diff --git a/remote/shared/messagehandler/MessageHandler.jsm b/remote/shared/messagehandler/MessageHandler.jsm
index 4bf3560a09f2..ffcdd44689b0 100644
--- a/remote/shared/messagehandler/MessageHandler.jsm
+++ b/remote/shared/messagehandler/MessageHandler.jsm
@@ -88,6 +88,28 @@ class MessageHandler extends EventEmitter {
     this.emit("message-handler-destroyed", this);
   }
 
+  /**
+   * Emit a message-handler-event. Such events should bubble up to the root of
+   * a MessageHandler network.
+   *
+   * @param {String} method
+   *     A string literal of the form [module name].[event name]. This is the
+   *     event name.
+   * @param {Object} params
+   *     The event parameters.
+   */
+  emitMessageHandlerEvent(method, params) {
+    this.emit("message-handler-event", {
+      // TODO: The messageHandlerInfo needs to be wrapped in the event so
+      // that consumers can check the type/context. Once MessageHandlerRegistry
+      // becomes context-specific (Bug 1722659), only the sessionId will be
+      // required.
+      messageHandlerInfo: this._messageHandlerInfo,
+      method,
+      params,
+    });
+  }
+
   /**
    * @typedef {Object} CommandDestination
    * @property {String} type - One of MessageHandler.type.
diff --git a/remote/shared/messagehandler/MessageHandlerRegistry.jsm b/remote/shared/messagehandler/MessageHandlerRegistry.jsm
index 31a384d2b1cd..4d8de1a459da 100644
--- a/remote/shared/messagehandler/MessageHandlerRegistry.jsm
+++ b/remote/shared/messagehandler/MessageHandlerRegistry.jsm
@@ -11,6 +11,8 @@ const { XPCOMUtils } = ChromeUtils.import(
 );
 
 XPCOMUtils.defineLazyModuleGetters(this, {
+  EventEmitter: "resource://gre/modules/EventEmitter.jsm",
+
   Log: "chrome://remote/content/shared/Log.jsm",
   MessageHandlerInfo:
     "chrome://remote/content/shared/messagehandler/MessageHandlerInfo.jsm",
@@ -61,8 +63,10 @@ function getMessageHandlerClass(type) {
  *
  * Note: this is still created as a class, but exposed as a singleton.
  */
-class MessageHandlerRegistryClass {
+class MessageHandlerRegistryClass extends EventEmitter {
   constructor() {
+    super();
+
     /**
      * Map of all message handlers registered in this process.
      * Keys are based on session id, message handler type and message handler
@@ -75,6 +79,7 @@ class MessageHandlerRegistryClass {
     this._onMessageHandlerDestroyed = this._onMessageHandlerDestroyed.bind(
       this
     );
+    this._onMessageHandlerEvent = this._onMessageHandlerEvent.bind(this);
   }
 
   /**
@@ -154,6 +159,30 @@ class MessageHandlerRegistryClass {
     return messageHandler;
   }
 
+  /**
+   * Retrieve an already registered RootMessageHandler instance matching the
+   * provided sessionId.
+   *
+   * @param {String} sessionId
+   *     ID of the session the handler is used for.
+   * @return {RootMessageHandler}
+   *     A RootMessageHandler instance.
+   * @throws {Error}
+   *     If no root MessageHandler can be found for the provided session id.
+   */
+  getRootMessageHandler(sessionId) {
+    const rootMessageHandler = this.getExistingMessageHandler(
+      sessionId,
+      RootMessageHandler.type
+    );
+    if (!rootMessageHandler) {
+      throw new Error(
+        `Unable to find a root MessageHandler for session id ${sessionId}`
+      );
+    }
+    return rootMessageHandler;
+  }
+
   toString() {
     return `[object ${this.constructor.name}]`;
   }
@@ -182,6 +211,7 @@ class MessageHandlerRegistryClass {
       "message-handler-destroyed",
       this._onMessageHandlerDestroyed
     );
+    messageHandler.on("message-handler-event", this._onMessageHandlerEvent);
     return messageHandler;
   }
 
