Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

65 files changed, 7010 insertions, 0 deletions

diff --git a/remote/shared/messagehandler/Errors.sys.mjs b/remote/shared/messagehandler/Errors.sys.mjs
new file mode 100644
index 0000000000..69c65acd09
--- /dev/null
+++ b/remote/shared/messagehandler/Errors.sys.mjs
@@ -0,0 +1,90 @@
+/* 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/. */
+
+import { RemoteError } from "chrome://remote/content/shared/RemoteError.sys.mjs";
+
+class MessageHandlerError extends RemoteError {
+  /**
+   * @param {(string|Error)=} x
+   *     Optional string describing error situation or Error instance
+   *     to propagate.
+   */
+  constructor(x) {
+    super(x);
+    this.name = this.constructor.name;
+    this.status = "message handler error";
+
+    // Error's ctor does not preserve x' stack
+    if (typeof x?.stack !== "undefined") {
+      this.stack = x.stack;
+    }
+  }
+
+  get isMessageHandlerError() {
+    return true;
+  }
+
+  /**
+   * @returns {Object<string, string>}
+   *     JSON serialisation of error prototype.
+   */
+  toJSON() {
+    return {
+      error: this.status,
+      message: this.message || "",
+      stacktrace: this.stack || "",
+    };
+  }
+
+  /**
+   * Unmarshals a JSON error representation to the appropriate MessageHandler
+   * error type.
+   *
+   * @param {Object<string, string>} json
+   *     Error object.
+   *
+   * @returns {Error}
+   *     Error prototype.
+   */
+  static fromJSON(json) {
+    if (typeof json.error == "undefined") {
+      let s = JSON.stringify(json);
+      throw new TypeError("Undeserialisable error type: " + s);
+    }
+    if (!STATUSES.has(json.error)) {
+      throw new TypeError("Not of MessageHandlerError descent: " + json.error);
+    }
+
+    let cls = STATUSES.get(json.error);
+    let err = new cls();
+    if ("message" in json) {
+      err.message = json.message;
+    }
+    if ("stacktrace" in json) {
+      err.stack = json.stacktrace;
+    }
+    return err;
+  }
+}
+
+/**
+ * A command could not be handled by the message handler network.
+ */
+class UnsupportedCommandError extends MessageHandlerError {
+  constructor(message) {
+    super(message);
+    this.status = "unsupported message handler command";
+  }
+}
+
+const STATUSES = new Map([
+  ["message handler error", MessageHandlerError],
+  ["unsupported message handler command", UnsupportedCommandError],
+]);
+
+/** @namespace */
+export const error = {
+  MessageHandlerError,
+  UnsupportedCommandError,
+};
diff --git a/remote/shared/messagehandler/EventsDispatcher.sys.mjs b/remote/shared/messagehandler/EventsDispatcher.sys.mjs
new file mode 100644
index 0000000000..b5db49ee7c
--- /dev/null
+++ b/remote/shared/messagehandler/EventsDispatcher.sys.mjs
@@ -0,0 +1,237 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ContextDescriptorType:
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  SessionDataCategory:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
+  SessionDataMethod:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+/**
+ * Helper to listen to events which rely on SessionData.
+ * In order to support the EventsDispatcher, a module emitting events should
+ * subscribe and unsubscribe to those events based on SessionData updates
+ * and should use the "event" SessionData category.
+ */
+export class EventsDispatcher {
+  // The MessageHandler owning this EventsDispatcher.
+  #messageHandler;
+
+  /**
+   * @typedef {object} EventListenerInfo
+   * @property {ContextDescriptor} contextDescriptor
+   *     The ContextDescriptor to which those callbacks are associated
+   * @property {Set<Function>} callbacks
+   *     The callbacks to trigger when an event matching the ContextDescriptor
+   *     is received.
+   */
+
+  // Map from event name to map of strings (context keys) to EventListenerInfo.
+  #listenersByEventName;
+
+  /**
+   * Create a new EventsDispatcher instance.
+   *
+   * @param {MessageHandler} messageHandler
+   *     The MessageHandler owning this EventsDispatcher.
+   */
+  constructor(messageHandler) {
+    this.#messageHandler = messageHandler;
+
+    this.#listenersByEventName = new Map();
+  }
+
+  destroy() {
+    for (const event of this.#listenersByEventName.keys()) {
+      this.#messageHandler.off(event, this.#onMessageHandlerEvent);
+    }
+
+    this.#listenersByEventName = null;
+  }
+
+  /**
+   * Stop listening for an event relying on SessionData and relayed by the
+   * message handler.
+   *
+   * @param {string} event
+   *     Name of the event to unsubscribe from.
+   * @param {ContextDescriptor} contextDescriptor
+   *     Context descriptor for this event.
+   * @param {Function} callback
+   *     Event listener callback.
+   * @returns {Promise}
+   *     Promise which resolves when the event fully unsubscribed, including
+   *     propagating the necessary session data.
+   */
+  async off(event, contextDescriptor, callback) {
+    return this.update([{ event, contextDescriptor, callback, enable: false }]);
+  }
+
+  /**
+   * Listen for an event relying on SessionData and relayed by the message
+   * handler.
+   *
+   * @param {string} event
+   *     Name of the event to subscribe to.
+   * @param {ContextDescriptor} contextDescriptor
+   *     Context descriptor for this event.
+   * @param {Function} callback
+   *     Event listener callback.
+   * @returns {Promise}
+   *     Promise which resolves when the event fully subscribed to, including
+   *     propagating the necessary session data.
+   */
+  async on(event, contextDescriptor, callback) {
+    return this.update([{ event, contextDescriptor, callback, enable: true }]);
+  }
+
+  /**
+   * An object that holds information about subscription/unsubscription
+   * of an event.
+   *
+   * @typedef Subscription
+   *
+   * @param {string} event
+   *     Name of the event to subscribe/unsubscribe to.
+   * @param {ContextDescriptor} contextDescriptor
+   *     Context descriptor for this event.
+   * @param {Function} callback
+   *     Event listener callback.
+   * @param {boolean} enable
+   *     True, if we need to subscribe to an event.
+   *     Otherwise false.
+   */
+
+  /**
+   * Start or stop listening to a list of events relying on SessionData
+   * and relayed by the message handler.
+   *
+   * @param {Array<Subscription>} subscriptions
+   *     The list of information to subscribe/unsubscribe to.
+   *
+   * @returns {Promise}
+   *     Promise which resolves when the events fully subscribed/unsubscribed to,
+   *     including propagating the necessary session data.
+   */
+  async update(subscriptions) {
+    const sessionDataItemUpdates = [];
+    subscriptions.forEach(({ event, contextDescriptor, callback, enable }) => {
+      if (enable) {
+        // Setup listeners.
+        if (!this.#listenersByEventName.has(event)) {
+          this.#listenersByEventName.set(event, new Map());
+          this.#messageHandler.on(event, this.#onMessageHandlerEvent);
+        }
+
+        const key = this.#getContextKey(contextDescriptor);
+        const listeners = this.#listenersByEventName.get(event);
+        if (listeners.has(key)) {
+          const { callbacks } = listeners.get(key);
+          callbacks.add(callback);
+        } else {
+          const callbacks = new Set([callback]);
+          listeners.set(key, { callbacks, contextDescriptor });
+
+          sessionDataItemUpdates.push({
+            ...this.#getSessionDataItem(event, contextDescriptor),
+            method: lazy.SessionDataMethod.Add,
+          });
+        }
+      } else {
+        // Remove listeners.
+        const listeners = this.#listenersByEventName.get(event);
+        if (!listeners) {
+          return;
+        }
+
+        const key = this.#getContextKey(contextDescriptor);
+        if (!listeners.has(key)) {
+          return;
+        }
+
+        const { callbacks } = listeners.get(key);
+        if (callbacks.has(callback)) {
+          callbacks.delete(callback);
+          if (callbacks.size === 0) {
+            listeners.delete(key);
+            if (listeners.size === 0) {
+              this.#messageHandler.off(event, this.#onMessageHandlerEvent);
+              this.#listenersByEventName.delete(event);
+            }
+
+            sessionDataItemUpdates.push({
+              ...this.#getSessionDataItem(event, contextDescriptor),
+              method: lazy.SessionDataMethod.Remove,
+            });
+          }
+        }
+      }
+    });
+
+    // Update all sessionData at once.
+    await this.#messageHandler.updateSessionData(sessionDataItemUpdates);
+  }
+
+  #getContextKey(contextDescriptor) {
+    const { id, type } = contextDescriptor;
+    return `${type}-${id}`;
+  }
+
+  #getSessionDataItem(event, contextDescriptor) {
+    const [moduleName] = event.split(".");
+    return {
+      moduleName,
+      category: lazy.SessionDataCategory.Event,
+      contextDescriptor,
+      values: [event],
+    };
+  }
+
+  #matchesContext(contextInfo, contextDescriptor) {
+    if (contextDescriptor.type === lazy.ContextDescriptorType.All) {
+      return true;
+    }
+
+    if (
+      contextDescriptor.type === lazy.ContextDescriptorType.TopBrowsingContext
+    ) {
+      const eventBrowsingContext = lazy.TabManager.getBrowsingContextById(
+        contextInfo.contextId
+      );
+      return eventBrowsingContext?.browserId === contextDescriptor.id;
+    }
+
+    return false;
+  }
+
+  #onMessageHandlerEvent = (name, event, contextInfo) => {
+    const listeners = this.#listenersByEventName.get(name);
+    for (const { callbacks, contextDescriptor } of listeners.values()) {
+      if (!this.#matchesContext(contextInfo, contextDescriptor)) {
+        continue;
+      }
+
+      for (const callback of callbacks) {
+        try {
+          callback(name, event);
+        } catch (e) {
+          lazy.logger.debug(
+            `Error while executing callback for ${name}: ${e.message}`
+          );
+        }
+      }
+    }
+  };
+}
diff --git a/remote/shared/messagehandler/MessageHandler.sys.mjs b/remote/shared/messagehandler/MessageHandler.sys.mjs
new file mode 100644
index 0000000000..ed15ed29b8
--- /dev/null
+++ b/remote/shared/messagehandler/MessageHandler.sys.mjs
@@ -0,0 +1,357 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { EventEmitter } from "resource://gre/modules/EventEmitter.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/messagehandler/Errors.sys.mjs",
+  EventsDispatcher:
+    "chrome://remote/content/shared/messagehandler/EventsDispatcher.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  ModuleCache:
+    "chrome://remote/content/shared/messagehandler/ModuleCache.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+/**
+ * A ContextDescriptor object provides information to decide if a broadcast or
+ * a session data item should be applied to a specific MessageHandler context.
+ *
+ * @typedef {object} ContextDescriptor
+ * @property {ContextDescriptorType} type
+ *     The type of context
+ * @property {string=} id
+ *     Unique id of a given context for the provided type.
+ *     For ContextDescriptorType.All, id can be ommitted.
+ *     For ContextDescriptorType.TopBrowsingContext, the id should be the
+ *     browserId corresponding to a top-level browsing context.
+ */
+
+/**
+ * Enum of ContextDescriptor types.
+ *
+ * @enum {string}
+ */
+export const ContextDescriptorType = {
+  All: "All",
+  TopBrowsingContext: "TopBrowsingContext",
+};
+
+/**
+ * A ContextInfo identifies a given context that can be linked to a MessageHandler
+ * instance. It should be used to identify events coming from this context.
+ *
+ * It can either be provided by the MessageHandler itself, when the event is
+ * emitted from the context it relates to.
+ *
+ * Or it can be assembled manually, for instance when emitting an event which
+ * relates to a window global from the root layer (eg browsingContext.contextCreated).
+ *
+ * @typedef {object} ContextInfo
+ * @property {string} contextId
+ *     Unique id of the MessageHandler corresponding to this context.
+ * @property {string} type
+ *     One of MessageHandler.type.
+ */
+
+/**
+ * MessageHandler instances are dedicated to handle both Commands and Events
+ * to enable automation and introspection for remote control protocols.
+ *
+ * MessageHandler instances are designed to form a network, where each instance
+ * should allow to inspect a specific context (eg. a BrowsingContext, a Worker,
+ * etc). Those instances might live in different processes and threads but
+ * should be linked together by the usage of a single sessionId, shared by all
+ * the instances of a single MessageHandler network.
+ *
+ * MessageHandler instances will be dynamically spawned depending on which
+ * Command or which Event needs to be processed and should therefore not be
+ * explicitly created by consumers, nor used directly.
+ *
+ * The only exception is the ROOT MessageHandler. This MessageHandler will be
+ * the entry point to send commands to the rest of the network. It will also
+ * emit all the relevant events captured by the network.
+ *
+ * However, even to create this ROOT MessageHandler, consumers should use the
+ * RootMessageHandlerRegistry. This singleton will ensure that MessageHandler
+ * instances are properly registered and can be retrieved based on a given
+ * session id as well as some other context information.
+ */
+export class MessageHandler extends EventEmitter {
+  #context;
+  #contextId;
+  #eventsDispatcher;
+  #moduleCache;
+  #registry;
+  #sessionId;
+
+  /**
+   * Create a new MessageHandler instance.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler is used for.
+   * @param {object} context
+   *     The context linked to this MessageHandler instance.
+   * @param {MessageHandlerRegistry} registry
+   *     The MessageHandlerRegistry which owns this MessageHandler instance.
+   */
+  constructor(sessionId, context, registry) {
+    super();
+
+    this.#moduleCache = new lazy.ModuleCache(this);
+
+    this.#sessionId = sessionId;
+    this.#context = context;
+    this.#contextId = this.constructor.getIdFromContext(context);
+    this.#eventsDispatcher = new lazy.EventsDispatcher(this);
+    this.#registry = registry;
+  }
+
+  get context() {
+    return this.#context;
+  }
+
+  get contextId() {
+    return this.#contextId;
+  }
+
+  get eventsDispatcher() {
+    return this.#eventsDispatcher;
+  }
+
+  get moduleCache() {
+    return this.#moduleCache;
+  }
+
+  get name() {
+    return [this.sessionId, this.constructor.type, this.contextId].join("-");
+  }
+
+  get registry() {
+    return this.#registry;
+  }
+
+  get sessionId() {
+    return this.#sessionId;
+  }
+
+  destroy() {
+    lazy.logger.trace(
+      `MessageHandler ${this.constructor.type} for session ${this.sessionId} is being destroyed`
+    );
+    this.#eventsDispatcher.destroy();
+    this.#moduleCache.destroy();
+
+    // At least the MessageHandlerRegistry will be expecting this event in order
+    // to remove the instance from the registry when destroyed.
+    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} name
+   *     Name of the event. Protocol level events should be of the
+   *     form [module name].[event name].
+   * @param {object} data
+   *     The event's data.
+   * @param {ContextInfo=} contextInfo
+   *     The event's context info, used to identify the origin of the event.
+   *     If not provided, the context info of the current MessageHandler will be
+   *     used.
+   */
+  emitEvent(name, data, contextInfo) {
+    // If no contextInfo field is provided on the event, extract it from the
+    // MessageHandler instance.
+    contextInfo = contextInfo || this.#getContextInfo();
+
+    // Events are emitted both under their own name for consumers listening to
+    // a specific and as `message-handler-event` for consumers which need to
+    // catch all events.
+    this.emit(name, data, contextInfo);
+    this.emit("message-handler-event", {
+      name,
+      contextInfo,
+      data,
+      sessionId: this.sessionId,
+    });
+  }
+
+  /**
+   * @typedef {object} CommandDestination
+   * @property {string} type
+   *     One of MessageHandler.type.
+   * @property {string=} id
+   *     Unique context identifier. The format depends on the type.
+   *     For WINDOW_GLOBAL destinations, this is a browsing context id.
+   *     Optional, should only be provided if `contextDescriptor` is missing.
+   * @property {ContextDescriptor=} contextDescriptor
+   *     Descriptor used to match several contexts, which will all receive the
+   *     command.
+   *     Optional, should only be provided if `id` is missing.
+   */
+
+  /**
+   * @typedef {object} Command
+   * @property {string} commandName
+   *     The name of the command to execute.
+   * @property {string} moduleName
+   *     The name of the module.
+   * @property {object} params
+   *     Optional command parameters.
+   * @property {CommandDestination} destination
+   *     The destination describing a debuggable context.
+   * @property {boolean=} retryOnAbort
+   *     Optional. When true, commands will be retried upon AbortError, which
+   *     can occur when the underlying JSWindowActor pair is destroyed.
+   *     Defaults to `false`.
+   */
+
+  /**
+   * Retrieve all module classes matching the moduleName and destination.
+   * See `getAllModuleClasses` (ModuleCache.jsm) for more details.
+   *
+   * @param {string} moduleName
+   *     The name of the module.
+   * @param {Destination} destination
+   *     The destination.
+   * @returns {Array.<class<Module>|null>}
+   *     An array of Module classes.
+   */
+  getAllModuleClasses(moduleName, destination) {
+    return this.#moduleCache.getAllModuleClasses(moduleName, destination);
+  }
+
+  /**
+   * Handle a command, either in one of the modules owned by this MessageHandler
+   * or in a another MessageHandler after forwarding the command.
+   *
+   * @param {Command} command
+   *     The command that should be either handled in this layer or forwarded to
+   *     the next layer leading to the destination.
+   * @returns {Promise} A Promise that will resolve with the return value of the
+   *     command once it has been executed.
+   */
+  handleCommand(command) {
+    const { moduleName, commandName, params, destination } = command;
+    lazy.logger.trace(
+      `Received command ${moduleName}.${commandName} for destination ${destination.type}`
+    );
+
+    if (!this.supportsCommand(moduleName, commandName, destination)) {
+      throw new lazy.error.UnsupportedCommandError(
+        `${moduleName}.${commandName} not supported for destination ${destination?.type}`
+      );
+    }
+
+    const module = this.#moduleCache.getModuleInstance(moduleName, destination);
+    if (module && module.supportsMethod(commandName)) {
+      return module[commandName](params, destination);
+    }
+
+    return this.forwardCommand(command);
+  }
+
+  toString() {
+    return `[object ${this.constructor.name} ${this.name}]`;
+  }
+
+  /**
+   * Apply the initial session data items provided to this MessageHandler on
+   * startup. Implementation is specific to each MessageHandler class.
+   *
+   * By default the implementation is a no-op.
+   *
+   * @param {Array<SessionDataItem>} sessionDataItems
+   *     Initial session data items for this MessageHandler.
+   */
+  async applyInitialSessionDataItems(sessionDataItems) {}
+
+  /**
+   * Returns the module path corresponding to this MessageHandler class.
+   *
+   * Needs to be implemented in the sub class.
+   */
+  static get modulePath() {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Returns the type corresponding to this MessageHandler class.
+   *
+   * Needs to be implemented in the sub class.
+   */
+  static get type() {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Returns the id corresponding to a context compatible with this
+   * MessageHandler class.
+   *
+   * Needs to be implemented in the sub class.
+   */
+  static getIdFromContext(context) {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Forward a command to other MessageHandlers.
+   *
+   * Needs to be implemented in the sub class.
+   */
+  forwardCommand(command) {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Check if contextDescriptor matches the context linked
+   * to this MessageHandler instance.
+   *
+   * Needs to be implemented in the sub class.
+   */
+  matchesContext(contextDescriptor) {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Check if the given command is supported in the module
+   * for the destination
+   *
+   * @param {string} moduleName
+   *     The name of the module.
+   * @param {string} commandName
+   *     The name of the command.
+   * @param {Destination} destination
+   *     The destination.
+   * @returns {boolean}
+   *     True if the command is supported.
+   */
+  supportsCommand(moduleName, commandName, destination) {
+    return this.getAllModuleClasses(moduleName, destination).some(cls =>
+      cls.supportsMethod(commandName)
+    );
+  }
+
+  /**
+   * Return the context information for this MessageHandler instance, which
+   * can be used to identify the origin of an event.
+   *
+   * @returns {ContextInfo}
+   *     The context information for this MessageHandler.
+   */
+  #getContextInfo() {
+    return {
+      contextId: this.contextId,
+      type: this.constructor.type,
+    };
+  }
+}
diff --git a/remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs b/remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs
new file mode 100644
index 0000000000..02a2514fee
--- /dev/null
+++ b/remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs
@@ -0,0 +1,238 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { EventEmitter } from "resource://gre/modules/EventEmitter.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  readSessionData:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs",
+  RootMessageHandler:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+/**
+ * Map of MessageHandler type to MessageHandler subclass.
+ */
+XPCOMUtils.defineLazyGetter(
+  lazy,
+  "MessageHandlerClasses",
+  () =>
+    new Map([
+      [lazy.RootMessageHandler.type, lazy.RootMessageHandler],
+      [lazy.WindowGlobalMessageHandler.type, lazy.WindowGlobalMessageHandler],
+    ])
+);
+
+/**
+ * Get the MessageHandler subclass corresponding to the provided type.
+
+ * @param {string} type
+ *     MessageHandler type, one of MessageHandler.type.
+ * @returns {Class}
+ *     A MessageHandler subclass
+ * @throws {Error}
+ *      Throws if no MessageHandler subclass is found for the provided type.
+ */
+export function getMessageHandlerClass(type) {
+  if (!lazy.MessageHandlerClasses.has(type)) {
+    throw new Error(`No MessageHandler class available for type "${type}"`);
+  }
+  return lazy.MessageHandlerClasses.get(type);
+}
+
+/**
+ * The MessageHandlerRegistry allows to create and retrieve MessageHandler
+ * instances for different session ids.
+ *
+ * A MessageHandlerRegistry instance is bound to a specific MessageHandler type
+ * and context. All MessageHandler instances created by the same registry will
+ * use the type and context of the registry, but each will be associated to a
+ * different session id.
+ *
+ * The registry is useful to retrieve the appropriate MessageHandler instance
+ * after crossing a technical boundary (eg process, thread...).
+ */
+export class MessageHandlerRegistry extends EventEmitter {
+  /*
+   * @param {String} type
+   *     MessageHandler type, one of MessageHandler.type.
+   * @param {Object} context
+   *     The context object, which depends on the type.
