
+        subject.QueryInterface(Ci.nsIDocShell);
+        this.onDocShellDestroyed(subject);
+        break;
+    }
+  }
+
+  onDocShellCreated(docShell) {
+    this.emit("frame-attached", {
+      frameId: docShell.browsingContext.id,
+      window: docShell.browsingContext.window,
+    });
+  }
+
+  onDocShellDestroyed(docShell) {
+    this.emit("frame-detached", {
+      frameId: docShell.browsingContext.id,
+    });
+  }
+}