@@ -198,13 +228,20 @@ class MessageHandlerRegistryClass {
 
   // Event handlers
 
-  _onMessageHandlerDestroyed(evt, messageHandler) {
+  _onMessageHandlerDestroyed(eventName, messageHandler) {
     messageHandler.off(
       "message-handler-destroyed",
       this._onMessageHandlerDestroyed
     );
+    messageHandler.off("message-handler-event", this._onMessageHandlerEvent);
     this._unregisterMessageHandler(messageHandler);
   }
+
+  _onMessageHandlerEvent(eventName, messageHandlerEvent) {
+    // The registry simply re-emits MessageHandler events so that consumers
+    // don't have to attach listeners to individual MessageHandler instances.
+    this.emit("message-handler-registry-event", messageHandlerEvent);
+  }
 }
 
 const MessageHandlerRegistry = new MessageHandlerRegistryClass();
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js b/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js
index 72479004fa0d..841bb412735d 100644
--- a/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js
@@ -6,37 +6,10 @@
 add_task(async function test_broadcasting_with_frames() {
   info("Navigate the initial tab to the test URL");
   const tab = gBrowser.selectedTab;
-
-  // Create a test page with 2 iframes:
-  // - one with a different eTLD+1 (example.com)
-  // - one with a nested iframe on a different eTLD+1 (example.net)
-  //
-  // Overall the document structure should look like:
-  //
-  // html (example.org)
-  //   iframe (example.org)
-  //     iframe (example.net)
-  //   iframe(example.com)
-  //
-  // Which means we should have 4 browsing contexts in total.
-
-  // Create the markup for an example.net frame nested in an example.com frame.
-  const NESTED_FRAME_MARKUP = createFrameForUri(
-    `http://example.org/document-builder.sjs?html=${createFrame("example.net")}`
-  );
-
-  // Combine the nested frame markup created above with an example.com frame.
-  const TEST_URI_MARKUP = `${NESTED_FRAME_MARKUP}${createFrame("example.com")}`;
-
-  // Create the test page URI on example.org.
-  const TEST_URI = `http://example.org/document-builder.sjs?html=${encodeURI(
-    TEST_URI_MARKUP
-  )}`;
-
-  await loadURL(tab.linkedBrowser, TEST_URI);
+  await loadURL(tab.linkedBrowser, createTestMarkupWithFrames());
 
   const contexts = tab.linkedBrowser.browsingContext.getAllBrowsingContextsInSubtree();
-  is(contexts.length, 4, "Test tab has 3 children contexts");
+  is(contexts.length, 4, "Test tab has 3 children contexts (4 in total)");
 
   const rootMessageHandler = createRootMessageHandler(
     "session-id-broadcasting_with_frames"
@@ -63,20 +36,3 @@ add_task(async function test_broadcasting_with_frames() {
 
   rootMessageHandler.destroy();
 });
-
-/**
- * Create inline markup for a simple iframe that can be used with
- * document-builder.sjs. The iframe will be served under the provided domain.
- *
- * @param {String} domain
- *     A domain (eg "example.com"), compatible with build/pgo/server-locations.txt
- */
-function createFrame(domain) {
-  return createFrameForUri(
-    `http://${domain}/document-builder.sjs?html=frame-${domain}`
-  );
-}
-
-function createFrameForUri(uri) {
-  return `<iframe src="${encodeURI(uri)}"></iframe>`;
-}
diff --git a/remote/shared/messagehandler/test/browser/browser.ini b/remote/shared/messagehandler/test/browser/browser.ini
index 6b1abc19df67..9423daa793cc 100644
--- a/remote/shared/messagehandler/test/browser/browser.ini
+++ b/remote/shared/messagehandler/test/browser/browser.ini
@@ -7,6 +7,7 @@ support-files =
 prefs =
   remote.messagehandler.modulecache.useBrowserTestRoot=true
 