+   */
+  constructor(type, context) {
+    super();
+
+    this._messageHandlerClass = getMessageHandlerClass(type);
+    this._context = context;
+    this._type = type;
+
+    /**
+     * Map of session id to MessageHandler instance
+     */
+    this._messageHandlersMap = new Map();
+
+    this._onMessageHandlerDestroyed =
+      this._onMessageHandlerDestroyed.bind(this);
+    this._onMessageHandlerEvent = this._onMessageHandlerEvent.bind(this);
+  }
+
+  /**
+   * Create all message handlers for the current context, based on the content
+   * of the session data.
+   * This should typically be called when the context is ready to be used and
+   * to receive/send commands.
+   */
+  createAllMessageHandlers() {
+    const data = lazy.readSessionData();
+    for (const [sessionId, sessionDataItems] of data) {
+      // Create a message handler for this context for each active message
+      // handler session.
+      // TODO: In the future, to support debugging use cases we might want to
+      // only create a message handler if there is relevant data.
+      // For automation scenarios, this is less critical.
+      this._createMessageHandler(sessionId, sessionDataItems);
+    }
+  }
+
+  destroy() {
+    this._messageHandlersMap.forEach(messageHandler => {
+      messageHandler.destroy();
+    });
+  }
+
+  /**
+   * Retrieve all MessageHandler instances held in this registry, for all
+   * session IDs.
+   *
+   * @returns {Iterable.<MessageHandler>}
+   *     Iterator of MessageHandler instances
+   */
+  getAllMessageHandlers() {
+    return this._messageHandlersMap.values();
+  }
+
+  /**
+   * Retrieve an existing MessageHandler instance matching the provided session
+   * id. Returns null if no MessageHandler was found.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler is used for.
+   * @returns {MessageHandler=}
+   *     A MessageHandler instance, null if not found.
+   */
+  getExistingMessageHandler(sessionId) {
+    return this._messageHandlersMap.get(sessionId);
+  }
+
+  /**
+   * Retrieve the MessageHandler instance registered for the provided session
+   * id. Will create and register a MessageHander if no instance was found.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler is used for.
+   * @returns {MessageHandler}
+   *     A MessageHandler instance.
+   */
+  getOrCreateMessageHandler(sessionId) {
+    let messageHandler = this.getExistingMessageHandler(sessionId);
+    if (!messageHandler) {
+      messageHandler = this._createMessageHandler(sessionId);
+    }
+
+    return messageHandler;
+  }
+
+  /**
+   * Retrieve an already registered RootMessageHandler instance matching the
+   * provided sessionId.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler is used for.
+   * @returns {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,
+      lazy.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}]`;
+  }
+
+  /**
+   * Create a new MessageHandler instance.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler will be used for.
+   * @param {Array<SessionDataItem>=} sessionDataItems
+   *     Optional array of session data items to be applied automatically to the
+   *     MessageHandler.
+   * @returns {MessageHandler}
+   *     A new MessageHandler instance.
+   */
+  _createMessageHandler(sessionId, sessionDataItems) {
+    const messageHandler = new this._messageHandlerClass(
+      sessionId,
+      this._context,
+      this
+    );
+
+    messageHandler.on(
+      "message-handler-destroyed",
+      this._onMessageHandlerDestroyed
+    );
+    messageHandler.on("message-handler-event", this._onMessageHandlerEvent);
+
+    messageHandler.applyInitialSessionDataItems(sessionDataItems);
+
+    this._messageHandlersMap.set(sessionId, messageHandler);
+
+    lazy.logger.trace(
+      `Created MessageHandler ${this._type} for session ${sessionId}`
+    );
+
+    return messageHandler;
+  }
+
+  // Event handlers
+
+  _onMessageHandlerDestroyed(eventName, messageHandler) {
+    messageHandler.off(
+      "message-handler-destroyed",
+      this._onMessageHandlerDestroyed
+    );
+    messageHandler.off("message-handler-event", this._onMessageHandlerEvent);
+    this._messageHandlersMap.delete(messageHandler.sessionId);
+
+    lazy.logger.trace(
+      `Unregistered MessageHandler ${messageHandler.constructor.type} for session ${messageHandler.sessionId}`
+    );
+  }
+
+  _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);
+  }
+}
diff --git a/remote/shared/messagehandler/Module.sys.mjs b/remote/shared/messagehandler/Module.sys.mjs
new file mode 100644
index 0000000000..7b121f054d
--- /dev/null
+++ b/remote/shared/messagehandler/Module.sys.mjs
@@ -0,0 +1,137 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "disabledExperimentalAPI", () => {
+  return !Services.prefs.getBoolPref("remote.experimental.enabled");
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+export class Module {
+  #messageHandler;
+
+  /**
+   * Create a new module instance.
+   *
+   * @param {MessageHandler} messageHandler
+   *     The MessageHandler instance which owns this Module instance.
+   */
+  constructor(messageHandler) {
+    this.#messageHandler = messageHandler;
+  }
+
+  /**
+   * Clean-up the module instance.
+   */
+  destroy() {
+    lazy.logger.warn(
+      `Module ${this.constructor.name} is missing a destroy method`
+    );
+  }
+
+  /**
+   * Emit a message handler event.
+   *
+   * Such events should bubble up to the root of a MessageHandler network.
+   *
+   * @param {string} name
+   *     Name of the event. Protocol level events should be of the
+   *     form [module name].[event name].
+   * @param {object} data
+   *     The event's data.
+   * @param {ContextInfo=} contextInfo
+   *     The event's context info, see MessageHandler:emitEvent. Optional.
+   */
+  emitEvent(name, data, contextInfo) {
+    this.messageHandler.emitEvent(name, data, contextInfo);
+  }
+
+  /**
+   * Intercept an event and modify the payload.
+   *
+   * It's required to be implemented in windowglobal-in-root modules.
+   *
+   * @param {string} name
+   *     Name of the event.
+   * @param {object} payload
+   *    The event's payload.
+   * @returns {object}
+   *     The modified event payload.
+   */
+  interceptEvent(name, payload) {
+    throw new Error(
+      `Could not intercept event ${name}, interceptEvent is not implemented in windowglobal-in-root module`
+    );
+  }
+
+  /**
+   * Assert if experimental commands are enabled.
+   *
+   * @param {string} methodName
+   *     Name of the command.
+   *
+   * @throws {UnknownCommandError}
+   *     If experimental commands are disabled.
+   */
+  assertExperimentalCommandsEnabled(methodName) {
+    // TODO: 1778987. Move it to a BiDi specific place.
+    if (lazy.disabledExperimentalAPI) {
+      throw new lazy.error.UnknownCommandError(methodName);
+    }
+  }
+
+  /**
+   * Assert if experimental events are enabled.
+   *
+   * @param {string} moduleName
+   *     Name of the module.
+   *
+   * @param {string} event
+   *     Name of the event.
+   *
+   * @throws {InvalidArgumentError}
+   *     If experimental events are disabled.
+   */
+  assertExperimentalEventsEnabled(moduleName, event) {
+    // TODO: 1778987. Move it to a BiDi specific place.
+    if (lazy.disabledExperimentalAPI) {
+      throw new lazy.error.InvalidArgumentError(
+        `Module ${moduleName} does not support event ${event}`
+      );
+    }
+  }
+
+  /**
+   * Instance shortcut for supportsMethod to avoid reaching the constructor for
+   * consumers which directly deal with an instance.
+   */
+  supportsMethod(methodName) {
+    return this.constructor.supportsMethod(methodName);
+  }
+
+  get messageHandler() {
+    return this.#messageHandler;
+  }
+
+  static get supportedEvents() {
+    return [];
+  }
+
+  static supportsEvent(event) {
+    return this.supportedEvents.includes(event);
+  }
+
+  static supportsMethod(methodName) {
+    return typeof this.prototype[methodName] === "function";
+  }
+}
diff --git a/remote/shared/messagehandler/ModuleCache.sys.mjs b/remote/shared/messagehandler/ModuleCache.sys.mjs
new file mode 100644
index 0000000000..559429ddd1
--- /dev/null
+++ b/remote/shared/messagehandler/ModuleCache.sys.mjs
@@ -0,0 +1,265 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  getMessageHandlerClass:
+    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  RootMessageHandler:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
+});
+
+const protocols = {
+  bidi: {},
+  test: {},
+};
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(protocols.bidi, {
+  // Additional protocols might use a different registry for their modules,
+  // in which case this will no longer be a constant but will instead depend on
+  // the protocol owning the MessageHandler. See Bug 1722464.
+  modules:
+    "chrome://remote/content/webdriver-bidi/modules/ModuleRegistry.sys.mjs",
+});
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(protocols.test, {
+  modules:
+    "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+/**
+ * ModuleCache instances are dedicated to lazily create and cache the instances
+ * of all the modules related to a specific MessageHandler instance.
+ *
+ * ModuleCache also implements the logic to resolve the path to the file for a
+ * given module, which depends both on the current MessageHandler context and on
+ * the expected destination.
+ *
+ * In order to implement module logic in any context, separate module files
+ * should be created for each situation. For instance, for a given module,
+ * - ${MODULES_FOLDER}/root/{ModuleName}.jsm contains the implementation for
+ *   commands intended for the destination ROOT, and will be created for a ROOT
+ *   MessageHandler only. Typically, they will run in the parent process.
+ * - ${MODULES_FOLDER}/windowglobal/{ModuleName}.jsm contains the implementation
+ *   for commands intended for a WINDOW_GLOBAL destination, and will be created
+ *   for a WINDOW_GLOBAL MessageHandler only. Those will usually run in a
+ *   content process.
+ * - ${MODULES_FOLDER}/windowglobal-in-root/{ModuleName}.jsm also handles
+ *   commands intended for a WINDOW_GLOBAL destination, but they will be created
+ *   for the ROOT MessageHandler and will run in the parent process. This can be
+ *   useful if some code has to be executed in the parent process, even though
+ *   the final destination is a WINDOW_GLOBAL.
+ * - And so on, as more MessageHandler types get added, more combinations will
+ *   follow based on the same pattern:
+ *   - {contextName}/{ModuleName}.jsm
+ *   - or {destinationType}-in-{currentType}/{ModuleName}.jsm
+ *
+ * All those implementations are optional. If a module cannot be found, based on
+ * the logic detailed above, the MessageHandler will assume that the command
+ * should simply be forwarded to the next layer of the network.
+ */
+export class ModuleCache {
+  #messageHandler;
+  #messageHandlerType;
+  #modules;
+  #protocol;
+
+  /*
+   * @param {MessageHandler} messageHandler
+   *     The MessageHandler instance which owns this ModuleCache instance.
+   */
+  constructor(messageHandler) {
+    this.#messageHandler = messageHandler;
+    this.#messageHandlerType = messageHandler.constructor.type;
+
+    // Map of absolute module paths to module instances.
+    this.#modules = new Map();
+
+    // Use the module class from the WebDriverBiDi ModuleRegistry if we
+    // are not using test modules.
+    this.#protocol = Services.prefs.getBoolPref(
+      "remote.messagehandler.modulecache.useBrowserTestRoot",
+      false
+    )
+      ? protocols.test
+      : protocols.bidi;
+  }
+
+  /**
+   * Destroy all instantiated modules.
+   */
+  destroy() {
+    this.#modules.forEach(module => module?.destroy());
+  }
+
+  /**
+   * Retrieve all module classes matching the provided module name to reach the
+   * provided destination, from the current context.
+   *
+   * This corresponds to the path a command can take to reach its destination.
+   * A command's method must be implemented in one of the classes returned by
+   * getAllModuleClasses in order to be successfully handled.
+   *
+   * @param {string} moduleName
+   *     The name of the module.
+   * @param {Destination} destination
+   *     The destination.
+   * @returns {Array<class<Module>|null>}
+   *     An array of Module classes.
+   */
+  getAllModuleClasses(moduleName, destination) {
+    const destinationType = destination.type;
+    const classes = [
+      this.#getModuleClass(
+        moduleName,
+        this.#messageHandlerType,
+        destinationType
+      ),
+    ];
+
+    // Bug 1733242: Extend the implementation of this method to handle workers.
+    // It assumes layers have at most one level of nesting, for instance
+    // "root -> windowglobal", but it wouldn't work for something such as
+    // "root -> windowglobal -> worker".
+    if (destinationType !== this.#messageHandlerType) {
+      classes.push(
+        this.#getModuleClass(moduleName, destinationType, destinationType)
+      );
+    }
+
+    return classes.filter(cls => !!cls);
+  }
+
+  /**
+   * Get a module instance corresponding to the provided moduleName and
+   * destination. If no existing module can be found in the cache, ModuleCache
+   * will attempt to import the module file and create a new instance, which
+   * will then be cached and returned for subsequent calls.
+   *
+   * @param {string} moduleName
+   *     The name of the module which should implement the command.
+   * @param {CommandDestination} destination
+   *     The destination of the command for which we need to instantiate a
+   *     module. See MessageHandler.jsm for the CommandDestination typedef.
+   * @returns {object=}
+   *     A module instance corresponding to the provided moduleName and
+   *     destination, or null if it could not be instantiated.
+   */
+  getModuleInstance(moduleName, destination) {
+    const key = `${moduleName}-${destination.type}`;
+
+    if (this.#modules.has(key)) {
+      // If there is already a cached instance (potentially null) for the
+      // module name + destination type pair, return it.
+      return this.#modules.get(key);
+    }
+
+    const ModuleClass = this.#getModuleClass(
+      moduleName,
+      this.#messageHandlerType,
+      destination.type
+    );
+
+    let module = null;
+    if (ModuleClass) {
+      module = new ModuleClass(this.#messageHandler);
+    }
+
+    this.#modules.set(key, module);
+    return module;
+  }
+
+  /**
+   * Check if the given module exists for the destination.
+   *
+   * @param {string} moduleName
+   *     The name of the module.
+   * @param {Destination} destination
+   *     The destination.
+   * @returns {boolean}
+   *     True if the module exists.
+   */
+  hasModuleClass(moduleName, destination) {
+    const classes = this.getAllModuleClasses(moduleName, destination);
+    return !!classes.length;
+  }
+
+  toString() {
+    return `[object ${this.constructor.name} ${this.#messageHandler.name}]`;
+  }
+
+  /**
+   * Retrieve the module class matching the provided module name and folder.
+   *
+   * @param {string} moduleName
+   *     The name of the module to get the class for.
+   * @param {string} originType
+   *     The MessageHandler type from where the command comes.
+   * @param {string} destinationType
+   *     The MessageHandler type where the command should go to.
+   * @returns {Class=}
+   *     The class corresponding to the module name and folder, null if no match
+   *     was found.
+   * @throws {Error}
+   *     If the provided module folder is unexpected.
+   */
+  #getModuleClass = function (moduleName, originType, destinationType) {
+    if (
+      destinationType === lazy.RootMessageHandler.type &&
+      originType !== destinationType
+    ) {
+      // If we are trying to reach the root layer from a lower layer, no module
+      // class should attempt to handle the command in the current layer and
+      // the command should be forwarded unconditionally.
+      return null;
+    }
+
+    const moduleFolder = this.#getModuleFolder(originType, destinationType);
+    if (!this.#protocol.modules[moduleFolder]) {
+      throw new Error(
+        `Invalid module folder "${moduleFolder}", expected one of "${Object.keys(
+          this.#protocol.modules
+        )}"`
+      );
+    }
+
+    let moduleClass = null;
+    if (this.#protocol.modules[moduleFolder][moduleName]) {
+      moduleClass = this.#protocol.modules[moduleFolder][moduleName];
+    }
+
+    if (moduleClass) {
+      lazy.logger.trace(
+        `Module ${moduleFolder}/${moduleName}.jsm found for ${destinationType}`
+      );
+    } else {
+      lazy.logger.trace(
+        `Module ${moduleFolder}/${moduleName}.jsm not found for ${destinationType}`
+      );
+    }
+
+    return moduleClass;
+  };
+
+  #getModuleFolder(originType, destinationType) {
+    const originPath = lazy.getMessageHandlerClass(originType).modulePath;
+    if (originType === destinationType) {
+      // If the command is targeting the current type, the module is expected to
+      // be in eg "windowglobal/${moduleName}.jsm".
+      return originPath;
+    }
+
+    // If the command is targeting another type, the module is expected to
+    // be in a composed folder eg "windowglobal-in-root/${moduleName}.jsm".
+    const destinationPath =
+      lazy.getMessageHandlerClass(destinationType).modulePath;
+    return `${destinationPath}-in-${originPath}`;
+  }
+}
diff --git a/remote/shared/messagehandler/RootMessageHandler.sys.mjs b/remote/shared/messagehandler/RootMessageHandler.sys.mjs
new file mode 100644
index 0000000000..17c2b86b65
--- /dev/null
+++ b/remote/shared/messagehandler/RootMessageHandler.sys.mjs
@@ -0,0 +1,154 @@
+/* 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/. */
+
+import { MessageHandler } from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  RootTransport:
+    "chrome://remote/content/shared/messagehandler/transports/RootTransport.sys.mjs",
+  SessionData:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
+  SessionDataMethod:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+/**
+ * A RootMessageHandler is the root node of a MessageHandler network. It lives
+ * in the parent process. It can forward commands to MessageHandlers in other
+ * layers (at the moment WindowGlobalMessageHandlers in content processes).
+ */
+export class RootMessageHandler extends MessageHandler {
+  #rootTransport;
+  #sessionData;
+
+  /**
+   * Returns the RootMessageHandler module path.
+   *
+   * @returns {string}
+   */
+  static get modulePath() {
+    return "root";
+  }
+
+  /**
+   * Returns the RootMessageHandler type.
+   *
+   * @returns {string}
+   */
+  static get type() {
+    return "ROOT";
+  }
+
+  /**
+   * The ROOT MessageHandler is unique for a given MessageHandler network
+   * (ie for a given sessionId). Reuse the type as context id here.
+   */
+  static getIdFromContext(context) {
+    return RootMessageHandler.type;
+  }
+
+  /**
+   * Create a new RootMessageHandler instance.
+   *
+   * @param {string} sessionId
+   *     ID of the session the handler is used for.
+   */
+  constructor(sessionId) {
+    super(sessionId, null);
+
+    this.#rootTransport = new lazy.RootTransport(this);
+    this.#sessionData = new lazy.SessionData(this);
+  }
+
+  get sessionData() {
+    return this.#sessionData;
+  }
+
+  destroy() {
+    this.#sessionData.destroy();
+    super.destroy();
+  }
+
+  /**
+   * Add new session data items of a given module, category and
+   * contextDescriptor.
+   *
+   * Forwards the call to the SessionData instance owned by this
+   * RootMessageHandler and propagates the information via a command to existing
+   * MessageHandlers.
+   */
+  addSessionDataItem(sessionData = {}) {
+    sessionData.method = lazy.SessionDataMethod.Add;
+    return this.updateSessionData([sessionData]);
+  }
+
+  /**
+   * Emit a public protocol event. This event will be sent over to the client.
+   *
+   * @param {string} name
+   *     Name of the event. Protocol level events should be of the
+   *     form [module name].[event name].
+   * @param {object} data
+   *     The event's data.
+   */
+  emitProtocolEvent(name, data) {
+    this.emit("message-handler-protocol-event", {
+      name,
+      data,
+      sessionId: this.sessionId,
+    });
+  }
+
+  /**
+   * Forward the provided command to WINDOW_GLOBAL MessageHandlers via the
+   * RootTransport.
+   *
+   * @param {Command} command
+   *     The command to forward. See type definition in MessageHandler.js
+   * @returns {Promise}
+   *     Returns a promise that resolves with the result of the command.
+   */
+  forwardCommand(command) {
+    switch (command.destination.type) {
+      case lazy.WindowGlobalMessageHandler.type:
+        return this.#rootTransport.forwardCommand(command);
+      default:
+        throw new Error(
+          `Cannot forward command to "${command.destination.type}" from "${this.constructor.type}".`
+        );
+    }
+  }
+
+  matchesContext() {
+    return true;
+  }
+
+  /**
+   * Remove session data items of a given module, category and
+   * contextDescriptor.
+   *
+   * Forwards the call to the SessionData instance owned by this
+   * RootMessageHandler and propagates the information via a command to existing
+   * MessageHandlers.
+   */
+  removeSessionDataItem(sessionData = {}) {
+    sessionData.method = lazy.SessionDataMethod.Remove;
+    return this.updateSessionData([sessionData]);
+  }
+
+  /**
+   * Update session data items of a given module, category and
+   * contextDescriptor.
+   *
+   * Forwards the call to the SessionData instance owned by this
+   * RootMessageHandler.
+   */
+  async updateSessionData(sessionData = []) {
+    await this.#sessionData.updateSessionData(sessionData);
+  }
+}
diff --git a/remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs b/remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs
new file mode 100644
index 0000000000..09ac489182
--- /dev/null
+++ b/remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs
@@ -0,0 +1,17 @@
+/* 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/. */
+
+import { MessageHandlerRegistry } from "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs";
+
+import { RootMessageHandler } from "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs";
+
+/**
+ * In the parent process, only one Root MessageHandlerRegistry should ever be
+ * created. All consumers can safely use this singleton to retrieve the Root
+ * registry and from there either create or retrieve Root MessageHandler
+ * instances for a specific session.
+ */
+export var RootMessageHandlerRegistry = new MessageHandlerRegistry(
+  RootMessageHandler.type
+);
diff --git a/remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs b/remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs
new file mode 100644
index 0000000000..1e0164cf44
--- /dev/null
+++ b/remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs
@@ -0,0 +1,242 @@
+/* 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/. */
+
+import {
+  ContextDescriptorType,
+  MessageHandler,
+} from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  getMessageHandlerFrameChildActor:
+    "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs",
+  RootMessageHandler:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
+  WindowRealm: "chrome://remote/content/shared/Realm.sys.mjs",
+});
+
+/**
+ * A WindowGlobalMessageHandler is dedicated to debugging a single window
+ * global. It follows the lifecycle of the corresponding window global and will
+ * therefore not survive any navigation. This MessageHandler cannot forward
+ * commands further to other MessageHandlers and represents a leaf node in a
+ * MessageHandler network.
+ */
+export class WindowGlobalMessageHandler extends MessageHandler {
+  #innerWindowId;
+  #realms;
+
+  constructor() {
+    super(...arguments);
+
+    this.#innerWindowId = this.context.window.windowGlobalChild.innerWindowId;
+
+    // Maps sandbox names to instances of window realms,
+    // the default realm is mapped to an empty string sandbox name.
+    this.#realms = new Map([["", new lazy.WindowRealm(this.context.window)]]);
+  }
+
+  destroy() {
+    for (const realm of this.#realms.values()) {
+      realm.destroy();
+    }
+    this.#realms = null;
+
+    super.destroy();
+  }
+
+  /**
+   * Returns the WindowGlobalMessageHandler module path.
+   *
+   * @returns {string}
+   */
+  static get modulePath() {
+    return "windowglobal";
+  }
+
+  /**
+   * Returns the WindowGlobalMessageHandler type.
+   *
+   * @returns {string}
+   */
+  static get type() {
+    return "WINDOW_GLOBAL";
+  }
+
+  /**
+   * For WINDOW_GLOBAL MessageHandlers, `context` is a BrowsingContext,
+   * and BrowsingContext.id can be used as the context id.
+   *
+   * @param {BrowsingContext} context
+   *     WindowGlobalMessageHandler contexts are expected to be
+   *     BrowsingContexts.
+   * @returns {string}
+   *     The browsing context id.
+   */
+  static getIdFromContext(context) {
+    return context.id;
+  }
+
+  get innerWindowId() {
+    return this.#innerWindowId;
+  }
+
+  get realms() {
+    return this.#realms;
+  }
+
+  get window() {
+    return this.context.window;
+  }
+
+  #getRealmFromSandboxName(sandboxName = null) {
+    if (sandboxName === null || sandboxName === "") {
+      return this.#realms.get("");
+    }
+
+    if (this.#realms.has(sandboxName)) {
+      return this.#realms.get(sandboxName);
+    }
+
+    const realm = new lazy.WindowRealm(this.context.window, {
+      sandboxName,
+    });
+
+    this.#realms.set(sandboxName, realm);
+
+    return realm;
+  }
+
+  async applyInitialSessionDataItems(sessionDataItems) {
+    if (!Array.isArray(sessionDataItems)) {
+      return;
+    }
+
+    const destination = {
+      type: WindowGlobalMessageHandler.type,
+    };
+
+    // Create a Map with the structure moduleName -> category -> relevant session data items.
+    const structuredUpdates = new Map();
+    for (const sessionDataItem of sessionDataItems) {
+      const { category, contextDescriptor, moduleName } = sessionDataItem;
+
+      if (!this.matchesContext(contextDescriptor)) {
+        continue;
+      }
+      if (!structuredUpdates.has(moduleName)) {
+        // Skip session data item if the module is not present
+        // for the destination.
+        if (!this.moduleCache.hasModuleClass(moduleName, destination)) {
+          continue;
+        }
+        structuredUpdates.set(moduleName, new Map());
+      }
+
+      if (!structuredUpdates.get(moduleName).has(category)) {
+        structuredUpdates.get(moduleName).set(category, new Set());
+      }
+
+      structuredUpdates.get(moduleName).get(category).add(sessionDataItem);
+    }
+
+    const sessionDataPromises = [];
+
+    for (const [moduleName, categories] of structuredUpdates.entries()) {
+      for (const [category, relevantSessionData] of categories.entries()) {
+        sessionDataPromises.push(
+          this.handleCommand({
+            moduleName,
+            commandName: "_applySessionData",
+            params: {
+              category,
+              initial: true,
+              sessionData: Array.from(relevantSessionData),
+            },
+            destination,
+          })
+        );
+      }
+    }
+
+    await Promise.all(sessionDataPromises);
+
+    // With the session data applied the handler is now ready to be used.
+    this.emitEvent("window-global-handler-created", {
+      contextId: this.contextId,
+      innerWindowId: this.#innerWindowId,
+    });
+  }
+
+  forwardCommand(command) {
+    switch (command.destination.type) {
+      case lazy.RootMessageHandler.type:
+        return lazy
+          .getMessageHandlerFrameChildActor(this)
+          .sendCommand(command, this.sessionId);
+      default:
+        throw new Error(
+          `Cannot forward command to "${command.destination.type}" from "${this.constructor.type}".`
+        );
+    }
+  }
+
+  /**
+   * If <var>realmId</var> is null or not provided get the realm for
+   * a given <var>sandboxName</var>, otherwise find the realm
+   * in the cache with the realm id equal given <var>realmId</var>.
+   *
+   * @param {object} options
+   * @param {string|null=} options.realmId
+   *     The realm id.
+   * @param {string=} options.sandboxName
+   *     The name of sandbox
+   *
+   * @returns {Realm}
+   *     The realm object.
+   */
+  getRealm(options = {}) {
+    const { realmId = null, sandboxName } = options;
+    if (realmId === null) {
+      return this.#getRealmFromSandboxName(sandboxName);
+    }
+
+    const realm = Array.from(this.#realms.values()).find(
+      realm => realm.id === realmId
+    );
+
+    if (realm) {
+      return realm;
+    }
+
+    throw new lazy.error.NoSuchFrameError(`Realm with id ${realmId} not found`);
+  }
+
+  matchesContext(contextDescriptor) {
+    return (
+      contextDescriptor.type === ContextDescriptorType.All ||
+      (contextDescriptor.type === ContextDescriptorType.TopBrowsingContext &&
+        contextDescriptor.id === this.context.browserId)
+    );
+  }
+
+  /**
+   * Send a command to the root MessageHandler.
+   *
+   * @param {Command} command
+   *     The command to send to the root MessageHandler.
+   * @returns {Promise}
+   *     A promise which resolves with the return value of the command.
+   */
+  sendRootCommand(command) {
+    return this.handleCommand({
+      ...command,
+      destination: {
+        type: lazy.RootMessageHandler.type,
+      },
+    });
+  }
+}
diff --git a/remote/shared/messagehandler/sessiondata/SessionData.sys.mjs b/remote/shared/messagehandler/sessiondata/SessionData.sys.mjs
new file mode 100644
index 0000000000..4a85585a90
--- /dev/null
+++ b/remote/shared/messagehandler/sessiondata/SessionData.sys.mjs
@@ -0,0 +1,394 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ContextDescriptorType:
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  RootMessageHandler:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+/**
+ * @typedef {string} SessionDataCategory
+ */
+
+/**
+ * Enum of session data categories.
+ *
+ * @readonly
+ * @enum {SessionDataCategory}
+ */
+export const SessionDataCategory = {
+  Event: "event",
+  PreloadScript: "preload-script",
+};
+
+/**
+ * @typedef {string} SessionDataMethod
+ */
+
+/**
+ * Enum of session data methods.
+ *
+ * @readonly
+ * @enum {SessionDataMethod}
+ */
+export const SessionDataMethod = {
+  Add: "add",
+  Remove: "remove",
+};
+
+export const SESSION_DATA_SHARED_DATA_KEY = "MessageHandlerSessionData";
+
+// This is a map from session id to session data, which will be persisted and
+// propagated to all processes using Services' sharedData.
+// We have to store this as a unique object under a unique shared data key
+// because new MessageHandlers in other processes will need to access this data
+// without any notion of a specific session.
+// This is a singleton.
+const sessionDataMap = new Map();
+
+/**
+ * @typedef {object} SessionDataItem
+ * @property {string} moduleName
+ *     The name of the module responsible for this data item.
+ * @property {SessionDataCategory} category
+ *     The category of data. The supported categories depend on the module.
+ * @property {(string|number|boolean)} value
+ *     Value of the session data item.
+ * @property {ContextDescriptor} contextDescriptor
+ *     ContextDescriptor to which this session data applies.
+ */
+
+/**
+ * @typedef SessionDataItemUpdate
+ * @property {SessionDataMethod} method
+ *     The way sessionData is updated.
+ * @property {string} moduleName
+ *     The name of the module responsible for this data item.
+ * @property {SessionDataCategory} category
+ *     The category of data. The supported categories depend on the module.
+ * @property {Array<(string|number|boolean)>} values
+ *     Values of the session data item update.
+ * @property {ContextDescriptor} contextDescriptor
+ *     ContextDescriptor to which this session data applies.
+ */
+
+/**
+ * SessionData provides APIs to read and write the session data for a specific
+ * ROOT message handler. It holds the session data as a property and acts as the
+ * source of truth for this session data.
+ *
+ * The session data of a given message handler network should contain all the
+ * information that might be needed to setup new contexts, for instance a list
+ * of subscribed events, a list of breakpoints etc.
+ *
+ * The actual session data is an array of SessionDataItems. Example below:
+ * ```
+ * data: [
+ *   {
+ *     moduleName: "log",
+ *     category: "event",
+ *     value: "log.entryAdded",
+ *     contextDescriptor: { type: "all" }
+ *   },
+ *   {
+ *     moduleName: "browsingContext",
+ *     category: "event",
+ *     value: "browsingContext.contextCreated",
+ *     contextDescriptor: { type: "browser-element", id: "7"}
+ *   },
+ *   {
+ *     moduleName: "browsingContext",
+ *     category: "event",
+ *     value: "browsingContext.contextCreated",
+ *     contextDescriptor: { type: "browser-element", id: "12"}
+ *   },
+ * ]
+ * ```
+ *
+ * The session data will be persisted using Services.ppmm.sharedData, so that
+ * new contexts living in different processes can also access the information
+ * during their startup.
+ *
+ * This class should only be used from a ROOT MessageHandler, or from modules
+ * owned by a ROOT MessageHandler. Other MessageHandlers should rely on
+ * SessionDataReader's readSessionData to get read-only access to session data.
+ *
+ */
+export class SessionData {
+  constructor(messageHandler) {
+    if (messageHandler.constructor.type != lazy.RootMessageHandler.type) {
+      throw new Error(
+        "SessionData should only be used from a ROOT MessageHandler"
+      );
+    }
+
+    this._messageHandler = messageHandler;
+
+    /*
+     * The actual data for this session. This is an array of SessionDataItems.
+     */
+    this._data = [];
+  }
+
+  destroy() {
+    // Update the sessionDataMap singleton.
+    sessionDataMap.delete(this._messageHandler.sessionId);
+
+    // Update sharedData and flush to force consistency.
+    Services.ppmm.sharedData.set(SESSION_DATA_SHARED_DATA_KEY, sessionDataMap);
+    Services.ppmm.sharedData.flush();
+  }
+
+  /**
+   * Update session data items of a given module, category and
+   * contextDescriptor.
+   *
+   * A SessionDataItem will be added or removed for each value of each update
+   * in the provided array.
+   *
+   * Attempting to add a duplicate SessionDataItem or to remove an unknown
+   * SessionDataItem will be silently skipped (no-op).
+   *
+   * The data will be persisted across processes at the end of this method.
+   *
+   * @param {Array<SessionDataItemUpdate>} sessionDataItemUpdates
+   *     Array of session data item updates.
+   *
+   * @returns {Array<SessionDataItemUpdate>}
+   *     The subset of session data item updates which want to be applied.
+   */
+  applySessionData(sessionDataItemUpdates = []) {
+    // The subset of session data item updates, which are cleaned up from
+    // duplicates and unknown items.
+    let updates = [];
+    for (const sessionDataItemUpdate of sessionDataItemUpdates) {
+      const { category, contextDescriptor, method, moduleName, values } =
+        sessionDataItemUpdate;
+      const updatedValues = [];
+      for (const value of values) {
+        const item = { moduleName, category, contextDescriptor, value };
+
+        if (method === SessionDataMethod.Add) {
+          const hasItem = this._findIndex(item) != -1;
+
+          if (!hasItem) {
+            this._data.push(item);
+            updatedValues.push(value);
+          } else {
+            lazy.logger.warn(
+              `Duplicated session data item was not added: ${JSON.stringify(
+                item
+              )}`
+            );
+          }
+        } else {
+          const itemIndex = this._findIndex(item);
+
+          if (itemIndex != -1) {
+            // The item was found in the session data, remove it.
+            this._data.splice(itemIndex, 1);
+            updatedValues.push(value);
+          } else {
+            lazy.logger.warn(
+              `Missing session data item was not removed: ${JSON.stringify(
+                item
+              )}`
+            );
+          }
+        }
+      }
+
+      if (updatedValues.length) {
+        updates.push({
+          ...sessionDataItemUpdate,
+          values: updatedValues,
+        });
+      }
+    }
+    // Persist the sessionDataMap.
+    this._persist();
+
+    return updates;
+  }
+
+  /**
+   * Retrieve the SessionDataItems for a given module and type.
+   *
+   * @param {string} moduleName
+   *     The name of the module responsible for this data item.
+   * @param {string} category
+   *     The session data category.
+   * @param {ContextDescriptor=} contextDescriptor
+   *     Optional context descriptor, to retrieve only session data items added
+   *     for a specific context descriptor.
+   * @returns {Array<SessionDataItem>}
+   *     Array of SessionDataItems for the provided module and type.
+   */
+  getSessionData(moduleName, category, contextDescriptor) {
+    return this._data.filter(
+      item =>
+        item.moduleName === moduleName &&
+        item.category === category &&
+        (!contextDescriptor ||
+          this._isSameContextDescriptor(
+            item.contextDescriptor,
+            contextDescriptor
+          ))
+    );
+  }
+
+  /**
+   * Update session data items of a given module, category and
+   * contextDescriptor and propagate the information
+   * via a command to existing MessageHandlers.
+   *
+   * @param {Array<SessionDataItemUpdate>} sessionDataItemUpdates
+   *     Array of session data item updates.
+   */
+  async updateSessionData(sessionDataItemUpdates = []) {
+    const updates = this.applySessionData(sessionDataItemUpdates);
+
+    if (!updates.length) {
+      // Avoid unnecessary broadcast if no items were updated.
+      return;
+    }
+
+    // Create a Map with the structure moduleName -> category -> list of descriptors.
+    const structuredUpdates = new Map();
+    for (const { moduleName, category, contextDescriptor } of updates) {
+      if (!structuredUpdates.has(moduleName)) {
+        structuredUpdates.set(moduleName, new Map());
+      }
+      if (!structuredUpdates.get(moduleName).has(category)) {
+        structuredUpdates.get(moduleName).set(category, new Set());
+      }
+      const descriptors = structuredUpdates.get(moduleName).get(category);
+      // If there is at least one update for all contexts,
+      // keep only this descriptor in the list of descriptors
+      if (contextDescriptor.type === lazy.ContextDescriptorType.All) {
+        structuredUpdates
+          .get(moduleName)
+          .set(category, new Set([contextDescriptor]));
+      }
+      // Add an individual descriptor if there is no descriptor for all contexts.
+      else if (
+        descriptors.size !== 1 ||
+        Array.from(descriptors)[0]?.type !== lazy.ContextDescriptorType.All
+      ) {
+        descriptors.add(contextDescriptor);
+      }
+    }
+
+    const rootDestination = {
+      type: lazy.RootMessageHandler.type,
+    };
+    const sessionDataPromises = [];
+
+    for (const [moduleName, categories] of structuredUpdates.entries()) {
+      for (const [category, contextDescriptors] of categories.entries()) {
+        // Find sessionData for the category and the moduleName.
+        const relevantSessionData = this._data.filter(
+          item => item.category == category && item.moduleName === moduleName
+        );
+        for (const contextDescriptor of contextDescriptors.values()) {
+          const windowGlobalDestination = {
+            type: lazy.WindowGlobalMessageHandler.type,
+            contextDescriptor,
+          };
+
+          for (const destination of [
+            windowGlobalDestination,
+            rootDestination,
+          ]) {
+            // Only apply session data if the module is present for the destination.
+            if (
+              this._messageHandler.supportsCommand(
+                moduleName,
+                "_applySessionData",
+                destination
+              )
+            ) {
+              sessionDataPromises.push(
+                this._messageHandler
+                  .handleCommand({
+                    moduleName,
+                    commandName: "_applySessionData",
+                    params: {
+                      sessionData: relevantSessionData,
+                      category,
+                      contextDescriptor,
+                    },
+                    destination,
+                  })
+                  ?.catch(reason =>
+                    lazy.logger.error(
+                      `_applySessionData for module: ${moduleName} failed, reason: ${reason}`
+                    )
+                  )
+              );
+            }
+          }
+        }
+      }
+    }
+
+    await Promise.allSettled(sessionDataPromises);
+  }
+
+  _isSameItem(item1, item2) {
+    const descriptor1 = item1.contextDescriptor;
+    const descriptor2 = item2.contextDescriptor;
+
+    return (
+      item1.moduleName === item2.moduleName &&
+      item1.category === item2.category &&
+      this._isSameContextDescriptor(descriptor1, descriptor2) &&
+      this._isSameValue(item1.category, item1.value, item2.value)
+    );
+  }
+
+  _isSameContextDescriptor(contextDescriptor1, contextDescriptor2) {
+    if (contextDescriptor1.type === lazy.ContextDescriptorType.All) {
+      // Ignore the id for type "all" since we made the id optional for this type.
+      return contextDescriptor1.type === contextDescriptor2.type;
+    }
+
+    return (
+      contextDescriptor1.type === contextDescriptor2.type &&
+      contextDescriptor1.id === contextDescriptor2.id
+    );
+  }
+
+  _isSameValue(category, value1, value2) {
+    if (category === SessionDataCategory.PreloadScript) {
+      return value1.script === value2.script;
+    }
+
+    return value1 === value2;
+  }
+
+  _findIndex(item) {
+    return this._data.findIndex(_item => this._isSameItem(item, _item));
+  }
+
+  _persist() {
+    // Update the sessionDataMap singleton.
+    sessionDataMap.set(this._messageHandler.sessionId, this._data);
+
+    // Update sharedData and flush to force consistency.
+    Services.ppmm.sharedData.set(SESSION_DATA_SHARED_DATA_KEY, sessionDataMap);
+    Services.ppmm.sharedData.flush();
+  }
+}
diff --git a/remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs b/remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs
new file mode 100644
index 0000000000..c96409fb94
--- /dev/null
+++ b/remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs
@@ -0,0 +1,29 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  SESSION_DATA_SHARED_DATA_KEY:
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "sharedData", () => {
+  const isInParent =
+    Services.appinfo.processType == Ci.nsIXULRuntime.PROCESS_TYPE_DEFAULT;
+
+  return isInParent ? Services.ppmm.sharedData : Services.cpmm.sharedData;
+});
+
+/**
+ * Returns a snapshot of the session data map, which is cloned from the
+ * sessionDataMap singleton of SessionData.jsm.
+ *
+ *  @returns {Map.<string, Array<SessionDataItem>>}
+ *     Map of session id to arrays of SessionDataItems.