+[browser_events.js]
 [browser_handle_command_errors.js]
 [browser_handle_simple_command.js]
 [browser_registry.js]
diff --git a/remote/shared/messagehandler/test/browser/browser_events.js b/remote/shared/messagehandler/test/browser/browser_events.js
new file mode 100644
index 000000000000..7901dadc5298
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_events.js
@@ -0,0 +1,248 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { MessageHandlerRegistry } = ChromeUtils.import(
+  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm"
+);
+const { RootMessageHandler } = ChromeUtils.import(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+);
+const { WindowGlobalMessageHandler } = ChromeUtils.import(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+);
+
+/**
+ * Emit an event from a WindowGlobal module triggered by a specific command.
+ * Check that the event is emitted on the RootMessageHandler as well as on
+ * the parent process MessageHandlerRegistry.
+ */
+add_task(async function test_event() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "http://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-event");
+
+  const onTestEvent = rootMessageHandler.once("message-handler-event");
+  // MessageHandlerRegistry should forward all the message-handler-events.
+  const onRegistryEvent = MessageHandlerRegistry.once(
+    "message-handler-registry-event"
+  );
+
+  callTestEmitEvent(rootMessageHandler, browsingContext.id);
+
+  const messageHandlerEvent = await onTestEvent;
+  is(
+    messageHandlerEvent.method,
+    "event.testEvent",
+    "Received event.testEvent on the ROOT MessageHandler"
+  );
+  is(
+    messageHandlerEvent.params.text,
+    `event from ${browsingContext.id}`,
+    "Received the expected data in testEvent"
+  );
+  const registryEvent = await onRegistryEvent;
+  is(
+    registryEvent,
+    messageHandlerEvent,
+    "The event forwarded by the MessageHandlerRegistry is identical to the MessageHandler event"
+  );
+
+  rootMessageHandler.destroy();
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Emit an event from a Root module triggered by a specific command.
+ * Check that the event is emitted on the RootMessageHandler.
+ */
+add_task(async function test_root_event() {
+  const rootMessageHandler = createRootMessageHandler("session-id-root_event");
+
+  const onTestEvent = rootMessageHandler.once("message-handler-event");
+  rootMessageHandler.handleCommand({
+    moduleName: "event",
+    commandName: "testEmitRootEvent",
+    destination: {
+      type: RootMessageHandler.type,
+    },
+  });
+
+  const { method, params } = await onTestEvent;
+  is(
+    method,
+    "event.testRootEvent",
+    "Received event.testRootEvent on the ROOT MessageHandler"
+  );
+  is(
+    params.text,
+    "event from root",
+    "Received the expected payload in testRootEvent"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+/**
+ * Emit an event from a windowglobal-in-root module triggered by a specific command.
+ * Check that the event is emitted on the RootMessageHandler.
+ */
+add_task(async function test_windowglobal_in_root_event() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "http://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-windowglobal_in_root_event"
+  );
+
+  const onTestEvent = rootMessageHandler.once("message-handler-event");
+  rootMessageHandler.handleCommand({
+    moduleName: "event",
+    commandName: "testEmitWindowGlobalInRootEvent",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+
+  const { method, params } = await onTestEvent;
+  is(
+    method,
+    "event.testWindowGlobalInRootEvent",
+    "Received event.testWindowGlobalInRoot on the ROOT MessageHandler"
+  );
+  is(
+    params.text,
+    `windowglobal-in-root event for ${browsingContext.id}`,
+    "Received the expected payload in testWindowGlobalInRoot"
+  );
+
+  rootMessageHandler.destroy();
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Emit an event from a windowglobal module, but from 2 different sessions.
+ * Check that the event is emitted by the corresponding RootMessageHandler as
+ * well as by the parent process MessageHandlerRegistry.