+ */
+export const readSessionData = () =>
+  lazy.sharedData.get(lazy.SESSION_DATA_SHARED_DATA_KEY) || new Map();
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser.ini b/remote/shared/messagehandler/test/browser/broadcast/browser.ini
new file mode 100644
index 0000000000..a2f989aaf0
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser.ini
@@ -0,0 +1,17 @@
+[DEFAULT]
+tags = remote
+subsuite = remote
+support-files =
+  doc_messagehandler_broadcasting_xul.xhtml
+  head.js
+  !/remote/shared/messagehandler/test/browser/head.js
+  !/remote/shared/messagehandler/test/browser/resources/*
+prefs =
+  remote.messagehandler.modulecache.useBrowserTestRoot=true
+
+[browser_filter_top_browsing_context.js]
+[browser_only_content_process.js]
+[browser_two_tabs.js]
+[browser_two_tabs_with_params.js]
+[browser_two_windows.js]
+[browser_with_frames.js]
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_filter_top_browsing_context.js b/remote/shared/messagehandler/test/browser/broadcast/browser_filter_top_browsing_context.js
new file mode 100644
index 0000000000..c140c26fc6
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_filter_top_browsing_context.js
@@ -0,0 +1,84 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const COM_TEST_PAGE = "https://example.com/document-builder.sjs?html=COM";
+const FRAME_TEST_PAGE = createTestMarkupWithFrames();
+
+add_task(async function test_broadcasting_filter_top_browsing_context() {
+  info("Navigate the initial tab to the COM test URL");
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, COM_TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  info("Open a second tab on the frame test URL");
+  const tab2 = await addTab(FRAME_TEST_PAGE);
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+
+  const contextsForTab2 =
+    tab2.linkedBrowser.browsingContext.getAllBrowsingContextsInSubtree();
+  is(
+    contextsForTab2.length,
+    4,
+    "Frame test tab has 3 children contexts (4 in total)"
+  );
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-broadcasting_filter_top_browsing_context"
+  );
+
+  const broadcastValue1 = await sendBroadcastForTopBrowsingContext(
+    browsingContext1,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue1),
+    "The broadcast returned an array of values"
+  );
+
+  is(broadcastValue1.length, 1, "The broadcast returned one value as expected");
+
+  ok(
+    broadcastValue1.includes("broadcast-" + browsingContext1.id),
+    "The broadcast returned the expected value from tab1"
+  );
+
+  const broadcastValue2 = await sendBroadcastForTopBrowsingContext(
+    browsingContext2,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue2),
+    "The broadcast returned an array of values"
+  );
+
+  is(broadcastValue2.length, 4, "The broadcast returned 4 values as expected");
+
+  for (const context of contextsForTab2) {
+    ok(
+      broadcastValue2.includes("broadcast-" + context.id),
+      "The broadcast contains the value for browsing context " + context.id
+    );
+  }
+
+  rootMessageHandler.destroy();
+});
+
+function sendBroadcastForTopBrowsingContext(
+  topBrowsingContext,
+  rootMessageHandler
+) {
+  return sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcast",
+    {},
+    {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: topBrowsingContext.browserId,
+    },
+    rootMessageHandler
+  );
+}
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_only_content_process.js b/remote/shared/messagehandler/test/browser/broadcast/browser_only_content_process.js
new file mode 100644
index 0000000000..d5090c701e
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_only_content_process.js
@@ -0,0 +1,46 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_broadcasting_only_content_process() {
+  info("Navigate the initial tab to the test URL");
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(
+    tab1.linkedBrowser,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  info("Open a new tab on a parent process about: page");
+  await addTab("about:robots");
+
+  info("Open a new tab on a XUL page");
+  await addTab(
+    getRootDirectory(gTestPath) + "doc_messagehandler_broadcasting_xul.xhtml"
+  );
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-broadcasting_only_content_process"
+  );
+  const broadcastValue = await sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcast",
+    {},
+    contextDescriptorAll,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue),
+    "The broadcast returned an array of values"
+  );
+
+  is(broadcastValue.length, 1, "The broadcast returned 1 value as expected");
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext1.id),
+    "The broadcast returned the expected value from tab1"
+  );
+
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs.js b/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs.js
new file mode 100644
index 0000000000..16b97e2a0a
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs.js
@@ -0,0 +1,46 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+add_task(async function test_broadcasting_two_tabs_command() {
+  info("Navigate the initial tab to the test URL");
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  info("Open a new tab on the same test URL");
+  const tab2 = await addTab(TEST_PAGE);
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-broadcasting_two_tabs_command"
+  );
+
+  const broadcastValue = await sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcast",
+    {},
+    contextDescriptorAll,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue),
+    "The broadcast returned an array of values"
+  );
+
+  is(broadcastValue.length, 2, "The broadcast returned 2 values as expected");
+
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext1.id),
+    "The broadcast returned the expected value from tab1"
+  );
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext2.id),
+    "The broadcast returned the expected value from tab2"
+  );
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs_with_params.js b/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs_with_params.js
new file mode 100644
index 0000000000..261b8c4cd6
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_two_tabs_with_params.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+add_task(async function test_broadcasting_two_tabs_with_params_command() {
+  info("Navigate the initial tab to the test URL");
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  info("Open a new tab on the same test URL");
+  const tab2 = await addTab(TEST_PAGE);
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-broadcasting_two_tabs_command"
+  );
+
+  const broadcastValue = await sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcastWithParameter",
+    {
+      value: "some-value",
+    },
+    contextDescriptorAll,
+    rootMessageHandler
+  );
+  ok(
+    Array.isArray(broadcastValue),
+    "The broadcast returned an array of values"
+  );
+
+  is(broadcastValue.length, 2, "The broadcast returned 2 values as expected");
+
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext1.id + "-some-value"),
+    "The broadcast returned the expected value from tab1"
+  );
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext2.id + "-some-value"),
+    "The broadcast returned the expected value from tab2"
+  );
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_two_windows.js b/remote/shared/messagehandler/test/browser/broadcast/browser_two_windows.js
new file mode 100644
index 0000000000..f59bebba69
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_two_windows.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+add_task(async function test_broadcasting_two_windows_command() {
+  const window1Browser = gBrowser.selectedTab.linkedBrowser;
+  await loadURL(window1Browser, TEST_PAGE);
+  const browsingContext1 = window1Browser.browsingContext;
+
+  const window2 = await BrowserTestUtils.openNewBrowserWindow();
+  registerCleanupFunction(() => BrowserTestUtils.closeWindow(window2));
+
+  const window2Browser = window2.gBrowser.selectedBrowser;
+  await loadURL(window2Browser, TEST_PAGE);
+  const browsingContext2 = window2Browser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-broadcasting_two_windows_command"
+  );
+  const broadcastValue = await sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcast",
+    {},
+    contextDescriptorAll,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue),
+    "The broadcast returned an array of values"
+  );
+  is(broadcastValue.length, 2, "The broadcast returned 2 values as expected");
+
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext1.id),
+    "The broadcast returned the expected value from tab1"
+  );
+  ok(
+    broadcastValue.includes("broadcast-" + browsingContext2.id),
+    "The broadcast returned the expected value from tab2"
+  );
+
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js b/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js
new file mode 100644
index 0000000000..50326d3885
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/browser_with_frames.js
@@ -0,0 +1,40 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_broadcasting_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-broadcasting_with_frames"
+  );
+  const broadcastValue = await sendTestBroadcastCommand(
+    "commandwindowglobalonly",
+    "testBroadcast",
+    {},
+    contextDescriptorAll,
+    rootMessageHandler
+  );
+
+  ok(
+    Array.isArray(broadcastValue),
+    "The broadcast returned an array of values"
+  );
+  is(broadcastValue.length, 4, "The broadcast returned 4 values as expected");
+
+  for (const context of contexts) {
+    ok(
+      broadcastValue.includes("broadcast-" + context.id),
+      "The broadcast contains the value for browsing context " + context.id
+    );
+  }
+
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/broadcast/doc_messagehandler_broadcasting_xul.xhtml b/remote/shared/messagehandler/test/browser/broadcast/doc_messagehandler_broadcasting_xul.xhtml
new file mode 100644
index 0000000000..91f3503ac3
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/doc_messagehandler_broadcasting_xul.xhtml
@@ -0,0 +1,3 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <box id="box" style="background-color: red;">Test chrome broadcasting</box>
+</window>
diff --git a/remote/shared/messagehandler/test/browser/broadcast/head.js b/remote/shared/messagehandler/test/browser/broadcast/head.js
new file mode 100644
index 0000000000..eb97549c26
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/broadcast/head.js
@@ -0,0 +1,48 @@
+/* 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";
+
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/head.js",
+  this
+);
+
+/**
+ * Broadcast the provided method to WindowGlobal contexts on a MessageHandler
+ * network.
+ * Returns a promise which will resolve the result of the command broadcast.
+ *
+ * @param {string} module
+ *     The name of the module implementing the command to broadcast.
+ * @param {string} command
+ *     The name of the command to broadcast.
+ * @param {object} params
+ *     The parameters for the command.
+ * @param {ContextDescriptor} contextDescriptor
+ *     The context descriptor to use for this broadcast
+ * @param {RootMessageHandler} rootMessageHandler
+ *     The root of the MessageHandler network.
+ * @returns {Promise.<Array>}
+ *     Promise which resolves an array where each item is the result of the
+ *     command handled by an individual context.
+ */
+function sendTestBroadcastCommand(
+  module,
+  command,
+  params,
+  contextDescriptor,
+  rootMessageHandler
+) {
+  info("Send a test broadcast command");
+  return rootMessageHandler.handleCommand({
+    moduleName: module,
+    commandName: command,
+    params,
+    destination: {
+      contextDescriptor,
+      type: WindowGlobalMessageHandler.type,
+    },
+  });
+}
diff --git a/remote/shared/messagehandler/test/browser/browser.ini b/remote/shared/messagehandler/test/browser/browser.ini
new file mode 100644
index 0000000000..8d93cdb795
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser.ini
@@ -0,0 +1,25 @@
+[DEFAULT]
+tags = remote
+subsuite = remote
+support-files =
+  head.js
+  resources/*
+prefs =
+  remote.messagehandler.modulecache.useBrowserTestRoot=true
+
+[browser_bfcache.js]
+[browser_events_dispatcher.js]
+[browser_events_handler.js]
+[browser_events_module.js]
+[browser_frame_context_utils.js]
+[browser_handle_command_errors.js]
+[browser_handle_command_retry.js]
+[browser_handle_simple_command.js]
+[browser_registry.js]
+[browser_session_data.js]
+[browser_session_data_browser_element.js]
+[browser_session_data_constructor_race.js]
+[browser_session_data_update.js]
+[browser_session_data_update_categories.js]
+[browser_session_data_update_contexts.js]
+[browser_windowglobal_to_root.js]
diff --git a/remote/shared/messagehandler/test/browser/browser_bfcache.js b/remote/shared/messagehandler/test/browser/browser_bfcache.js
new file mode 100644
index 0000000000..f829d8b58d
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_bfcache.js
@@ -0,0 +1,98 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+const TEST_PREF = "remote.messagehandler.test.pref";
+
+// Check that pages in bfcache no longer have message handlers attached to them,
+// and that they will not emit unexpected events.
+add_task(async function test_bfcache_broadcast() {
+  const tab = await addTab("https://example.com/document-builder.sjs?html=tab");
+  const rootMessageHandler = createRootMessageHandler("session-id-bfcache");
+
+  try {
+    const browsingContext = tab.linkedBrowser.browsingContext;
+    const contextDescriptor = {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext.browserId,
+    };
+
+    // Whenever a "preference-changed" event from the eventonprefchange module
+    // will be received on the root MessageHandler, increment a counter.
+    let preferenceChangeEventCount = 0;
+    const onEvent = (evtName, wrappedEvt) => {
+      if (wrappedEvt.name === "preference-changed") {
+        preferenceChangeEventCount++;
+      }
+    };
+    rootMessageHandler.on("message-handler-event", onEvent);
+
+    // Initialize the preference, no eventonprefchange module should be created
+    // yet so preferenceChangeEventCount is not expected to be updated.
+    Services.prefs.setIntPref(TEST_PREF, 0);
+    await TestUtils.waitForCondition(() => preferenceChangeEventCount >= 0);
+    is(preferenceChangeEventCount, 0);
+
+    // Broadcast a "ping" command to force the creation of the eventonprefchange
+    // module
+    let values = await sendPingCommand(rootMessageHandler, contextDescriptor);
+    is(values.length, 1, "Broadcast returned a single value");
+
+    Services.prefs.setIntPref(TEST_PREF, 1);
+    await TestUtils.waitForCondition(() => preferenceChangeEventCount >= 1);
+    is(preferenceChangeEventCount, 1);
+
+    info("Navigate to another page");
+    await loadURL(
+      tab.linkedBrowser,
+      "https://example.com/document-builder.sjs?html=othertab"
+    );
+
+    values = await sendPingCommand(rootMessageHandler, contextDescriptor);
+    is(values.length, 1, "Broadcast returned a single value after navigation");
+
+    info("Update the preference and check we only receive 1 event");
+    Services.prefs.setIntPref(TEST_PREF, 2);
+    await TestUtils.waitForCondition(() => preferenceChangeEventCount >= 2);
+    is(preferenceChangeEventCount, 2);
+
+    info("Navigate to another origin");
+    await loadURL(
+      tab.linkedBrowser,
+      "https://example.org/document-builder.sjs?html=otherorigin"
+    );
+
+    values = await sendPingCommand(rootMessageHandler, contextDescriptor);
+    is(
+      values.length,
+      1,
+      "Broadcast returned a single value after cross origin navigation"
+    );
+
+    info("Update the preference and check again that we only receive 1 event");
+    Services.prefs.setIntPref(TEST_PREF, 3);
+    await TestUtils.waitForCondition(() => preferenceChangeEventCount >= 3);
+    is(preferenceChangeEventCount, 3);
+  } finally {
+    rootMessageHandler.destroy();
+    gBrowser.removeTab(tab);
+    Services.prefs.clearUserPref(TEST_PREF);
+  }
+});
+
+function sendPingCommand(rootMessageHandler, contextDescriptor) {
+  return rootMessageHandler.handleCommand({
+    moduleName: "eventonprefchange",
+    commandName: "ping",
+    params: {},
+    destination: {
+      contextDescriptor,
+      type: WindowGlobalMessageHandler.type,
+    },
+  });
+}
diff --git a/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js b/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js
new file mode 100644
index 0000000000..ea75167bf2
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js
@@ -0,0 +1,449 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check the basic behavior of on/off.
+ */
+add_task(async function test_add_remove_event_listener() {
+  const tab = await addTab("https://example.com/document-builder.sjs?html=tab");
+  const browsingContext = tab.linkedBrowser.browsingContext;
+  const contextDescriptor = {
+    type: ContextDescriptorType.TopBrowsingContext,
+    id: browsingContext.browserId,
+  };
+
+  const root = createRootMessageHandler("session-id-event");
+  const monitoringEvents = await setupEventMonitoring(root);
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(await isSubscribed(root, browsingContext), false);
+
+  info("Add an listener for eventemitter.testEvent");
+  const events = [];
+  const onEvent = (event, data) => events.push(data.text);
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+  is(await isSubscribed(root, browsingContext), true);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 1);
+
+  info(
+    "Remove a listener for a callback not added before and check that the first one is still registered"
+  );
+  const anotherCallback = () => {};
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    anotherCallback
+  );
+  is(await isSubscribed(root, browsingContext), true);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 2);
+
+  info("Remove the listener for eventemitter.testEvent");
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+  is(await isSubscribed(root, browsingContext), false);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 2);
+
+  info("Add the listener for eventemitter.testEvent again");
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+  is(await isSubscribed(root, browsingContext), true);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 3);
+
+  info("Remove the listener for eventemitter.testEvent");
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+  is(await isSubscribed(root, browsingContext), false);
+
+  info("Remove the listener again to check the API will not throw");
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+
+  root.destroy();
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Check that two callbacks can subscribe to the same event in the same context
+ * in parallel.
+ */
+add_task(async function test_two_callbacks() {
+  const tab = await addTab("https://example.com/document-builder.sjs?html=tab");
+  const browsingContext = tab.linkedBrowser.browsingContext;
+  const contextDescriptor = {
+    type: ContextDescriptorType.TopBrowsingContext,
+    id: browsingContext.browserId,
+  };
+
+  const root = createRootMessageHandler("session-id-event");
+  const monitoringEvents = await setupEventMonitoring(root);
+
+  info("Add an listener for eventemitter.testEvent");
+  const events = [];
+  const onEvent = (event, data) => events.push(data.text);
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 1);
+
+  info("Add another listener for eventemitter.testEvent");
+  const otherevents = [];
+  const otherCallback = (event, data) => otherevents.push(data.text);
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    otherCallback
+  );
+  is(await isSubscribed(root, browsingContext), true);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 2);
+  is(otherevents.length, 1);
+
+  info("Remove the other listener for eventemitter.testEvent");
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    otherCallback
+  );
+  is(await isSubscribed(root, browsingContext), true);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 3);
+  is(otherevents.length, 1);
+
+  info("Remove the first listener for eventemitter.testEvent");
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor,
+    onEvent
+  );
+  is(await isSubscribed(root, browsingContext), false);
+
+  await emitTestEvent(root, browsingContext, monitoringEvents);
+  is(events.length, 3);
+  is(otherevents.length, 1);
+
+  root.destroy();
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Check that two callbacks can subscribe to the same event in the two contexts.
+ */
+add_task(async function test_two_contexts() {
+  const tab1 = await addTab("https://example.com/document-builder.sjs?html=1");
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  const tab2 = await addTab("https://example.com/document-builder.sjs?html=2");
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+
+  const contextDescriptor1 = {
+    type: ContextDescriptorType.TopBrowsingContext,
+    id: browsingContext1.browserId,
+  };
+  const contextDescriptor2 = {
+    type: ContextDescriptorType.TopBrowsingContext,
+    id: browsingContext2.browserId,
+  };
+
+  const root = createRootMessageHandler("session-id-event");
+
+  const monitoringEvents = await setupEventMonitoring(root);
+
+  const events1 = [];
+  const onEvent1 = (event, data) => events1.push(data.text);
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor1,
+    onEvent1
+  );
+  is(await isSubscribed(root, browsingContext1), true);
+  is(await isSubscribed(root, browsingContext2), false);
+
+  const events2 = [];
+  const onEvent2 = (event, data) => events2.push(data.text);
+  await root.eventsDispatcher.on(
+    "eventemitter.testEvent",
+    contextDescriptor2,
+    onEvent2
+  );
+  is(await isSubscribed(root, browsingContext1), true);
+  is(await isSubscribed(root, browsingContext2), true);
+
+  await emitTestEvent(root, browsingContext1, monitoringEvents);
+  is(events1.length, 1);
+  is(events2.length, 0);
+
+  await emitTestEvent(root, browsingContext2, monitoringEvents);
+  is(events1.length, 1);
+  is(events2.length, 1);
+
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor1,
+    onEvent1
+  );
+  is(await isSubscribed(root, browsingContext1), false);
+  is(await isSubscribed(root, browsingContext2), true);
+
+  // No event expected here since the module for browsingContext1 is no longer
+  // subscribed
+  await emitTestEvent(root, browsingContext1, monitoringEvents);
+  is(events1.length, 1);
+  is(events2.length, 1);
+
+  // Whereas the module for browsingContext2 is still subscribed
+  await emitTestEvent(root, browsingContext2, monitoringEvents);
+  is(events1.length, 1);
+  is(events2.length, 2);
+
+  await root.eventsDispatcher.off(
+    "eventemitter.testEvent",
+    contextDescriptor2,
+    onEvent2
+  );
+  is(await isSubscribed(root, browsingContext1), false);
+  is(await isSubscribed(root, browsingContext2), false);
+
+  await emitTestEvent(root, browsingContext1, monitoringEvents);
+  await emitTestEvent(root, browsingContext2, monitoringEvents);
+  is(events1.length, 1);
+  is(events2.length, 2);
+
+  root.destroy();
+  gBrowser.removeTab(tab2);
+  gBrowser.removeTab(tab1);
+});
+
+/**
+ * Check that adding and removing first listener for the specific context and then
+ * for the global context works as expected.
+ */
+add_task(
+  async function test_remove_context_event_listener_and_then_global_event_listener() {
+    const tab = await addTab(
+      "https://example.com/document-builder.sjs?html=tab"
+    );
+    const browsingContext = tab.linkedBrowser.browsingContext;
+    const contextDescriptor = {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext.browserId,
+    };
+    const contextDescriptorAll = {
+      type: ContextDescriptorType.All,
+    };
+
+    const root = createRootMessageHandler("session-id-event");
+    const monitoringEvents = await setupEventMonitoring(root);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(await isSubscribed(root, browsingContext), false);
+
+    info("Add an listener for eventemitter.testEvent");
+    const events = [];
+    const onEvent = (event, data) => events.push(data.text);
+    await root.eventsDispatcher.on(
+      "eventemitter.testEvent",
+      contextDescriptor,
+      onEvent
+    );
+    is(await isSubscribed(root, browsingContext), true);
+
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(events.length, 1);
+
+    info(
+      "Add another listener for eventemitter.testEvent, using global context"
+    );
+    const eventsAll = [];
+    const onEventAll = (event, data) => eventsAll.push(data.text);
+    await root.eventsDispatcher.on(
+      "eventemitter.testEvent",
+      contextDescriptorAll,
+      onEventAll
+    );
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 1);
+    is(events.length, 2);
+
+    info("Remove the first listener for eventemitter.testEvent");
+    await root.eventsDispatcher.off(
+      "eventemitter.testEvent",
+      contextDescriptor,
+      onEvent
+    );
+
+    info("Check that we are still subscribed to eventemitter.testEvent");
+    is(await isSubscribed(root, browsingContext), true);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 2);
+    is(events.length, 2);
+
+    await root.eventsDispatcher.off(
+      "eventemitter.testEvent",
+      contextDescriptorAll,
+      onEventAll
+    );
+    is(await isSubscribed(root, browsingContext), false);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 2);
+    is(events.length, 2);
+
+    root.destroy();
+    gBrowser.removeTab(tab);
+  }
+);
+
+/**
+ * Check that adding and removing first listener for the global context and then
+ * for the specific context works as expected.
+ */
+add_task(
+  async function test_global_event_listener_and_then_remove_context_event_listener() {
+    const tab = await addTab(
+      "https://example.com/document-builder.sjs?html=tab"
+    );
+    const browsingContext = tab.linkedBrowser.browsingContext;
+    const contextDescriptor = {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext.browserId,
+    };
+    const contextDescriptorAll = {
+      type: ContextDescriptorType.All,
+    };
+
+    const root = createRootMessageHandler("session-id-event");
+    const monitoringEvents = await setupEventMonitoring(root);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(await isSubscribed(root, browsingContext), false);
+
+    info("Add an listener for eventemitter.testEvent");
+    const events = [];
+    const onEvent = (event, data) => events.push(data.text);
+    await root.eventsDispatcher.on(
+      "eventemitter.testEvent",
+      contextDescriptor,
+      onEvent
+    );
+    is(await isSubscribed(root, browsingContext), true);
+
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(events.length, 1);
+
+    info(
+      "Add another listener for eventemitter.testEvent, using global context"
+    );
+    const eventsAll = [];
+    const onEventAll = (event, data) => eventsAll.push(data.text);
+    await root.eventsDispatcher.on(
+      "eventemitter.testEvent",
+      contextDescriptorAll,
+      onEventAll
+    );
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 1);
+    is(events.length, 2);
+
+    info("Remove the global listener for eventemitter.testEvent");
+    await root.eventsDispatcher.off(
+      "eventemitter.testEvent",
+      contextDescriptorAll,
+      onEventAll
+    );
+
+    info(
+      "Check that we are still subscribed to eventemitter.testEvent for the specific context"
+    );
+    is(await isSubscribed(root, browsingContext), true);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 1);
+    is(events.length, 3);
+
+    await root.eventsDispatcher.off(
+      "eventemitter.testEvent",
+      contextDescriptor,
+      onEvent
+    );
+
+    is(await isSubscribed(root, browsingContext), false);
+    await emitTestEvent(root, browsingContext, monitoringEvents);
+    is(eventsAll.length, 1);
+    is(events.length, 3);
+
+    root.destroy();
+    gBrowser.removeTab(tab);
+  }
+);
+
+async function setupEventMonitoring(root) {
+  const monitoringEvents = [];
+  const onMonitoringEvent = (event, data) => monitoringEvents.push(data.text);
+  root.on("eventemitter.monitoringEvent", onMonitoringEvent);
+
+  registerCleanupFunction(() =>
+    root.off("eventemitter.monitoringEvent", onMonitoringEvent)
+  );
+
+  return monitoringEvents;
+}
+
+async function emitTestEvent(root, browsingContext, monitoringEvents) {
+  const count = monitoringEvents.length;
+  info("Call eventemitter.emitTestEvent");
+  await root.handleCommand({
+    moduleName: "eventemitter",
+    commandName: "emitTestEvent",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+
+  // The monitoring event is always emitted, regardless of the status of the
+  // module. Wait for catching this event before resuming the assertions.
+  info("Wait for the monitoring event");
+  await BrowserTestUtils.waitForCondition(
+    () => monitoringEvents.length >= count + 1
+  );
+  is(monitoringEvents.length, count + 1);
+}
+
+function isSubscribed(root, browsingContext) {
+  info("Call eventemitter.isSubscribed");
+  return root.handleCommand({
+    moduleName: "eventemitter",
+    commandName: "isSubscribed",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+}
diff --git a/remote/shared/messagehandler/test/browser/browser_events_handler.js b/remote/shared/messagehandler/test/browser/browser_events_handler.js
new file mode 100644
index 0000000000..705c306de3
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_events_handler.js
@@ -0,0 +1,57 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test that the window-global-handler-created event gets emitted for each
+ * individual frame's browsing context.
+ */
+add_task(async function test_windowGlobalHandlerCreated() {
+  const events = [];
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-event_with_frames"
+  );
+
+  info("Add a new session data item to get window global handlers created");
+  await rootMessageHandler.addSessionDataItem({
+    moduleName: "command",
+    category: "browser_session_data_browser_element",
+    contextDescriptor: {
+      type: ContextDescriptorType.All,
+    },
+    values: [true],
+  });
+
+  const onEvent = (evtName, wrappedEvt) => {
+    if (wrappedEvt.name === "window-global-handler-created") {
+      console.info(`Received event for context ${wrappedEvt.data.contextId}`);
+      events.push(wrappedEvt.data);
+    }
+  };
+  rootMessageHandler.on("message-handler-event", onEvent);
+
+  info("Navigate the initial tab to the test URL");
+  const browser = gBrowser.selectedTab.linkedBrowser;
+  await loadURL(browser, createTestMarkupWithFrames());
+
+  const contexts = browser.browsingContext.getAllBrowsingContextsInSubtree();
+  is(contexts.length, 4, "Test tab has 3 children contexts (4 in total)");
+
+  // Wait for all the events
+  await TestUtils.waitForCondition(() => events.length >= 4);
+
+  for (const context of contexts) {
+    const contextEvents = events.filter(evt => {
+      return (
+        evt.contextId === context.id &&
+        evt.innerWindowId === context.currentWindowGlobal.innerWindowId
+      );
+    });
+    is(contextEvents.length, 1, `Found event for context ${context.id}`);
+  }
+
+  rootMessageHandler.off("message-handler-event", onEvent);
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_events_module.js b/remote/shared/messagehandler/test/browser/browser_events_module.js
new file mode 100644
index 0000000000..536c1d9754
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_events_module.js
@@ -0,0 +1,281 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { RootMessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs"
+);
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+/**
+ * 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,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-event");
+
+  // Events are emitted both as generic message-handler-event events as well
+  // as under their own name. We expect to receive the event for both.
+  const onHandlerEvent = rootMessageHandler.once("message-handler-event");
+  const onNamedEvent = rootMessageHandler.once("event-from-window-global");
+  // MessageHandlerRegistry should forward all the message-handler-events.
+  const onRegistryEvent = RootMessageHandlerRegistry.once(
+    "message-handler-registry-event"
+  );
+
+  callTestEmitEvent(rootMessageHandler, browsingContext.id);
+
+  const messageHandlerEvent = await onHandlerEvent;
+  is(
+    messageHandlerEvent.name,
+    "event-from-window-global",
+    "Received event on the ROOT MessageHandler"
+  );
+  is(
+    messageHandlerEvent.data.text,
+    `event from ${browsingContext.id}`,
+    "Received the expected payload"
+  );
+
+  const namedEvent = await onNamedEvent;
+  is(
+    namedEvent.text,
+    `event from ${browsingContext.id}`,
+    "Received the expected payload"
+  );
+
+  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");
+
+  // events are emitted both as generic message-handler-event events as
+  // well as under their own name. We expect to receive the event for both.
+  const onHandlerEvent = rootMessageHandler.once("message-handler-event");
+  const onNamedEvent = rootMessageHandler.once("event-from-root");
+
+  rootMessageHandler.handleCommand({
+    moduleName: "event",
+    commandName: "testEmitRootEvent",
+    destination: {
+      type: RootMessageHandler.type,
+    },
+  });
+
+  const { name, data } = await onHandlerEvent;
+  is(name, "event-from-root", "Received event on the ROOT MessageHandler");
+  is(data.text, "event from root", "Received the expected payload");
+
+  const namedEvent = await onNamedEvent;
+  is(namedEvent.text, "event from root", "Received the expected payload");
+
+  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,
+    "https://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"
+  );
+
+  // events are emitted both as generic message-handler-event events as
+  // well as under their own name. We expect to receive the event for both.
+  const onHandlerEvent = rootMessageHandler.once("message-handler-event");
+  const onNamedEvent = rootMessageHandler.once(
+    "event-from-window-global-in-root"
+  );
+  rootMessageHandler.handleCommand({
+    moduleName: "event",
+    commandName: "testEmitWindowGlobalInRootEvent",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+
+  const { name, data } = await onHandlerEvent;
+  is(
+    name,
+    "event-from-window-global-in-root",
+    "Received event on the ROOT MessageHandler"
+  );
+  is(
+    data.text,
+    `windowglobal-in-root event for ${browsingContext.id}`,
+    "Received the expected payload"
+  );
+
+  const namedEvent = await onNamedEvent;
+  is(
+    namedEvent.text,
+    `windowglobal-in-root event for ${browsingContext.id}`,
+    "Received the expected payload"
+  );
+
+  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,
+    "https://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.name === "event-from-window-global") {
+      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.name === "event-from-window-global") {
+      root2Events++;
+    }
+  };
+  root2.on("message-handler-event", onRoot2Event);
+
+  let registryEvents = 0;
+  const onRegistryEvent = function (evtName, wrappedEvt) {
+    if (wrappedEvt.name === "event-from-window-global") {
+      registryEvents++;
+    }
+  };
+  RootMessageHandlerRegistry.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);
+  RootMessageHandlerRegistry.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"
+  );
+
+  const rootEvents = [];
+  const onRootEvent = function (evtName, wrappedEvt) {
+    if (wrappedEvt.name === "event-from-window-global") {
+      rootEvents.push(wrappedEvt.data.text);
+    }
+  };
+  rootMessageHandler.on("message-handler-event", onRootEvent);
+
+  const namedEvents = [];
+  const onNamedEvent = (name, event) => namedEvents.push(event.text);
+  rootMessageHandler.on("event-from-window-global", onNamedEvent);
+
+  for (const context of contexts) {
+    callTestEmitEvent(rootMessageHandler, context.id);
+    info("Wait for root event to be received in both event arrays");
+    await TestUtils.waitForCondition(() =>
+      [namedEvents, rootEvents].every(events =>
+        events.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.off("event-from-window-global", onNamedEvent);
+  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/browser_frame_context_utils.js b/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js
new file mode 100644
index 0000000000..cddcba3529
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js
@@ -0,0 +1,98 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { isBrowsingContextCompatible } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs"
+);
+const TEST_COM_PAGE = "https://example.com/document-builder.sjs?html=com";
+const TEST_NET_PAGE = "https://example.net/document-builder.sjs?html=net";
+
+// Test helpers from BrowsingContextUtils in various processes.
+add_task(async function () {
+  const tab1 = BrowserTestUtils.addTab(gBrowser, TEST_COM_PAGE);
+  const contentBrowser1 = tab1.linkedBrowser;
+  await BrowserTestUtils.browserLoaded(contentBrowser1);
+  const browserId1 = contentBrowser1.browsingContext.browserId;
+
+  const tab2 = BrowserTestUtils.addTab(gBrowser, TEST_NET_PAGE);
+  const contentBrowser2 = tab2.linkedBrowser;
+  await BrowserTestUtils.browserLoaded(contentBrowser2);
+  const browserId2 = contentBrowser2.browsingContext.browserId;
+
+  const { extension, sidebarBrowser } = await installSidebarExtension();
+
+  const tab3 = BrowserTestUtils.addTab(
+    gBrowser,
+    `moz-extension://${extension.uuid}/tab.html`
+  );
+  const { bcId } = await extension.awaitMessage("tab-loaded");
+  const tabExtensionBrowser = BrowsingContext.get(bcId).top.embedderElement;
+
+  const parentBrowser1 = createParentBrowserElement(tab1, "content");
+  const parentBrowser2 = createParentBrowserElement(tab1, "chrome");
+
+  info("Check browsing context compatibility for content browser 1");
+  await checkBrowsingContextCompatible(contentBrowser1, undefined, true);
+  await checkBrowsingContextCompatible(contentBrowser1, browserId1, true);
+  await checkBrowsingContextCompatible(contentBrowser1, browserId2, false);
+
+  info("Check browsing context compatibility for content browser 2");
+  await checkBrowsingContextCompatible(contentBrowser2, undefined, true);
+  await checkBrowsingContextCompatible(contentBrowser2, browserId1, false);
+  await checkBrowsingContextCompatible(contentBrowser2, browserId2, true);
+
+  info("Check browsing context compatibility for parent browser 1");
+  await checkBrowsingContextCompatible(parentBrowser1, undefined, false);
+  await checkBrowsingContextCompatible(parentBrowser1, browserId1, false);
+  await checkBrowsingContextCompatible(parentBrowser1, browserId2, false);
+
+  info("Check browsing context compatibility for parent browser 2");
+  await checkBrowsingContextCompatible(parentBrowser2, undefined, false);
+  await checkBrowsingContextCompatible(parentBrowser2, browserId1, false);
+  await checkBrowsingContextCompatible(parentBrowser2, browserId2, false);
+
+  info("Check browsing context compatibility for extension");
+  await checkBrowsingContextCompatible(sidebarBrowser, undefined, false);
+  await checkBrowsingContextCompatible(sidebarBrowser, browserId1, false);
+  await checkBrowsingContextCompatible(sidebarBrowser, browserId2, false);
+
+  info("Check browsing context compatibility for extension viewed in a tab");
+  await checkBrowsingContextCompatible(tabExtensionBrowser, undefined, false);
+  await checkBrowsingContextCompatible(tabExtensionBrowser, browserId1, false);
+  await checkBrowsingContextCompatible(tabExtensionBrowser, browserId2, false);
+
+  gBrowser.removeTab(tab1);
+  gBrowser.removeTab(tab2);
+  gBrowser.removeTab(tab3);
+  await extension.unload();
+});
+
+async function checkBrowsingContextCompatible(browser, browserId, expected) {
+  const options = { browserId };
+  info("Check browsing context compatibility from the parent process");
+  is(isBrowsingContextCompatible(browser.browsingContext, options), expected);
+
+  info(
+    "Check browsing context compatibility from the browsing context's process"
+  );
+  await SpecialPowers.spawn(
+    browser,
+    [browserId, expected],
+    (_browserId, _expected) => {
+      const BrowsingContextUtils = ChromeUtils.importESModule(
+        "chrome://remote/content/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs"
+      );
+      is(
+        BrowsingContextUtils.isBrowsingContextCompatible(
+          content.browsingContext,
+          {
+            browserId: _browserId,
+          }
+        ),
+        _expected
+      );
+    }
+  );
+}
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js b/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js
new file mode 100644
index 0000000000..5f0bdb78df
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js
@@ -0,0 +1,218 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+// Check that errors from WindowGlobal modules can be caught by the consumer
+// of the RootMessageHandler.
+add_task(async function test_module_error() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-error");
+
+  info("Call a module method which will throw");
+  try {
+    await rootMessageHandler.handleCommand({
+      moduleName: "commandwindowglobalonly",
+      commandName: "testError",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContextId,
+      },
+    });
+    ok(false, "Error from window global module was not caught");
+  } catch (e) {
+    ok(true, "Error from window global module caught");
+  }
+
+  rootMessageHandler.destroy();
+});
+
+// Check that sending commands to incorrect destinations creates an error which
+// can be caught by the consumer of the RootMessageHandler.
+add_task(async function test_destination_error() {
+  const rootMessageHandler = createRootMessageHandler("session-id-error");
+
+  const fakeBrowsingContextId = -1;
+  ok(
+    !BrowsingContext.get(fakeBrowsingContextId),
+    "No browsing context matches fakeBrowsingContextId"
+  );
+
+  info("Call a valid module method, but on a non-existent browsing context id");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "commandwindowglobalonly",
+        commandName: "testOnlyInWindowGlobal",
+        destination: {
+          type: WindowGlobalMessageHandler.type,
+          id: fakeBrowsingContextId,
+        },
+      }),
+    err => err.message == `Unable to find a BrowsingContext for id -1`
+  );
+
+  rootMessageHandler.destroy();
+});
+
+add_task(async function test_invalid_module_error() {
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_module"
+  );
+
+  info("Attempt to call a Root module which has a syntax error");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "invalid",
+        commandName: "someMethod",
+        destination: {
+          type: RootMessageHandler.type,
+        },
+      }),
+    err =>
+      err.name === "SyntaxError" &&
+      err.message == "expected expression, got ';'"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+add_task(async function test_missing_root_module_error() {
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_module"
+  );
+
+  info("Attempt to call a Root module which doesn't exist");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "missingmodule",
+        commandName: "someMethod",
+        destination: {
+          type: RootMessageHandler.type,
+        },
+      }),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message ==
+        `missingmodule.someMethod not supported for destination ROOT`
+  );
+
+  rootMessageHandler.destroy();
+});
+
+add_task(async function test_missing_windowglobal_module_error() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_windowglobal_module"
+  );
+
+  info("Attempt to call a WindowGlobal module which doesn't exist");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "missingmodule",
+        commandName: "someMethod",
+        destination: {
+          type: WindowGlobalMessageHandler.type,
+          id: browsingContextId,
+        },
+      }),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message ==
+        `missingmodule.someMethod not supported for destination WINDOW_GLOBAL`
+  );
+
+  rootMessageHandler.destroy();
+});
+
+add_task(async function test_missing_root_method_error() {
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_root_method"
+  );
+
+  info("Attempt to call an invalid method on a Root module");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "command",
+        commandName: "wrongMethod",
+        destination: {
+          type: RootMessageHandler.type,
+        },
+      }),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message == `command.wrongMethod not supported for destination ROOT`
+  );
+
+  rootMessageHandler.destroy();
+});
+
+add_task(async function test_missing_windowglobal_method_error() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_windowglobal_method"
+  );
+
+  info("Attempt to call an invalid method on a WindowGlobal module");
+  Assert.throws(
+    () =>
+      rootMessageHandler.handleCommand({
+        moduleName: "commandwindowglobalonly",
+        commandName: "wrongMethod",
+        destination: {
+          type: WindowGlobalMessageHandler.type,
+          id: browsingContextId,
+        },
+      }),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message ==
+        `commandwindowglobalonly.wrongMethod not supported for destination WINDOW_GLOBAL`
+  );
+
+  rootMessageHandler.destroy();
+});
+
+/**
+ * This test checks that even if a command is rerouted to another command after
+ * the RootMessageHandler, we still check the new command and log a useful
+ * error message.
+ *
+ * This illustrates why it is important to perform the command check at each
+ * layer of the MessageHandler network.
+ */
+add_task(async function test_missing_intermediary_method_error() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-missing_intermediary_method"
+  );
+
+  info(
+    "Call a (valid) command that relies on another (missing) command on a WindowGlobal module"
+  );
+  await Assert.rejects(
+    rootMessageHandler.handleCommand({
+      moduleName: "commandwindowglobalonly",
+      commandName: "testMissingIntermediaryMethod",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContextId,
+      },
+    }),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message ==
+        `commandwindowglobalonly.missingMethod not supported for destination WINDOW_GLOBAL`
+  );
+
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js b/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js
new file mode 100644
index 0000000000..fc70ae8c4e
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js
@@ -0,0 +1,239 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// We are forcing the actors to shutdown while queries are unresolved.
+const { PromiseTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PromiseTestUtils.sys.mjs"
+);
+PromiseTestUtils.allowMatchingRejectionsGlobally(
+  /Actor 'MessageHandlerFrame' destroyed before query 'MessageHandlerFrameParent:sendCommand' was resolved/
+);
+
+// The tests in this file assert the retry behavior for MessageHandler commands.
+// We call "blocked" commands from resources/modules/windowglobal/retry.jsm and
+// then trigger reload and navigations to simulate AbortErrors and force the
+// MessageHandler to retry the commands, when possible.
+
+// Test that without retry behavior, a pending command rejects when the
+// underlying JSWindowActor pair is destroyed.
+add_task(async function test_no_retry() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-no-retry");
+
+  try {
+    info("Call a module method which will throw");
+    const onBlockedOneTime = rootMessageHandler.handleCommand({
+      moduleName: "retry",
+      commandName: "blockedOneTime",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContext.id,
+      },
+    });
+
+    // Reloading the tab will reject the pending query with an AbortError.
+    await BrowserTestUtils.reloadTab(tab);
+
+    try {
+      await onBlockedOneTime;
+      ok("false", "onBlockedOneTime should not have resolved");
+    } catch (e) {
+      is(
+        e.name,
+        "AbortError",
+        "Caught the expected abort error when reloading"
+      );
+    }
+  } finally {
+    await cleanup(rootMessageHandler, tab);
+  }
+});
+
+// Test various commands, which all need a different number of "retries" to
+// succeed. Check that they only resolve when the expected number of "retries"
+// was reached. For commands which require more "retries" than we allow, check
+// that we still fail with an AbortError once all the attempts are consumed.
+add_task(async function test_retry() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-retry");
+
+  try {
+    // This command will return if called twice.
+    const onBlockedOneTime = rootMessageHandler.handleCommand({
+      moduleName: "retry",
+      commandName: "blockedOneTime",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContext.id,
+      },
+      params: {
+        foo: "bar",
+      },
+      retryOnAbort: true,
+    });
+
+    // This command will return if called three times.
+    const onBlockedTenTimes = rootMessageHandler.handleCommand({
+      moduleName: "retry",
+      commandName: "blockedTenTimes",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContext.id,
+      },
+      params: {
+        foo: "baz",
+      },
+      retryOnAbort: true,
+    });
+
+    // This command will return if called twelve times, which is greater than the
+    // maximum amount of retries allowed.
+    const onBlockedElevenTimes = rootMessageHandler.handleCommand({
+      moduleName: "retry",
+      commandName: "blockedElevenTimes",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContext.id,
+      },
+      retryOnAbort: true,
+    });
+
+    info("Reload one time");
+    await BrowserTestUtils.reloadTab(tab);
+
+    info("blockedOneTime should resolve on the first retry");
+    let { callsToCommand, foo } = await onBlockedOneTime;
+    is(
+      callsToCommand,
+      2,
+      "The command was called twice (initial call + 1 retry)"
+    );
+    is(foo, "bar", "The parameter was sent when the command was retried");
+
+    // We already reloaded 1 time. Reload 9 more times to unblock blockedTenTimes.
+    for (let i = 2; i < 11; i++) {
+      info("blockedTenTimes/blockedElevenTimes should not have resolved yet");
+      ok(!(await hasPromiseResolved(onBlockedTenTimes)));
+      ok(!(await hasPromiseResolved(onBlockedElevenTimes)));
+
+      info(`Reload the tab (time: ${i})`);
+      await BrowserTestUtils.reloadTab(tab);
+    }
+
+    info("blockedTenTimes should resolve on the 10th reload");
+    ({ callsToCommand, foo } = await onBlockedTenTimes);
+    is(
+      callsToCommand,
+      11,
+      "The command was called 11 times (initial call + 10 retry)"
+    );
+    is(foo, "baz", "The parameter was sent when the command was retried");
+
+    info("Reload one more time");
+    await BrowserTestUtils.reloadTab(tab);
+
+    try {
+      info(
+        "The call to blockedElevenTimes now exceeds the maximum attempts allowed"
+      );
+      await onBlockedElevenTimes;
+      ok("false", "blockedElevenTimes should not have resolved");
+    } catch (e) {
+      is(
+        e.name,
+        "AbortError",
+        "Caught the expected abort error when reloading"
+      );
+    }
+  } finally {
+    await cleanup(rootMessageHandler, tab);
+  }
+});
+
+// Test cross-group navigations to check that the retry mechanism will
+// transparently switch to the new Browsing Context created by the cross-group
+// navigation.
+add_task(async function test_retry_cross_group() {
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "https://example.com/document-builder.sjs?html=COM" +
+      // Attach an unload listener to prevent the page from going into bfcache,
+      // so that pending queries will be rejected with an AbortError.
+      "<script type='text/javascript'>window.onunload = function() {};</script>"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-retry-cross-group"
+  );
+
+  try {
+    // This command hangs and only returns if the current domain is example.net.
+    // We send the command while on example.com, perform a series of reload and
+    // navigations, and the retry mechanism should allow onBlockedOnNetDomain to
+    // resolve.
+    const onBlockedOnNetDomain = rootMessageHandler.handleCommand({
+      moduleName: "retry",
+      commandName: "blockedOnNetDomain",
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContext.id,
+      },
+      params: {
+        foo: "bar",
+      },
+      retryOnAbort: true,
+    });
+
+    info("Reload one time");
+    await BrowserTestUtils.reloadTab(tab);
+
+    info("blockedOnNetDomain should not have resolved yet");
+    ok(!(await hasPromiseResolved(onBlockedOnNetDomain)));
+
+    info(
+      "Navigate to example.net with COOP headers to destroy browsing context"
+    );
+    await loadURL(
+      tab.linkedBrowser,
+      "https://example.net/document-builder.sjs?headers=Cross-Origin-Opener-Policy:same-origin&html=NET"
+    );
+
+    info("blockedOnNetDomain should resolve now");
+    let { foo } = await onBlockedOnNetDomain;
+    is(foo, "bar", "The parameter was sent when the command was retried");
+  } finally {
+    await cleanup(rootMessageHandler, tab);
+  }
+});
+
+async function cleanup(rootMessageHandler, tab) {
+  const browsingContext = tab.linkedBrowser.browsingContext;
+  // Cleanup global JSM state in the test module.
+  await rootMessageHandler.handleCommand({
+    moduleName: "retry",
+    commandName: "cleanup",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+
+  rootMessageHandler.destroy();
+  gBrowser.removeTab(tab);
+}
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js b/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js
new file mode 100644
index 0000000000..0a086d6f09
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js
@@ -0,0 +1,203 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+// Test calling methods only implemented in the root version of a module.
+add_task(async function test_rootModule_command() {
+  const rootMessageHandler = createRootMessageHandler("session-id-rootModule");
+  const rootValue = await rootMessageHandler.handleCommand({
+    moduleName: "command",
+    commandName: "testRootModule",
+    destination: {
+      type: RootMessageHandler.type,
+    },
+  });
+
+  is(
+    rootValue,
+    "root-value",
+    "Retrieved the expected value from testRootModule"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+// Test calling methods only implemented in the windowglobal-in-root version of
+// a module.
+add_task(async function test_windowglobalInRootModule_command() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-windowglobalInRootModule"
+  );
+  const interceptedValue = await rootMessageHandler.handleCommand({
+    moduleName: "command",
+    commandName: "testInterceptModule",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    interceptedValue,
+    "intercepted-value",
+    "Retrieved the expected value from testInterceptModule"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+// Test calling methods only implemented in the windowglobal version of a
+// module.
+add_task(async function test_windowglobalModule_command() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-windowglobalModule"
+  );
+  const windowGlobalValue = await rootMessageHandler.handleCommand({
+    moduleName: "command",
+    commandName: "testWindowGlobalModule",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    windowGlobalValue,
+    "windowglobal-value",
+    "Retrieved the expected value from testWindowGlobalModule"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+// Test calling a method on a module which is only available in the "windowglobal"
+// folder. This will check that the MessageHandler/ModuleCache correctly moves
+// on to the next layer when no implementation can be found in the root layer.