+ */
+add_task(async function test_event_multisession() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "http://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContextId = tab.linkedBrowser.browsingContext.id;
+
+  const root1 = createRootMessageHandler("session-id-event_multisession-1");
+  let root1Events = 0;
+  const onRoot1Event = function(evtName, wrappedEvt) {
+    if (wrappedEvt.method === "event.testEvent") {
+      root1Events++;
+    }
+  };
+  root1.on("message-handler-event", onRoot1Event);
+
+  const root2 = createRootMessageHandler("session-id-event_multisession-2");
+  let root2Events = 0;
+  const onRoot2Event = function(evtName, wrappedEvt) {
+    if (wrappedEvt.method === "event.testEvent") {
+      root2Events++;
+    }
+  };
+  root2.on("message-handler-event", onRoot2Event);
+
+  let registryEvents = 0;
+  const onRegistryEvent = function(evtName, wrappedEvt) {
+    if (wrappedEvt.method === "event.testEvent") {
+      registryEvents++;
+    }
+  };
+  MessageHandlerRegistry.on("message-handler-registry-event", onRegistryEvent);
+
+  callTestEmitEvent(root1, browsingContextId);
+  callTestEmitEvent(root2, browsingContextId);
+
+  info("Wait for root1 event to be received");
+  await TestUtils.waitForCondition(() => root1Events === 1);
+  info("Wait for root2 event to be received");
+  await TestUtils.waitForCondition(() => root2Events === 1);
+
+  await TestUtils.waitForTick();
+  is(root1Events, 1, "Session 1 only received 1 event");
+  is(root2Events, 1, "Session 2 only received 1 event");
+  is(
+    registryEvents,
+    2,
+    "MessageHandlerRegistry forwarded events from both sessions"
+  );
+
+  root1.off("message-handler-event", onRoot1Event);
+  root2.off("message-handler-event", onRoot2Event);
+  MessageHandlerRegistry.off("message-handler-registry-event", onRegistryEvent);
+  root1.destroy();
+  root2.destroy();
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Test that events can be emitted from individual frame contexts and that
+ * events going through a shared content process MessageHandlerRegistry are not
+ * duplicated.
+ */
+add_task(async function test_event_with_frames() {
+  info("Navigate the initial tab to the test URL");
+  const tab = gBrowser.selectedTab;
+  await loadURL(tab.linkedBrowser, createTestMarkupWithFrames());
+
+  const contexts = tab.linkedBrowser.browsingContext.getAllBrowsingContextsInSubtree();
+  is(contexts.length, 4, "Test tab has 3 children contexts (4 in total)");
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-event_with_frames"
+  );
+
+  let rootEvents = [];
+  const onRootEvent = function(evtName, wrappedEvt) {
+    if (wrappedEvt.method === "event.testEvent") {
+      rootEvents.push(wrappedEvt.params.text);
+    }
+  };
+  rootMessageHandler.on("message-handler-event", onRootEvent);
+
+  for (const context of contexts) {
+    callTestEmitEvent(rootMessageHandler, context.id);
+    info("Wait for root event to be received");
+    await TestUtils.waitForCondition(() =>
+      rootEvents.includes(`event from ${context.id}`)
+    );
+  }
+
+  info("Wait for a bit and check that we did not receive duplicated events");
+  await TestUtils.waitForTick();
+  is(rootEvents.length, 4, "Only received 4 events");
+
+  rootMessageHandler.off("message-handler-event", onRootEvent);
+  rootMessageHandler.destroy();
+});
+
+function callTestEmitEvent(rootMessageHandler, browsingContextId) {
+  rootMessageHandler.handleCommand({
+    moduleName: "event",
+    commandName: "testEmitEvent",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+}
diff --git a/remote/shared/messagehandler/test/browser/head.js b/remote/shared/messagehandler/test/browser/head.js
index 0e6c3469c25e..0da8fb430fe9 100644
--- a/remote/shared/messagehandler/test/browser/head.js
+++ b/remote/shared/messagehandler/test/browser/head.js
@@ -46,3 +46,47 @@ async function addTab(url) {
   });
   return tab;
 }
+
+/**
+ * Create inline markup for a simple iframe that can be used with
+ * document-builder.sjs. The iframe will be served under the provided domain.
+ *
+ * @param {String} domain