+add_task(async function test_windowglobalOnlyModule_command() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-windowglobalOnlyModule"
+  );
+  const windowGlobalOnlyValue = await rootMessageHandler.handleCommand({
+    moduleName: "commandwindowglobalonly",
+    commandName: "testOnlyInWindowGlobal",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    windowGlobalOnlyValue,
+    "only-in-windowglobal",
+    "Retrieved the expected value from testOnlyInWindowGlobal"
+  );
+
+  rootMessageHandler.destroy();
+});
+
+// Try to create 2 sessions which will both set values in individual modules
+// via a command `testSetValue`, and then retrieve the values via another
+// command `testGetValue`.
+// This will ensure that different sessions use different module instances.
+add_task(async function test_multisession() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler1 = createRootMessageHandler(
+    "session-id-multisession-1"
+  );
+  const rootMessageHandler2 = createRootMessageHandler(
+    "session-id-multisession-2"
+  );
+
+  info("Set value for session 1");
+  await rootMessageHandler1.handleCommand({
+    moduleName: "command",
+    commandName: "testSetValue",
+    params: { value: "session1-value" },
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  info("Set value for session 2");
+  await rootMessageHandler2.handleCommand({
+    moduleName: "command",
+    commandName: "testSetValue",
+    params: { value: "session2-value" },
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  const session1Value = await rootMessageHandler1.handleCommand({
+    moduleName: "command",
+    commandName: "testGetValue",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    session1Value,
+    "session1-value",
+    "Retrieved the expected value for session 1"
+  );
+
+  const session2Value = await rootMessageHandler2.handleCommand({
+    moduleName: "command",
+    commandName: "testGetValue",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    session2Value,
+    "session2-value",
+    "Retrieved the expected value for session 2"
+  );
+
+  rootMessageHandler1.destroy();
+  rootMessageHandler2.destroy();
+});
+
+// Test calling a method from the windowglobal-in-root module which will
+// internally forward to the windowglobal module and will return a composite
+// result built both in parent and content process.
+add_task(async function test_forwarding_command() {
+  const browsingContextId = gBrowser.selectedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler("session-id-forwarding");
+  const interceptAndForwardValue = await rootMessageHandler.handleCommand({
+    moduleName: "command",
+    commandName: "testInterceptAndForwardModule",
+    params: { id: "value" },
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContextId,
+    },
+  });
+
+  is(
+    interceptAndForwardValue,
+    "intercepted-and-forward+forward-to-windowglobal-value",
+    "Retrieved the expected value from testInterceptAndForwardModule"
+  );
+
+  rootMessageHandler.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_registry.js b/remote/shared/messagehandler/test/browser/browser_registry.js
new file mode 100644
index 0000000000..945ac06c19
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_registry.js
@@ -0,0 +1,37 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { MessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs"
+);
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+add_task(async function test_messageHandlerRegistry_API() {
+  const sessionId = 1;
+  const type = RootMessageHandler.type;
+
+  const rootMessageHandlerRegistry = new MessageHandlerRegistry(type);
+
+  const rootMessageHandler =
+    rootMessageHandlerRegistry.getOrCreateMessageHandler(sessionId);
+  ok(rootMessageHandler, "Valid ROOT MessageHandler created");
+
+  const contextId = rootMessageHandler.contextId;
+  ok(contextId, "ROOT MessageHandler has a valid contextId");
+
+  is(
+    rootMessageHandler,
+    rootMessageHandlerRegistry.getExistingMessageHandler(sessionId),
+    "ROOT MessageHandler can be retrieved from the registry"
+  );
+
+  rootMessageHandler.destroy();
+  ok(
+    !rootMessageHandlerRegistry.getExistingMessageHandler(sessionId),
+    "Destroyed ROOT MessageHandler is no longer returned by the Registry"
+  );
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data.js b/remote/shared/messagehandler/test/browser/browser_session_data.js
new file mode 100644
index 0000000000..f0fef31883
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data.js
@@ -0,0 +1,271 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { MessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs"
+);
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+const { SessionData } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs"
+);
+
+const TEST_PAGE = "http://example.com/document-builder.sjs?html=tab";
+
+add_task(async function test_sessionData() {
+  info("Navigate the initial tab to the test URL");
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+
+  const sessionId = "sessionData-test";
+
+  const rootMessageHandlerRegistry = new MessageHandlerRegistry(
+    RootMessageHandler.type
+  );
+
+  const rootMessageHandler =
+    rootMessageHandlerRegistry.getOrCreateMessageHandler(sessionId);
+  ok(rootMessageHandler, "Valid ROOT MessageHandler created");
+
+  const sessionData = rootMessageHandler.sessionData;
+  ok(
+    sessionData instanceof SessionData,
+    "ROOT MessageHandler has a valid sessionData"
+  );
+
+  let sessionDataSnapshot = await getSessionDataFromContent();
+  is(sessionDataSnapshot.size, 0, "session data is empty");
+
+  info("Store a string value in session data");
+  sessionData.updateSessionData([
+    {
+      method: "add",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: ["value-1"],
+    },
+  ]);
+
+  sessionDataSnapshot = await getSessionDataFromContent();
+  is(sessionDataSnapshot.size, 1, "session data contains 1 session");
+  ok(sessionDataSnapshot.has(sessionId));
+  let snapshot = sessionDataSnapshot.get(sessionId);
+  ok(Array.isArray(snapshot));
+  is(snapshot.length, 1);
+
+  const stringDataItem = snapshot[0];
+  checkSessionDataItem(
+    stringDataItem,
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    "value-1"
+  );
+
+  info("Store a number value in session data");
+  sessionData.updateSessionData([
+    {
+      method: "add",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: [12],
+    },
+  ]);
+  snapshot = (await getSessionDataFromContent()).get(sessionId);
+  is(snapshot.length, 2);
+
+  const numberDataItem = snapshot[1];
+  checkSessionDataItem(
+    numberDataItem,
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    12
+  );
+
+  info("Store a boolean value in session data");
+  sessionData.updateSessionData([
+    {
+      method: "add",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: [true],
+    },
+  ]);
+  snapshot = (await getSessionDataFromContent()).get(sessionId);
+  is(snapshot.length, 3);
+
+  const boolDataItem = snapshot[2];
+  checkSessionDataItem(
+    boolDataItem,
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    true
+  );
+
+  info("Remove one value");
+  sessionData.updateSessionData([
+    {
+      method: "remove",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: [12],
+    },
+  ]);
+  snapshot = (await getSessionDataFromContent()).get(sessionId);
+  is(snapshot.length, 2);
+  checkSessionDataItem(
+    snapshot[0],
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    "value-1"
+  );
+  checkSessionDataItem(
+    snapshot[1],
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    true
+  );
+
+  info("Remove all values");
+  sessionData.updateSessionData([
+    {
+      method: "remove",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: ["value-1", true],
+    },
+  ]);
+  snapshot = (await getSessionDataFromContent()).get(sessionId);
+  is(snapshot.length, 0, "Session data is now empty");
+
+  info("Add another value before destroy");
+  sessionData.updateSessionData([
+    {
+      method: "add",
+      moduleName: "fakemodule",
+      category: "testCategory",
+      contextDescriptor: contextDescriptorAll,
+      values: ["value-2"],
+    },
+  ]);
+  snapshot = (await getSessionDataFromContent()).get(sessionId);
+  is(snapshot.length, 1);
+  checkSessionDataItem(
+    snapshot[0],
+    "fakemodule",
+    "testCategory",
+    ContextDescriptorType.All,
+    "value-2"
+  );
+
+  sessionData.destroy();
+  sessionDataSnapshot = await getSessionDataFromContent();
+  is(sessionDataSnapshot.size, 0, "session data should be empty again");
+});
+
+add_task(async function test_sessionDataRootOnlyModule() {
+  const sessionId = "sessionData-test-rootOnly";
+
+  const rootMessageHandler = createRootMessageHandler(sessionId);
+  ok(rootMessageHandler, "Valid ROOT MessageHandler created");
+
+  await BrowserTestUtils.loadURIString(
+    gBrowser,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+
+  const windowGlobalCreated = rootMessageHandler.once("message-handler-event");
+
+  info("Test that adding SessionData items works the root module");
+  // Updating the session data on the root message handler should not cause
+  // failures for other message handlers if the module only exists for root.
+  await rootMessageHandler.addSessionDataItem({
+    moduleName: "rootOnly",
+    category: "session_data_root_only",
+    contextDescriptor: {
+      type: ContextDescriptorType.All,
+    },
+    values: [true],
+  });
+
+  await windowGlobalCreated;
+  ok(true, "Window global has been initialized");
+
+  let sessionDataReceivedByRoot = await rootMessageHandler.handleCommand({
+    moduleName: "rootOnly",
+    commandName: "getSessionDataReceived",
+    destination: {
+      type: RootMessageHandler.type,
+    },
+  });
+
+  is(sessionDataReceivedByRoot.length, 1);
+  is(sessionDataReceivedByRoot[0].category, "session_data_root_only");
+  is(sessionDataReceivedByRoot[0].added.length, 1);
+  is(sessionDataReceivedByRoot[0].added[0], true);
+  is(
+    sessionDataReceivedByRoot[0].contextDescriptor.type,
+    ContextDescriptorType.All
+  );
+
+  info("Now test that removing items also works on the root module");
+  await rootMessageHandler.removeSessionDataItem({
+    moduleName: "rootOnly",
+    category: "session_data_root_only",
+    contextDescriptor: {
+      type: ContextDescriptorType.All,
+    },
+    values: [true],
+  });
+
+  sessionDataReceivedByRoot = await rootMessageHandler.handleCommand({
+    moduleName: "rootOnly",
+    commandName: "getSessionDataReceived",
+    destination: {
+      type: RootMessageHandler.type,
+    },
+  });
+
+  is(sessionDataReceivedByRoot.length, 2);
+  is(sessionDataReceivedByRoot[1].category, "session_data_root_only");
+  is(sessionDataReceivedByRoot[1].removed.length, 1);
+  is(sessionDataReceivedByRoot[1].removed[0], true);
+  is(
+    sessionDataReceivedByRoot[1].contextDescriptor.type,
+    ContextDescriptorType.All
+  );
+
+  rootMessageHandler.destroy();
+});
+
+function checkSessionDataItem(item, moduleName, category, contextType, value) {
+  is(item.moduleName, moduleName, "Data item has the expected module name");
+  is(item.category, category, "Data item has the expected category");
+  is(
+    item.contextDescriptor.type,
+    contextType,
+    "Data item has the expected context type"
+  );
+  is(item.value, value, "Data item has the expected value");
+}
+
+function getSessionDataFromContent() {
+  return SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    const { readSessionData } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs"
+    );
+    return readSessionData();
+  });
+}
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_browser_element.js b/remote/shared/messagehandler/test/browser/browser_session_data_browser_element.js
new file mode 100644
index 0000000000..9c15974ae6
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_browser_element.js
@@ -0,0 +1,94 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+/**
+ * Check that message handlers are not created for parent process browser
+ * elements, even if they have the type="content" attribute (eg used for the
+ * DevTools toolbox), as well as for webextension contexts.
+ */
+add_task(async function test_session_data_broadcast() {
+  // Prepare:
+  // - one content tab
+  // - one browser type content
+  // - one browser type chrome
+  // - one sidebar webextension
+  // We only expect session data to be applied to the content tab
+  const tab1 = BrowserTestUtils.addTab(gBrowser, TEST_PAGE);
+  const contentBrowser1 = tab1.linkedBrowser;
+  await BrowserTestUtils.browserLoaded(contentBrowser1);
+  const parentBrowser1 = createParentBrowserElement(tab1, "content");
+  const parentBrowser2 = createParentBrowserElement(tab1, "chrome");
+  const { extension: extension1, sidebarBrowser: extSidebarBrowser1 } =
+    await installSidebarExtension();
+
+  const root = createRootMessageHandler("session-id-event");
+
+  // When the windowglobal command.jsm module applies the session data
+  // browser_session_data_browser_element, it will emit an event.
+  // Collect the events to detect which MessageHandlers have been started.
+  info("Watch events emitted when session data is applied");
+  const sessionDataEvents = [];
+  const onRootEvent = function (evtName, wrappedEvt) {
+    if (wrappedEvt.name === "received-session-data") {
+      sessionDataEvents.push(wrappedEvt.data.contextId);
+    }
+  };
+  root.on("message-handler-event", onRootEvent);
+
+  info("Add a new session data item, expect one return value");
+  await root.addSessionDataItem({
+    moduleName: "command",
+    category: "browser_session_data_browser_element",
+    contextDescriptor: {
+      type: ContextDescriptorType.All,
+    },
+    values: [true],
+  });
+
+  function hasSessionData(browsingContext) {
+    return sessionDataEvents.includes(browsingContext.id);
+  }
+
+  info(
+    "Check that only the content tab window global received the session data"
+  );
+  is(hasSessionData(contentBrowser1.browsingContext), true);
+  is(hasSessionData(parentBrowser1.browsingContext), false);
+  is(hasSessionData(parentBrowser2.browsingContext), false);
+  is(hasSessionData(extSidebarBrowser1.browsingContext), false);
+
+  const tab2 = BrowserTestUtils.addTab(gBrowser, TEST_PAGE);
+  const contentBrowser2 = tab2.linkedBrowser;
+  await BrowserTestUtils.browserLoaded(contentBrowser2);
+  const parentBrowser3 = createParentBrowserElement(contentBrowser2, "content");
+  const parentBrowser4 = createParentBrowserElement(contentBrowser2, "chrome");
+
+  const { extension: extension2, sidebarBrowser: extSidebarBrowser2 } =
+    await installSidebarExtension();
+
+  info("Wait until the session data was applied to the new tab");
+  await TestUtils.waitForCondition(() =>
+    sessionDataEvents.includes(contentBrowser2.browsingContext.id)
+  );
+
+  info("Check that parent browser elements did not apply the session data");
+  is(hasSessionData(parentBrowser3.browsingContext), false);
+  is(hasSessionData(parentBrowser4.browsingContext), false);
+
+  info(
+    "Check that extension did not apply the session data, " +
+      extSidebarBrowser2.browsingContext.id
+  );
+  is(hasSessionData(extSidebarBrowser2.browsingContext), false);
+
+  root.destroy();
+
+  gBrowser.removeTab(tab1);
+  gBrowser.removeTab(tab2);
+  await extension1.unload();
+  await extension2.unload();
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js b/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js
new file mode 100644
index 0000000000..03ed59166f
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js
@@ -0,0 +1,50 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+/**
+ * Check that modules created early for session data are still created with a
+ * fully initialized MessageHandler. See Bug 1743083.
+ */
+add_task(async function () {
+  const tab = BrowserTestUtils.addTab(gBrowser, TEST_PAGE);
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContext = tab.linkedBrowser.browsingContext;
+
+  const root = createRootMessageHandler("session-id-event");
+
+  info("Add some session data for the command module");
+  await root.addSessionDataItem({
+    moduleName: "command",
+    category: "testCategory",
+    contextDescriptor: contextDescriptorAll,
+    values: ["some-value"],
+  });
+
+  info("Reload the current tab to create new message handlers and modules");
+  await BrowserTestUtils.reloadTab(tab);
+
+  info(
+    "Check if the command module was created by the MessageHandler constructor"
+  );
+  const isCreatedByMessageHandlerConstructor = await root.handleCommand({
+    moduleName: "command",
+    commandName: "testIsCreatedByMessageHandlerConstructor",
+    destination: {
+      type: WindowGlobalMessageHandler.type,
+      id: browsingContext.id,
+    },
+  });
+
+  is(
+    isCreatedByMessageHandlerConstructor,
+    false,
+    "The command module from session data should not be created by the MessageHandler constructor"
+  );
+  root.destroy();
+
+  gBrowser.removeTab(tab);
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_update.js b/remote/shared/messagehandler/test/browser/browser_session_data_update.js
new file mode 100644
index 0000000000..342a4a6139
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_update.js
@@ -0,0 +1,113 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+const { assertUpdate, createSessionDataUpdate, getUpdates } =
+  SessionDataUpdateHelpers;
+
+// Test various session data update scenarios against a single browsing context.
+add_task(async function test_session_data_update() {
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  const root = createRootMessageHandler("session-data-update");
+
+  info("Add a new session data item, expect one return value");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-1"], "add", "category1"),
+  ]);
+  let processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 1);
+  assertUpdate(processedUpdates.at(-1), ["text-1"], "category1");
+
+  info("Add two session data items, expect one return value with both items");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-2"], "add", "category1"),
+    createSessionDataUpdate(["text-3"], "add", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 2);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3"],
+    "category1"
+  );
+
+  info("Try to add an existing data item, expect no update broadcast");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-1"], "add", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 2);
+
+  info("Add an existing and a new item");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-2", "text-4"], "add", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 3);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3", "text-4"],
+    "category1"
+  );
+
+  info("Remove an item, expect only the new item to return");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-3"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 4);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-4"],
+    "category1"
+  );
+
+  info("Remove a unknown item, expect no return value");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-unknown"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 4);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-4"],
+    "category1"
+  );
+
+  info("Remove an existing and a unknown item");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-2"], "remove", "category1"),
+    createSessionDataUpdate(["text-unknown"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 5);
+  assertUpdate(processedUpdates.at(-1), ["text-1", "text-4"], "category1");
+
+  info("Add and remove at once");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-5"], "add", "category1"),
+    createSessionDataUpdate(["text-4"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 6);
+  assertUpdate(processedUpdates.at(-1), ["text-1", "text-5"], "category1");
+
+  info("Adding and removing an item does not trigger any update");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-6"], "add", "category1"),
+    createSessionDataUpdate(["text-6"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  // TODO: We could detect transactions which can't have any impact and fully
+  // ignore them. See Bug 1810807.
+  todo_is(processedUpdates.length, 6);
+  assertUpdate(processedUpdates.at(-1), ["text-1", "text-5"], "category1");
+
+  root.destroy();
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_update_categories.js b/remote/shared/messagehandler/test/browser/browser_session_data_update_categories.js
new file mode 100644
index 0000000000..b1cadcf095
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_update_categories.js
@@ -0,0 +1,91 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+const { assertUpdate, createSessionDataUpdate, getUpdates } =
+  SessionDataUpdateHelpers;
+
+// Test session data update scenarios involving different session data item
+// categories.
+add_task(async function test_session_data_update_categories() {
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  const root = createRootMessageHandler("session-data-update-categories");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value1-1"], "add", "category1"),
+    createSessionDataUpdate(["value1-2"], "add", "category1"),
+  ]);
+
+  let processedUpdates = await getUpdates(root, browsingContext1);
+
+  is(processedUpdates.length, 1);
+  assertUpdate(processedUpdates.at(-1), ["value1-1", "value1-2"], "category1");
+
+  info("Adding a new item in category1 broadcasts all category1 items");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value1-3"], "add", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 2);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["value1-1", "value1-2", "value1-3"],
+    "category1"
+  );
+
+  info("Removing a new item in category1 broadcasts all category1 items");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value1-1"], "remove", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 3);
+  assertUpdate(processedUpdates.at(-1), ["value1-2", "value1-3"], "category1");
+
+  info("Adding a new category does not broadcast category1 items");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value2-1"], "add", "category2"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 4);
+  assertUpdate(processedUpdates.at(-1), ["value2-1"], "category2");
+
+  info("Adding an item in 2 categories triggers an update for each category");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value1-4"], "add", "category1"),
+    createSessionDataUpdate(["value2-2"], "add", "category2"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 6);
+  assertUpdate(
+    processedUpdates.at(-2),
+    ["value1-2", "value1-3", "value1-4"],
+    "category1"
+  );
+  assertUpdate(processedUpdates.at(-1), ["value2-1", "value2-2"], "category2");
+
+  info("Removing an item in 2 categories triggers an update for each category");
+  await root.updateSessionData([
+    createSessionDataUpdate(["value1-4"], "remove", "category1"),
+    createSessionDataUpdate(["value2-2"], "remove", "category2"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 8);
+  assertUpdate(processedUpdates.at(-2), ["value1-2", "value1-3"], "category1");
+  assertUpdate(processedUpdates.at(-1), ["value2-1"], "category2");
+
+  info("Opening a new tab triggers an update for each category");
+  const tab2 = await addTab(TEST_PAGE);
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 2);
+  assertUpdate(processedUpdates.at(-2), ["value1-2", "value1-3"], "category1");
+  assertUpdate(processedUpdates.at(-1), ["value2-1"], "category2");
+
+  root.destroy();
+  gBrowser.removeTab(tab2);
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_update_contexts.js b/remote/shared/messagehandler/test/browser/browser_session_data_update_contexts.js
new file mode 100644
index 0000000000..711df1fc56
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_update_contexts.js
@@ -0,0 +1,194 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
+
+const { assertUpdate, createSessionDataUpdate, getUpdates } =
+  SessionDataUpdateHelpers;
+
+// Test session data update scenarios involving 2 browsing contexts, and using
+// the TopBrowsingContext ContextDescriptor type.
+add_task(async function test_session_data_update_contexts() {
+  const tab1 = gBrowser.selectedTab;
+  await loadURL(tab1.linkedBrowser, TEST_PAGE);
+  const browsingContext1 = tab1.linkedBrowser.browsingContext;
+
+  const root = createRootMessageHandler("session-data-update-contexts");
+
+  info("Add several items over 2 separate updates for all contexts");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-1"], "add", "category1"),
+  ]);
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-2"], "add", "category1"),
+    createSessionDataUpdate(["text-3"], "add", "category1"),
+  ]);
+
+  info("Check we processed two distinct updates in browsingContext 1");
+  let processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 2);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3"],
+    "category1"
+  );
+
+  info("Open a new tab on the same test URL");
+  const tab2 = await addTab(TEST_PAGE);
+  const browsingContext2 = tab2.linkedBrowser.browsingContext;
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 1);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3"],
+    "category1"
+  );
+
+  info("Add two items: one globally and one in a single context");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-4"], "add", "category1"),
+    createSessionDataUpdate(["text-5"], "add", "category1", {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext2.browserId,
+    }),
+  ]);
+
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 3);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3", "text-4"],
+    "category1"
+  );
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 2);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-1", "text-2", "text-3", "text-4", "text-5"],
+    "category1"
+  );
+
+  info("Remove two items: one globally and one in a single context");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-1"], "remove", "category1"),
+    createSessionDataUpdate(["text-5"], "remove", "category1", {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext2.browserId,
+    }),
+  ]);
+
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 4);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4"],
+    "category1"
+  );
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 3);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4"],
+    "category1"
+  );
+
+  info(
+    "Add session data item to all contexts and remove this event for one context (2 steps)"
+  );
+
+  info("First step: add an item to browsingContext1");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-6"], "add", "category1", {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext1.browserId,
+    }),
+  ]);
+
+  info(
+    "Second step: remove the item from browsingContext1, and add it globally"
+  );
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-6"], "remove", "category1", {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext1.browserId,
+    }),
+    createSessionDataUpdate(["text-6"], "add", "category1"),
+  ]);
+
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 6);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6"],
+    "category1"
+  );
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 4);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6"],
+    "category1"
+  );
+
+  info(
+    "Remove the event, which has also an individual subscription, for all contexts (2 steps)"
+  );
+
+  info("First step: Add the same item for browsingContext1 and globally");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-7"], "add", "category1", {
+      type: ContextDescriptorType.TopBrowsingContext,
+      id: browsingContext1.browserId,
+    }),
+    createSessionDataUpdate(["text-7"], "add", "category1"),
+  ]);
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 7);
+  // We will find text-7 twice here, the module is responsible for not applying
+  // the same session data item twice. Each item corresponds to a different
+  // descriptor which matched browsingContext1.
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6", "text-7", "text-7"],
+    "category1"
+  );
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 5);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6", "text-7"],
+    "category1"
+  );
+
+  info("Second step: Remove the item globally");
+  await root.updateSessionData([
+    createSessionDataUpdate(["text-7"], "remove", "category1"),
+  ]);
+
+  processedUpdates = await getUpdates(root, browsingContext1);
+  is(processedUpdates.length, 8);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6", "text-7"],
+    "category1"
+  );
+
+  processedUpdates = await getUpdates(root, browsingContext2);
+  is(processedUpdates.length, 6);
+  assertUpdate(
+    processedUpdates.at(-1),
+    ["text-2", "text-3", "text-4", "text-6"],
+    "category1"
+  );
+
+  root.destroy();
+
+  gBrowser.removeTab(tab2);
+});
diff --git a/remote/shared/messagehandler/test/browser/browser_windowglobal_to_root.js b/remote/shared/messagehandler/test/browser/browser_windowglobal_to_root.js
new file mode 100644
index 0000000000..57629e5485
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/browser_windowglobal_to_root.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+
+add_task(async function test_windowGlobal_to_root_command() {
+  // Navigate to a page to make sure that the windowglobal modules run in a
+  // different process than the root module.
+  const tab = BrowserTestUtils.addTab(
+    gBrowser,
+    "https://example.com/document-builder.sjs?html=tab"
+  );
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  const browsingContextId = tab.linkedBrowser.browsingContext.id;
+
+  const rootMessageHandler = createRootMessageHandler(
+    "session-id-windowglobal-to-rootModule"
+  );
+
+  for (const commandName of [
+    "testHandleCommandToRoot",
+    "testSendRootCommand",
+  ]) {
+    const valueFromRoot = await rootMessageHandler.handleCommand({
+      moduleName: "windowglobaltoroot",
+      commandName,
+      destination: {
+        type: WindowGlobalMessageHandler.type,
+        id: browsingContextId,
+      },
+    });
+
+    is(
+      valueFromRoot,
+      "root-value-called-from-windowglobal",
+      "Retrieved the expected value from windowglobaltoroot using " +
+        commandName
+    );
+  }
+
+  rootMessageHandler.destroy();
+  gBrowser.removeTab(tab);
+});
diff --git a/remote/shared/messagehandler/test/browser/head.js b/remote/shared/messagehandler/test/browser/head.js
new file mode 100644
index 0000000000..5397fc0761
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/head.js
@@ -0,0 +1,236 @@
+/* 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";
+
+var { ContextDescriptorType } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs"
+);
+
+var { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
+);
+
+var contextDescriptorAll = {
+  type: ContextDescriptorType.All,
+};
+
+function createRootMessageHandler(sessionId) {
+  const { RootMessageHandlerRegistry } = ChromeUtils.importESModule(
+    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs"
+  );
+  return RootMessageHandlerRegistry.getOrCreateMessageHandler(sessionId);
+}
+
+/**
+ * Load the provided url in an existing browser.
+ * Returns a promise which will resolve when the page is loaded.
+ *
+ * @param {Browser} browser
+ *     The browser element where the URL should be loaded.
+ * @param {string} url
+ *     The URL to load in the new tab
+ */
+async function loadURL(browser, url) {
+  const loaded = BrowserTestUtils.browserLoaded(browser);
+  BrowserTestUtils.loadURIString(browser, url);
+  return loaded;
+}
+
+/**
+ * Create a new foreground tab loading the provided url.
+ * Returns a promise which will resolve when the page is loaded.
+ *
+ * @param {string} url
+ *     The URL to load in the new tab
+ */
+async function addTab(url) {
+  const tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
+  registerCleanupFunction(() => {
+    gBrowser.removeTab(tab);
+  });
+  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(
+    `https://${domain}/document-builder.sjs?html=frame-${domain}`
+  );
+}
+
+function createFrameForUri(uri) {
+  return `<iframe src="${encodeURI(uri)}"></iframe>`;
+}
+
+/**
+ * Create a XUL browser element in the provided XUL tab, with the provided type.
+ *
+ * @param {XULTab} tab
+ *     The XUL tab in which the browser element should be inserted.
+ * @param {string} type
+ *     The type attribute of the browser element, "chrome" or "content".
+ * @returns {XULBrowser}
+ *     The created browser element.
+ */
+function createParentBrowserElement(tab, type) {
+  const parentBrowser = gBrowser.ownerDocument.createXULElement("browser");
+  parentBrowser.setAttribute("type", type);
+  const container = gBrowser.getBrowserContainer(tab.linkedBrowser);
+  container.appendChild(parentBrowser);
+
+  return parentBrowser;
+}
+
+// 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(
+    `https://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}${createFrame("example.com")}`;
+
+  // Create the test page URI on example.org.
+  return `https://example.org/document-builder.sjs?html=${encodeURI(
+    TEST_URI_MARKUP
+  )}`;
+}
+
+const hasPromiseResolved = async function (promise) {
+  let resolved = false;
+  promise.finally(() => (resolved = true));
+  // Make sure microtasks have time to run.
+  await new Promise(resolve => Services.tm.dispatchToMainThread(resolve));
+  return resolved;
+};
+
+/**
+ * Install a sidebar extension.
+ *
+ * @returns {object}
+ *     Return value with two properties:
+ *     - extension: test wrapper as returned by SpecialPowers.loadExtension.
+ *       Make sure to explicitly call extension.unload() before the end of the test.
+ *     - sidebarBrowser: the browser element containing the extension sidebar.
+ */
+async function installSidebarExtension() {
+  info("Load the test extension");
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      sidebar_action: {
+        default_panel: "sidebar.html",
+      },
+    },
+    useAddonManager: "temporary",
+
+    files: {
+      "sidebar.html": `
+        <!DOCTYPE html>
+        <html>
+          Test extension
+          <script src="sidebar.js"></script>
+        </html>
+      `,
+      "sidebar.js": function () {
+        const { browser } = this;
+        browser.test.sendMessage("sidebar-loaded", {
+          bcId: SpecialPowers.wrap(window).browsingContext.id,
+        });
+      },
+      "tab.html": `
+        <!DOCTYPE html>
+        <html>
+          Test extension (tab)
+          <script src="tab.js"></script>
+        </html>
+      `,
+      "tab.js": function () {
+        const { browser } = this;
+        browser.test.sendMessage("tab-loaded", {
+          bcId: SpecialPowers.wrap(window).browsingContext.id,
+        });
+      },
+    },
+  });
+
+  info("Wait for the extension to start");
+  await extension.startup();
+
+  info("Wait for the extension browsing context");
+  const { bcId } = await extension.awaitMessage("sidebar-loaded");
+  const sidebarBrowser = BrowsingContext.get(bcId).top.embedderElement;
+  ok(sidebarBrowser, "Got a browser element for the extension sidebar");
+
+  return {
+    extension,
+    sidebarBrowser,
+  };
+}
+
+const SessionDataUpdateHelpers = {
+  getUpdates(rootMessageHandler, browsingContext) {
+    return rootMessageHandler.handleCommand({
+      moduleName: "sessiondataupdate",
+      commandName: "getSessionDataUpdates",
+      destination: {
+        id: browsingContext.id,
+        type: WindowGlobalMessageHandler.type,
+      },
+    });
+  },
+
+  createSessionDataUpdate(
+    values,
+    method,
+    category,
+    descriptor = { type: ContextDescriptorType.All }
+  ) {
+    return {
+      method,
+      values,
+      moduleName: "sessiondataupdate",
+      category,
+      contextDescriptor: descriptor,
+    };
+  },
+
+  assertUpdate(update, expectedValues, expectedCategory) {
+    is(
+      update.length,
+      expectedValues.length,
+      "Update has the expected number of values"
+    );
+
+    for (const item of update) {
+      info(`Check session data update item '${item.value}'`);
+      is(item.category, expectedCategory, "Item has the expected category");
+      is(
+        expectedValues[update.indexOf(item)],
+        item.value,
+        "Item has the expected value"
+      );
+    }
+  },
+};
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs
new file mode 100644
index 0000000000..7d93f45b33
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export const modules = {
+  root: {},
+  "windowglobal-in-root": {},
+  windowglobal: {},
+};
+
+const BASE_FOLDER =
+  "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules";
+
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(modules.root, {
+  command: `${BASE_FOLDER}/root/command.sys.mjs`,
+  event: `${BASE_FOLDER}/root/event.sys.mjs`,
+  invalid: `${BASE_FOLDER}/root/invalid.sys.mjs`,
+  rootOnly: `${BASE_FOLDER}/root/rootOnly.sys.mjs`,
+  windowglobaltoroot: `${BASE_FOLDER}/root/windowglobaltoroot.sys.mjs`,
+});
+
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(modules["windowglobal-in-root"], {
+  command: `${BASE_FOLDER}/windowglobal-in-root/command.sys.mjs`,
+  event: `${BASE_FOLDER}/windowglobal-in-root/event.sys.mjs`,
+});
+
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(modules.windowglobal, {
+  command: `${BASE_FOLDER}/windowglobal/command.sys.mjs`,
+  commandwindowglobalonly: `${BASE_FOLDER}/windowglobal/commandwindowglobalonly.sys.mjs`,
+  event: `${BASE_FOLDER}/windowglobal/event.sys.mjs`,
+  eventemitter: `${BASE_FOLDER}/windowglobal/eventemitter.sys.mjs`,
+  eventnointercept: `${BASE_FOLDER}/windowglobal/eventnointercept.sys.mjs`,
+  eventonprefchange: `${BASE_FOLDER}/windowglobal/eventonprefchange.sys.mjs`,
+  retry: `${BASE_FOLDER}/windowglobal/retry.sys.mjs`,
+  sessiondataupdate: `${BASE_FOLDER}/windowglobal/sessiondataupdate.sys.mjs`,
+  windowglobaltoroot: `${BASE_FOLDER}/windowglobal/windowglobaltoroot.sys.mjs`,
+});
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs
new file mode 100644
index 0000000000..29e4a75828
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs
@@ -0,0 +1,29 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class CommandModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testRootModule() {
+    return "root-value";
+  }
+
+  testMissingIntermediaryMethod(params, destination) {
+    // Spawn a new internal command, but with a commandName which doesn't match
+    // any method.
+    return this.messageHandler.handleCommand({
+      moduleName: "command",
+      commandName: "missingMethod",
+      destination,
+    });
+  }
+}
+
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs
new file mode 100644
index 0000000000..e49437e80d
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs
@@ -0,0 +1,21 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class EventModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testEmitRootEvent() {
+    this.emitEvent("event-from-root", {
+      text: "event from root",
+    });
+  }
+}
+
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs
new file mode 100644
index 0000000000..3b74769d06
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs
@@ -0,0 +1,4 @@
+// This module is meant to check error reporting when importing a module fails
+// due to an actual issue (syntax error etc...).
+
+SyntaxError(;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs
new file mode 100644
index 0000000000..0931a7ee8e
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs
@@ -0,0 +1,70 @@
+/* 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/. */
+
+import { ContextDescriptorType } from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class RootOnlyModule extends Module {
+  #sessionDataReceived;
+  #subscribedEvents;
+
+  constructor(messageHandler) {
+    super(messageHandler);
+
+    this.#sessionDataReceived = [];
+    this.#subscribedEvents = new Set();
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  getSessionDataReceived() {
+    return this.#sessionDataReceived;
+  }
+
+  testCommand(params = {}) {
+    return params;
+  }
+
+  _applySessionData(params) {
+    const added = [];
+    const removed = [];
+
+    const filteredSessionData = params.sessionData.filter(item =>
+      this.messageHandler.matchesContext(item.contextDescriptor)
+    );
+    for (const event of this.#subscribedEvents.values()) {
+      const hasSessionItem = filteredSessionData.some(
+        item => item.value === event
+      );
+      // If there are no session items for this context, we should unsubscribe from the event.
+      if (!hasSessionItem) {
+        this.#subscribedEvents.delete(event);
+        removed.push(event);
+      }
+    }
+
+    // Subscribe to all events, which have an item in SessionData
+    for (const { value } of filteredSessionData) {
+      if (!this.#subscribedEvents.has(value)) {
+        this.#subscribedEvents.add(value);
+        added.push(value);
+      }
+    }
+
+    this.#sessionDataReceived.push({
+      category: params.category,
+      added,
+      removed,
+      contextDescriptor: {
+        type: ContextDescriptorType.All,
+      },
+    });
+  }
+}
+
+export const rootOnly = RootOnlyModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/windowglobaltoroot.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/root/windowglobaltoroot.sys.mjs
new file mode 100644
index 0000000000..0975c4abd5
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/windowglobaltoroot.sys.mjs
@@ -0,0 +1,29 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class WindowGlobalToRootModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  getValueFromRoot() {
+    this.#assertParentProcess();
+    return "root-value-called-from-windowglobal";
+  }
+
+  #assertParentProcess() {
+    const isParent =
+      Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_DEFAULT;
+
+    if (!isParent) {
+      throw new Error("Can only run in the parent process");
+    }
+  }
+}
+
+export const windowglobaltoroot = WindowGlobalToRootModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs
new file mode 100644
index 0000000000..f9a2e5d4eb
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs
@@ -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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class CommandModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testInterceptModule() {
+    return "intercepted-value";
+  }
+
+  async testInterceptAndForwardModule(params, destination) {
+    const windowGlobalValue = await this.messageHandler.handleCommand({
+      moduleName: "command",
+      commandName: "testForwardToWindowGlobal",
+      destination,
+    });
+    return "intercepted-and-forward+" + windowGlobalValue;
+  }
+}
+
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs
new file mode 100644
index 0000000000..2969e0a3e8
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs
@@ -0,0 +1,31 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class EventModule extends Module {
+  destroy() {}
+
+  interceptEvent(name, payload) {
+    if (name === "event.testEventWithInterception") {
+      return {
+        ...payload,
+        additionalInformation: "information added through interception",
+      };
+    }
+    return payload;
+  }
+
+  /**
+   * Commands
+   */
+
+  testEmitWindowGlobalInRootEvent(params, destination) {
+    this.emitEvent("event-from-window-global-in-root", {
+      text: `windowglobal-in-root event for ${destination.id}`,
+    });
+  }
+}
+
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs
new file mode 100644
index 0000000000..99ee76a4b8
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs
@@ -0,0 +1,85 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class CommandModule extends Module {
+  constructor(messageHandler) {
+    super(messageHandler);
+    this._subscribedEvents = new Set();
+
+    this._createdByMessageHandlerConstructor =
+      this._isCreatedByMessageHandlerConstructor();
+  }
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  _applySessionData(params) {
+    if (params.category === "testCategory") {
+      const filteredSessionData = params.sessionData.filter(item =>
+        this.messageHandler.matchesContext(item.contextDescriptor)
+      );
+      for (const event of this._subscribedEvents.values()) {
+        const hasSessionItem = filteredSessionData.some(
+          item => item.value === event
+        );
+        // If there are no session items for this context, we should unsubscribe from the event.
+        if (!hasSessionItem) {
+          this._subscribedEvents.delete(event);
+        }
+      }
+
+      // Subscribe to all events, which have an item in SessionData
+      for (const { value } of filteredSessionData) {
+        if (!this._subscribedEvents.has(value)) {
+          this._subscribedEvents.add(value);
+        }
+      }
+    }
+
+    if (params.category === "browser_session_data_browser_element") {
+      this.emitEvent("received-session-data", {
+        contextId: this.messageHandler.contextId,
+      });
+    }
+  }
+
+  testWindowGlobalModule() {
+    return "windowglobal-value";
+  }
+
+  testSetValue(params) {
+    const { value } = params;
+
+    this._testValue = value;
+  }
+
+  testGetValue() {
+    return this._testValue;
+  }
+
+  testForwardToWindowGlobal() {
+    return "forward-to-windowglobal-value";
+  }
+
+  testIsCreatedByMessageHandlerConstructor() {
+    return this._createdByMessageHandlerConstructor;
+  }
+
+  _isCreatedByMessageHandlerConstructor() {
+    let caller = Components.stack.caller;
+    while (caller) {
+      if (caller.name === this.messageHandler.constructor.name) {
+        return true;
+      }
+      caller = caller.caller;
+    }
+    return false;
+  }
+}
+
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs
new file mode 100644
index 0000000000..1e4e6c1574
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs
@@ -0,0 +1,41 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class CommandWindowGlobalOnlyModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testOnlyInWindowGlobal() {
+    return "only-in-windowglobal";
+  }
+
+  testBroadcast() {
+    return `broadcast-${this.messageHandler.contextId}`;
+  }
+
+  testBroadcastWithParameter(params) {
+    return `broadcast-${this.messageHandler.contextId}-${params.value}`;
+  }
+
+  testError() {
+    throw new Error("error-from-module");
+  }
+
+  testMissingIntermediaryMethod(params, destination) {
+    // Spawn a new internal command, but with a commandName which doesn't match
+    // any method.
+    return this.messageHandler.handleCommand({
+      moduleName: "commandwindowglobalonly",
+      commandName: "missingMethod",
+      destination,
+    });
+  }
+}
+
+export const commandwindowglobalonly = CommandWindowGlobalOnlyModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs
new file mode 100644
index 0000000000..5bb50cb83d
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs
@@ -0,0 +1,26 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class EventModule extends Module {
+  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.emitEvent("event-from-window-global", { text });
+  }
+
+  testEmitEventWithInterception() {
+    this.emitEvent("event.testEventWithInterception", {});
+  }
+}
+
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs
new file mode 100644
index 0000000000..c86954c5e0
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs
@@ -0,0 +1,81 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class EventEmitterModule extends Module {
+  #isSubscribed;
+  #subscribedEvents;
+
+  constructor(messageHandler) {
+    super(messageHandler);
+    this.#isSubscribed = false;
+    this.#subscribedEvents = new Set();
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  emitTestEvent() {
+    if (this.#isSubscribed) {
+      const text = `event from ${this.messageHandler.contextId}`;
+      this.emitEvent("eventemitter.testEvent", { text });
+    }
+
+    // Emit another event consistently for monitoring during the test.
+    this.emitEvent("eventemitter.monitoringEvent", {});
+  }
+
+  isSubscribed() {
+    return this.#isSubscribed;
+  }
+
+  _applySessionData(params) {
+    const { category } = params;
+    if (category === "event") {
+      const filteredSessionData = params.sessionData.filter(item =>
+        this.messageHandler.matchesContext(item.contextDescriptor)
+      );
+      for (const event of this.#subscribedEvents.values()) {
+        const hasSessionItem = filteredSessionData.some(
+          item => item.value === event
+        );
+        // If there are no session items for this context, we should unsubscribe from the event.
+        if (!hasSessionItem) {
+          this.#unsubscribeEvent(event);
+        }
+      }
+
+      // Subscribe to all events, which have an item in SessionData
+      for (const { value } of filteredSessionData) {
+        this.#subscribeEvent(value);
+      }
+    }
+  }
+
+  #subscribeEvent(event) {
+    if (event === "eventemitter.testEvent") {
+      if (this.#isSubscribed) {
+        throw new Error("Already subscribed to eventemitter.testEvent");
+      }
+      this.#isSubscribed = true;
+      this.#subscribedEvents.add(event);
+    }
+  }
+
+  #unsubscribeEvent(event) {
+    if (event === "eventemitter.testEvent") {
+      if (!this.#isSubscribed) {
+        throw new Error("Not subscribed to eventemitter.testEvent");
+      }
+      this.#isSubscribed = false;
+      this.#subscribedEvents.delete(event);
+    }
+  }
+}
+
+export const eventemitter = EventEmitterModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs
new file mode 100644
index 0000000000..48bbfbf951
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs
@@ -0,0 +1,16 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class EventNoInterceptModule extends Module {
+  destroy() {}
+
+  testEvent() {
+    const text = `event no interception`;
+    this.emitEvent("eventnointercept.testEvent", { text });
+  }
+}
+
+export const eventnointercept = EventNoInterceptModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventonprefchange.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventonprefchange.sys.mjs
new file mode 100644
index 0000000000..33cb25d10b
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventonprefchange.sys.mjs
@@ -0,0 +1,33 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+const TEST_PREF = "remote.messagehandler.test.pref";
+
+class EventOnPrefChangeModule extends Module {
+  constructor(messageHandler) {
+    super(messageHandler);
+    Services.prefs.addObserver(TEST_PREF, this.#onPreferenceUpdated);
+  }
+
+  destroy() {
+    Services.prefs.removeObserver(TEST_PREF, this.#onPreferenceUpdated);
+  }
+
+  #onPreferenceUpdated = () => {
+    this.emitEvent("preference-changed");
+  };
+
+  /**
+   * Commands
+   */
+
+  ping() {
+    // We only use this command to force creating the module.