+ *     A domain (eg "example.com"), compatible with build/pgo/server-locations.txt
+ */
+function createFrame(domain) {
+  return createFrameForUri(
+    `http://${domain}/document-builder.sjs?html=frame-${domain}`
+  );
+}
+
+function createFrameForUri(uri) {
+  return `<iframe src="${encodeURI(uri)}"></iframe>`;
+}
+
+// Create a test page with 2 iframes:
+// - one with a different eTLD+1 (example.com)
+// - one with a nested iframe on a different eTLD+1 (example.net)
+//
+// Overall the document structure should look like:
+//
+// html (example.org)
+//   iframe (example.org)
+//     iframe (example.net)
+//   iframe(example.com)
+//
+// Which means we should have 4 browsing contexts in total.
+function createTestMarkupWithFrames() {
+  // Create the markup for an example.net frame nested in an example.com frame.
+  const NESTED_FRAME_MARKUP = createFrameForUri(
+    `http://example.org/document-builder.sjs?html=${createFrame("example.net")}`
+  );
+
+  // Combine the nested frame markup created above with an example.com frame.
+  const TEST_URI_MARKUP = `${NESTED_FRAME_MARKUP}${createFrame("example.com")}`;
+
+  // Create the test page URI on example.org.
+  return `http://example.org/document-builder.sjs?html=${encodeURI(
+    TEST_URI_MARKUP
+  )}`;
+}
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm
new file mode 100644
index 000000000000..8cb13790ea59
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm
@@ -0,0 +1,27 @@
+/* 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/. */
+
+"use strict";
+
+const EXPORTED_SYMBOLS = ["event"];
+
+class Event {
+  constructor(messageHandler) {
+    this.messageHandler = messageHandler;
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testEmitRootEvent() {
+    this.messageHandler.emitMessageHandlerEvent("event.testRootEvent", {
+      text: "event from root",
+    });
+  }
+}
+
+const event = Event;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm
new file mode 100644
index 000000000000..5bc4fb6e346a
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm
@@ -0,0 +1,28 @@
+/* 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/. */
+
+"use strict";
+
+const EXPORTED_SYMBOLS = ["event"];
+
+class Event {
+  constructor(messageHandler) {
+    this.messageHandler = messageHandler;
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testEmitWindowGlobalInRootEvent(params, destination) {
+    this.messageHandler.emitMessageHandlerEvent(
+      "event.testWindowGlobalInRootEvent",
+      { text: `windowglobal-in-root event for ${destination.id}` }
+    );
+  }
+}
+
+const event = Event;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm
new file mode 100644
index 000000000000..9acad9afc35e
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm
@@ -0,0 +1,28 @@
+/* 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/. */
+
+"use strict";
+
+const EXPORTED_SYMBOLS = ["event"];
+
+class Event {
+  constructor(messageHandler) {
+    this.messageHandler = messageHandler;
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testEmitEvent() {
+    // Emit a payload including the contextId to check which context emitted
+    // a specific event.
+    const text = `event from ${this.messageHandler.contextId}`;
+    this.messageHandler.emitMessageHandlerEvent("event.testEvent", { text });
+  }
+}
+
+const event = Event;
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm
index 5cdf55bdff17..0b746ac67e4f 100644
--- a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm
@@ -26,6 +26,17 @@ class MessageHandlerFrameChild extends JSWindowActorChild {
   actorCreated() {
     this.type = WindowGlobalMessageHandler.type;
     this.context = this.manager.browsingContext;
+
+    this._onRegistryEvent = this._onRegistryEvent.bind(this);
+
+    // MessageHandlerFrameChild is responsible for forwarding events from
+    // WindowGlobalMessageHandler to the parent process.
+    // Such events are re-emitted on the MessageHandlerRegistry to avoid
+    // setting up listeners on individual MessageHandler instances.
+    MessageHandlerRegistry.on(
+      "message-handler-registry-event",
+      this._onRegistryEvent
+    );
   }
 
   receiveMessage(message) {
@@ -53,7 +64,31 @@ class MessageHandlerFrameChild extends JSWindowActorChild {
     );
   }
 
+  _onRegistryEvent(eventName, wrappedEvent) {
+    const { messageHandlerInfo, method, params } = wrappedEvent;
+    const { contextId, sessionId, type } = messageHandlerInfo;
+
+    // TODO: With a single MessageHandlerRegistry per process, we might receive
+    // events intended for other contexts. Consequently we have to filter out
+    // unrelevant events. Once Registry becomes context-specific (Bug 1722659)
+    // this filtering should be removed.
+    if (
+      type === this.type &&
+      contextId === WindowGlobalMessageHandler.getIdFromContext(this.context)
+    ) {
+      this.sendAsyncMessage("MessageHandlerFrameChild:messageHandlerEvent", {
+        method,
+        params,
+        sessionId,
+      });
+    }
+  }
+
   didDestroy() {
     MessageHandlerRegistry.contextDestroyed(this.context, this.type);
+    MessageHandlerRegistry.off(
+      "message-handler-registry-event",
+      this._onRegistryEvent
+    );
   }
 }
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm
index a96e80121c4e..36e8e87db397 100644
--- a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm
@@ -6,12 +6,39 @@
 
 var EXPORTED_SYMBOLS = ["MessageHandlerFrameParent"];
 
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+
+XPCOMUtils.defineLazyModuleGetters(this, {
+  MessageHandlerRegistry:
+    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm",
+});
+
 /**
  * Parent actor for the MessageHandlerFrame JSWindowActor. The
  * MessageHandlerFrame actor is used by FrameTransport to communicate between
  * ROOT MessageHandlers and WINDOW_GLOBAL MessageHandlers.
  */
 class MessageHandlerFrameParent extends JSWindowActorParent {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "MessageHandlerFrameChild:messageHandlerEvent":
+        const { method, params, sessionId } = message.data;
+
+        const messageHandler = MessageHandlerRegistry.getRootMessageHandler(
+          sessionId
+        );
+
+        // Re-emit the event on the RootMessageHandler.
+        messageHandler.emitMessageHandlerEvent(method, params);
+
+        break;
+      default:
+        throw new Error("Unsupported message:" + message.name);
+    }
+  }
+
   /**
    * Send a command to the corresponding MessageHandlerFrameChild actor via a
    * JSWindowActor query.
diff --git a/remote/shared/webdriver/Session.jsm b/remote/shared/webdriver/Session.jsm
index b7c39d6579d8..d91be26cf9d0 100644
--- a/remote/shared/webdriver/Session.jsm
+++ b/remote/shared/webdriver/Session.jsm
@@ -211,7 +211,13 @@ class WebDriverSession {
     this._connections.clear();
 
     // Destroy the dedicated MessageHandler instance if we created one.
-    this._messageHandler?.destroy();
+    if (this._messageHandler) {
+      this._messageHandler.off(
+        "message-handler-event",
+        this._onMessageHandlerEvent
+      );
+      this._messageHandler.destroy();
+    }
   }
 
   execute(module, command, params) {
@@ -242,6 +248,11 @@ class WebDriverSession {
         this.id,
         RootMessageHandler.type
       );
+      this._onMessageHandlerEvent = this._onMessageHandlerEvent.bind(this);
+      this._messageHandler.on(
+        "message-handler-event",
+        this._onMessageHandlerEvent
+      );
     }
 
     return this._messageHandler;
@@ -296,6 +307,13 @@ class WebDriverSession {
     this._connections.add(conn);
   }
 
+  _onMessageHandlerEvent(eventName, messageHandlerEvent) {
+    const { method, params } = messageHandlerEvent;
+    this._connections.forEach(connection =>
+      connection.sendEvent(method, params)
+    );
+  }
+
   // XPCOM
 
   get QueryInterface() {
diff --git a/remote/webdriver-bidi/WebDriverBiDiConnection.jsm b/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
index 3a3670f9f8f9..9bd9f9f04231 100644
--- a/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
+++ b/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
@@ -93,7 +93,9 @@ class WebDriverBiDiConnection extends WebSocketConnection {
    * @param {Object} params
    *     A JSON-serializable object, which is the payload of this event.
    */
-  sendEvent(method, params) {}
+  sendEvent(method, params) {
+    this.send({ method, params });
+  }
 
   /**
    * Send the result of a call to a module's method back to the