+    return 1;
+  }
+}
+
+export const eventonprefchange = EventOnPrefChangeModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs
new file mode 100644
index 0000000000..f7b2279018
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs
@@ -0,0 +1,84 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+// Store counters in the JSM scope to persist them across reloads.
+let callsToBlockedOneTime = 0;
+let callsToBlockedTenTimes = 0;
+let callsToBlockedElevenTimes = 0;
+
+// This module provides various commands which all hang for various reasons.
+// The test is supposed to trigger the command and then destroy the
+// JSWindowActor pair by any mean (eg a navigation) in order to trigger an
+// AbortError and a retry.
+class RetryModule extends Module {
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  // Resolves only if called while on the example.net domain.
+  async blockedOnNetDomain(params) {
+    // Note: we do not store a call counter here, because this is used for a
+    // cross-group navigation test, and the JSM will be loaded in different
+    // processes.
+    const uri = this.messageHandler.window.document.baseURI;
+    if (!uri.includes("example.net")) {
+      await new Promise(r => {});
+    }
+
+    return { ...params };
+  }
+
+  // Resolves only if called more than once.
+  async blockedOneTime(params) {
+    callsToBlockedOneTime++;
+    if (callsToBlockedOneTime < 2) {
+      await new Promise(r => {});
+    }
+
+    // Return:
+    // - params sent to the command to check that retries have correct params
+    // - the call counter
+    return { ...params, callsToCommand: callsToBlockedOneTime };
+  }
+
+  // Resolves only if called more than ten times (which is exactly the maximum
+  // of retry attempts).
+  async blockedTenTimes(params) {
+    callsToBlockedTenTimes++;
+    if (callsToBlockedTenTimes < 11) {
+      await new Promise(r => {});
+    }
+
+    // Return:
+    // - params sent to the command to check that retries have correct params
+    // - the call counter
+    return { ...params, callsToCommand: callsToBlockedTenTimes };
+  }
+
+  // Resolves only if called more than eleven times (which is greater than the
+  // maximum of retry attempts).
+  async blockedElevenTimes(params) {
+    callsToBlockedElevenTimes++;
+    if (callsToBlockedElevenTimes < 12) {
+      await new Promise(r => {});
+    }
+
+    // Return:
+    // - params sent to the command to check that retries have correct params
+    // - the call counter
+    return { ...params, callsToCommand: callsToBlockedElevenTimes };
+  }
+
+  cleanup() {
+    callsToBlockedOneTime = 0;
+    callsToBlockedTenTimes = 0;
+    callsToBlockedElevenTimes = 0;
+  }
+}
+
+export const retry = RetryModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/sessiondataupdate.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/sessiondataupdate.sys.mjs
new file mode 100644
index 0000000000..5e9ce00b46
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/sessiondataupdate.sys.mjs
@@ -0,0 +1,33 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+
+class SessionDataUpdateModule extends Module {
+  #sessionDataUpdates;
+
+  constructor(messageHandler) {
+    super(messageHandler);
+    this.#sessionDataUpdates = [];
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  _applySessionData(params) {
+    const filteredSessionData = params.sessionData.filter(item =>
+      this.messageHandler.matchesContext(item.contextDescriptor)
+    );
+    this.#sessionDataUpdates.push(filteredSessionData);
+  }
+
+  getSessionDataUpdates() {
+    return this.#sessionDataUpdates;
+  }
+}
+
+export const sessiondataupdate = SessionDataUpdateModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/windowglobaltoroot.sys.mjs b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/windowglobaltoroot.sys.mjs
new file mode 100644
index 0000000000..815a836d9c
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/windowglobaltoroot.sys.mjs
@@ -0,0 +1,47 @@
+/* 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/. */
+
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+import { RootMessageHandler } from "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs";
+
+class WindowGlobalToRootModule extends Module {
+  constructor(messageHandler) {
+    super(messageHandler);
+    this.#assertContentProcess();
+  }
+
+  destroy() {}
+
+  /**
+   * Commands
+   */
+
+  testHandleCommandToRoot(params, destination) {
+    return this.messageHandler.handleCommand({
+      moduleName: "windowglobaltoroot",
+      commandName: "getValueFromRoot",
+      destination: {
+        type: RootMessageHandler.type,
+      },
+    });
+  }
+
+  testSendRootCommand(params, destination) {
+    return this.messageHandler.sendRootCommand({
+      moduleName: "windowglobaltoroot",
+      commandName: "getValueFromRoot",
+    });
+  }
+
+  #assertContentProcess() {
+    const isContent =
+      Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_CONTENT;
+
+    if (!isContent) {
+      throw new Error("Can only run in a content process");
+    }
+  }
+}
+
+export const windowglobaltoroot = WindowGlobalToRootModule;
diff --git a/remote/shared/messagehandler/test/browser/webdriver/browser.ini b/remote/shared/messagehandler/test/browser/webdriver/browser.ini
new file mode 100644
index 0000000000..a7ece66163
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/webdriver/browser.ini
@@ -0,0 +1,9 @@
+[DEFAULT]
+tags = remote
+subsuite = remote
+support-files =
+  !/remote/shared/messagehandler/test/browser/resources/*
+prefs =
+  remote.messagehandler.modulecache.useBrowserTestRoot=true
+
+[browser_session_execute_command_errors.js]
diff --git a/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js b/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js
new file mode 100644
index 0000000000..36a510bb29
--- /dev/null
+++ b/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js
@@ -0,0 +1,40 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { WebDriverSession } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Session.sys.mjs"
+);
+
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
+);
+
+add_task(async function test_execute_missing_command_error() {
+  const session = new WebDriverSession();
+
+  info("Attempt to execute an unknown protocol command");
+  await Assert.rejects(
+    session.execute("command", "missingCommand"),
+    err =>
+      err.name == "UnknownCommandError" &&
+      err.message == `command.missingCommand`
+  );
+});
+
+add_task(async function test_execute_missing_internal_command_error() {
+  const session = new WebDriverSession();
+
+  info(
+    "Attempt to execute a protocol command which relies on an unknown internal method"
+  );
+  await Assert.rejects(
+    session.execute("command", "testMissingIntermediaryMethod"),
+    err =>
+      err.name == "UnsupportedCommandError" &&
+      err.message ==
+        `command.missingMethod not supported for destination ROOT` &&
+      !error.isWebDriverError(err)
+  );
+});
diff --git a/remote/shared/messagehandler/test/xpcshell/test_Errors.js b/remote/shared/messagehandler/test/xpcshell/test_Errors.js
new file mode 100644
index 0000000000..26187dac11
--- /dev/null
+++ b/remote/shared/messagehandler/test/xpcshell/test_Errors.js
@@ -0,0 +1,91 @@
+/* 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/. */
+
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Errors.sys.mjs"
+);
+
+// Note: this test file is similar to remote/shared/webdriver/test/xpcshell/test_Errors.js
+// because shared/webdriver/Errors.jsm and shared/messagehandler/Errors.jsm share
+// similar helpers.
+
+add_task(function test_toJSON() {
+  let e0 = new error.MessageHandlerError();
+  let e0s = e0.toJSON();
+  equal(e0s.error, "message handler error");
+  equal(e0s.message, "");
+
+  let e1 = new error.MessageHandlerError("a");
+  let e1s = e1.toJSON();
+  equal(e1s.message, e1.message);
+
+  let e2 = new error.UnsupportedCommandError("foo");
+  let e2s = e2.toJSON();
+  equal(e2.status, e2s.error);
+  equal(e2.message, e2s.message);
+});
+
+add_task(function test_fromJSON() {
+  Assert.throws(
+    () => error.MessageHandlerError.fromJSON({ error: "foo" }),
+    /Not of MessageHandlerError descent/
+  );
+  Assert.throws(
+    () => error.MessageHandlerError.fromJSON({ error: "Error" }),
+    /Not of MessageHandlerError descent/
+  );
+  Assert.throws(
+    () => error.MessageHandlerError.fromJSON({}),
+    /Undeserialisable error type/
+  );
+  Assert.throws(
+    () => error.MessageHandlerError.fromJSON(undefined),
+    /TypeError/
+  );
+
+  let e1 = new error.MessageHandlerError("1");
+  let e1r = error.MessageHandlerError.fromJSON({
+    error: "message handler error",
+    message: "1",
+  });
+  ok(e1r instanceof error.MessageHandlerError);
+  equal(e1r.name, e1.name);
+  equal(e1r.status, e1.status);
+  equal(e1r.message, e1.message);
+
+  let e2 = new error.UnsupportedCommandError("foo");
+  let e2r = error.MessageHandlerError.fromJSON({
+    error: "unsupported message handler command",
+    message: "foo",
+  });
+  ok(e2r instanceof error.MessageHandlerError);
+  ok(e2r instanceof error.UnsupportedCommandError);
+  equal(e2r.name, e2.name);
+  equal(e2r.status, e2.status);
+  equal(e2r.message, e2.message);
+
+  // parity with toJSON
+  let e3 = new error.UnsupportedCommandError("foo");
+  let e3toJSON = e3.toJSON();
+  let e3fromJSON = error.MessageHandlerError.fromJSON(e3toJSON);
+  equal(e3toJSON.error, e3fromJSON.status);
+  equal(e3toJSON.message, e3fromJSON.message);
+  equal(e3toJSON.stacktrace, e3fromJSON.stack);
+});
+
+add_task(function test_MessageHandlerError() {
+  let err = new error.MessageHandlerError("foo");
+  equal("MessageHandlerError", err.name);
+  equal("foo", err.message);
+  equal("message handler error", err.status);
+  ok(err instanceof error.MessageHandlerError);
+});
+
+add_task(function test_UnsupportedCommandError() {
+  let e = new error.UnsupportedCommandError("foo");
+  equal("UnsupportedCommandError", e.name);
+  equal("foo", e.message);
+  equal("unsupported message handler command", e.status);
+  ok(e instanceof error.MessageHandlerError);
+});
diff --git a/remote/shared/messagehandler/test/xpcshell/test_SessionData.js b/remote/shared/messagehandler/test/xpcshell/test_SessionData.js
new file mode 100644
index 0000000000..ef61ce27d4
--- /dev/null
+++ b/remote/shared/messagehandler/test/xpcshell/test_SessionData.js
@@ -0,0 +1,296 @@
+/* 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/. */
+
+const { ContextDescriptorType } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs"
+);
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
+);
+const { SessionData, SessionDataMethod } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs"
+);
+
+add_task(async function test_sessionData() {
+  const sessionData = new SessionData(new RootMessageHandler("session-id-1"));
+  equal(sessionData.getSessionData("mod", "event").length, 0);
+
+  const globalContext = {
+    type: ContextDescriptorType.All,
+  };
+  const otherContext = { type: "other-type", id: "some-id" };
+
+  info("Add a first event for the global context");
+  let updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, globalContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "One item added");
+  let updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value added");
+  equal(updatedValues[0], "first.event", "Expected value was added");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+  ]);
+
+  info("Add the exact same data (same module, type, context, value)");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, globalContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 0, "No new item updated");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+  ]);
+
+  info("Add another context for the same event");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, otherContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "One item added");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value added");
+  equal(updatedValues[0], "first.event", "Expected value was added");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+  ]);
+
+  info("Add a second event for the global context");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, globalContext, ["second.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "One item added");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value added");
+  equal(updatedValues[0], "second.event", "Expected value was added");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+    {
+      value: "second.event",
+      contextDescriptor: globalContext,
+    },
+  ]);
+
+  info("Add two events for the global context");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, globalContext, [
+      "third.event",
+      "fourth.event",
+    ]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "One item added");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 2, "Two values added");
+  equal(updatedValues[0], "third.event", "Expected value was added");
+  equal(updatedValues[1], "fourth.event", "Expected value was added");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+    {
+      value: "second.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "third.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "fourth.event",
+      contextDescriptor: globalContext,
+    },
+  ]);
+
+  info("Remove the second, third and fourth events");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Remove, globalContext, [
+      "second.event",
+      "third.event",
+      "fourth.event",
+    ]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Remove, "One item removed");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 3, "Three values removed");
+  equal(updatedValues[0], "second.event", "Expected value was removed");
+  equal(updatedValues[1], "third.event", "Expected value was removed");
+  equal(updatedValues[2], "fourth.event", "Expected value was removed");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: globalContext,
+    },
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+  ]);
+
+  info("Remove the global context from the first event");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Remove, globalContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Remove, "One item removed");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value removed");
+  equal(updatedValues[0], "first.event", "Expected value was removed");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+  ]);
+
+  info("Remove the other context from the first event");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Remove, otherContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Remove, "One item removed");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value removed");
+  equal(updatedValues[0], "first.event", "Expected value was removed");
+  checkEvents(sessionData.getSessionData("mod", "event"), []);
+
+  info("Add two events for different contexts");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, otherContext, ["first.event"]),
+    createUpdate(SessionDataMethod.Add, globalContext, ["second.event"]),
+  ]);
+  equal(updatedItems.length, 2, "Two items updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "First item added");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value for first item added");
+  equal(updatedValues[0], "first.event", "Expected value first item was added");
+  equal(updatedItems[1].method, SessionDataMethod.Add, "Second item added");
+  updatedValues = updatedItems[1].values;
+  equal(updatedValues.length, 1, "One value for second item added");
+  equal(
+    updatedValues[0],
+    "second.event",
+    "Expected value second item was added"
+  );
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+    {
+      value: "second.event",
+      contextDescriptor: globalContext,
+    },
+  ]);
+
+  info("Remove two events for different contexts");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Remove, otherContext, ["first.event"]),
+    createUpdate(SessionDataMethod.Remove, globalContext, ["second.event"]),
+  ]);
+  equal(updatedItems.length, 2, "Two items updated");
+  equal(updatedItems[0].method, SessionDataMethod.Remove, "First item removed");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value for first item removed");
+  equal(
+    updatedValues[0],
+    "first.event",
+    "Expected value first item was removed"
+  );
+  equal(
+    updatedItems[1].method,
+    SessionDataMethod.Remove,
+    "Second item removed"
+  );
+  updatedValues = updatedItems[1].values;
+  equal(updatedValues.length, 1, "One value for second item removed");
+  equal(
+    updatedValues[0],
+    "second.event",
+    "Expected value second item was removed"
+  );
+  checkEvents(sessionData.getSessionData("mod", "event"), []);
+
+  info("Add and remove event in different order");
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Remove, otherContext, ["first.event"]),
+    createUpdate(SessionDataMethod.Add, otherContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "One item updated");
+  equal(updatedItems[0].method, SessionDataMethod.Add, "One item added");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value added");
+  equal(updatedValues[0], "first.event", "Expected value was added");
+  checkEvents(sessionData.getSessionData("mod", "event"), [
+    {
+      value: "first.event",
+      contextDescriptor: otherContext,
+    },
+  ]);
+
+  updatedItems = sessionData.applySessionData([
+    createUpdate(SessionDataMethod.Add, otherContext, ["first.event"]),
+    createUpdate(SessionDataMethod.Remove, otherContext, ["first.event"]),
+  ]);
+  equal(updatedItems.length, 1, "No item update");
+  equal(updatedItems[0].method, SessionDataMethod.Remove, "One item removed");
+  updatedValues = updatedItems[0].values;
+  equal(updatedValues.length, 1, "One value removed");
+  equal(updatedValues[0], "first.event", "Expected value was removed");
+  checkEvents(sessionData.getSessionData("mod", "event"), []);
+});
+
+function checkEvents(events, expectedEvents) {
+  // Check the arrays have the same size.
+  equal(events.length, expectedEvents.length);
+
+  // Check all the expectedEvents can be found in the events array.
+  for (const expected of expectedEvents) {
+    ok(
+      events.some(
+        event =>
+          expected.contextDescriptor.type === event.contextDescriptor.type &&
+          expected.contextDescriptor.id === event.contextDescriptor.id &&
+          expected.value == event.value
+      )
+    );
+  }
+}
+
+function createUpdate(method, contextDescriptor, values) {
+  return {
+    method,
+    moduleName: "mod",
+    category: "event",
+    contextDescriptor,
+    values,
+  };
+}
diff --git a/remote/shared/messagehandler/test/xpcshell/xpcshell.ini b/remote/shared/messagehandler/test/xpcshell/xpcshell.ini
new file mode 100644
index 0000000000..9199459e0a
--- /dev/null
+++ b/remote/shared/messagehandler/test/xpcshell/xpcshell.ini
@@ -0,0 +1,6 @@
+# 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/.
+
+[test_Errors.js]
+[test_SessionData.js]
diff --git a/remote/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs b/remote/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs
new file mode 100644
index 0000000000..482f90948a
--- /dev/null
+++ b/remote/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs
@@ -0,0 +1,57 @@
+/* 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/. */
+
+function isExtensionContext(browsingContext) {
+  let principal;
+  if (CanonicalBrowsingContext.isInstance(browsingContext)) {
+    principal = browsingContext.currentWindowGlobal.documentPrincipal;
+  } else {
+    principal = browsingContext.window.document.nodePrincipal;
+  }
+
+  // In practice, note that the principal will never be an expanded principal.
+  // The are only used for content scripts executed in a Sandbox, and do not
+  // have a browsing context on their own.
+  // But we still use this flag because there is no isAddonPrincipal flag.
+  return principal.isAddonOrExpandedAddonPrincipal;
+}
+
+function isParentProcess(browsingContext) {
+  if (CanonicalBrowsingContext.isInstance(browsingContext)) {
+    return browsingContext.currentWindowGlobal.osPid === -1;
+  }
+
+  // If `browsingContext` is not a `CanonicalBrowsingContext`, then we are
+  // necessarily in a content process page.
+  return false;
+}
+
+/**
+ * Check if the given browsing context is valid for the message handler
+ * to use.
+ *
+ * @param {BrowsingContext} browsingContext
+ *     The browsing context to check.
+ * @param {object=} options
+ * @param {string=} options.browserId
+ *    The id of the browser to filter the browsing contexts by (optional).
+ * @returns {boolean}
+ *     True if the browsing context is valid, false otherwise.
+ */
+export function isBrowsingContextCompatible(browsingContext, options = {}) {
+  const { browserId } = options;
+
+  // If a browserId was provided, skip browsing contexts which are not
+  // associated with this browserId.
+  if (browserId !== undefined && browsingContext.browserId !== browserId) {
+    return false;
+  }
+
+  // Skip:
+  // - extension contexts until we support debugging webextensions, see Bug 1755014.
+  // - privileged contexts until we support debugging Chrome context, see Bug 1713440.
+  return (
+    !isExtensionContext(browsingContext) && !isParentProcess(browsingContext)
+  );
+}
diff --git a/remote/shared/messagehandler/transports/RootTransport.sys.mjs b/remote/shared/messagehandler/transports/RootTransport.sys.mjs
new file mode 100644
index 0000000000..2fb206ab93
--- /dev/null
+++ b/remote/shared/messagehandler/transports/RootTransport.sys.mjs
@@ -0,0 +1,190 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ContextDescriptorType:
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  isBrowsingContextCompatible:
+    "chrome://remote/content/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  MessageHandlerFrameActor:
+    "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+const MAX_RETRY_ATTEMPTS = 10;
+
+/**
+ * RootTransport is intended to be used from a ROOT MessageHandler to communicate
+ * with WINDOW_GLOBAL MessageHandlers via the MessageHandlerFrame JSWindow
+ * actors.
+ */
+export class RootTransport {
+  /**
+   * @param {MessageHandler} messageHandler
+   *     The MessageHandler instance which owns this RootTransport instance.
+   */
+  constructor(messageHandler) {
+    this._messageHandler = messageHandler;
+
+    // RootTransport will rely on the MessageHandlerFrame JSWindow actors.
+    // Make sure they are registered when instanciating a RootTransport.
+    lazy.MessageHandlerFrameActor.register();
+  }
+
+  /**
+   * Forward the provided command to WINDOW_GLOBAL MessageHandlers via the
+   * MessageHandlerFrame actors.
+   *
+   * @param {Command} command
+   *     The command to forward. See type definition in MessageHandler.js
+   * @returns {Promise}
+   *     Returns a promise that resolves with the result of the command after
+   *     being processed by WINDOW_GLOBAL MessageHandlers.
+   */
+  forwardCommand(command) {
+    if (command.destination.id && command.destination.contextDescriptor) {
+      throw new Error(
+        "Invalid command destination with both 'id' and 'contextDescriptor' properties"
+      );
+    }
+
+    // With an id given forward the command to only this specific destination.
+    if (command.destination.id) {
+      const browsingContext = BrowsingContext.get(command.destination.id);
+      if (!browsingContext) {
+        throw new Error(
+          "Unable to find a BrowsingContext for id " + command.destination.id
+        );
+      }
+      return this._sendCommandToBrowsingContext(command, browsingContext);
+    }
+
+    // ... otherwise broadcast to destinations matching the contextDescriptor.
+    if (command.destination.contextDescriptor) {
+      return this._broadcastCommand(command);
+    }
+
+    throw new Error(
+      "Unrecognized command destination, missing 'id' or 'contextDescriptor' properties"
+    );
+  }
+
+  _broadcastCommand(command) {
+    const { contextDescriptor } = command.destination;
+    const browsingContexts =
+      this._getBrowsingContextsForDescriptor(contextDescriptor);
+
+    return Promise.all(
+      browsingContexts.map(async browsingContext => {
+        try {
+          return await this._sendCommandToBrowsingContext(
+            command,
+            browsingContext
+          );
+        } catch (e) {
+          console.error(
+            `Failed to broadcast a command to browsingContext ${browsingContext.id}`,
+            e
+          );
+          return null;
+        }
+      })
+    );
+  }
+
+  async _sendCommandToBrowsingContext(command, browsingContext) {
+    const name = `${command.moduleName}.${command.commandName}`;
+
+    // The browsing context might be destroyed by a navigation. Keep a reference
+    // to the webProgress, which will persist, and always use it to retrieve the
+    // currently valid browsing context.
+    const webProgress = browsingContext.webProgress;
+
+    const { retryOnAbort = false } = command;
+
+    let attempts = 0;
+    while (true) {
+      try {
+        return await webProgress.browsingContext.currentWindowGlobal
+          .getActor("MessageHandlerFrame")
+          .sendCommand(command, this._messageHandler.sessionId);
+      } catch (e) {
+        if (!retryOnAbort || e.name != "AbortError") {
+          // Only retry if the command supports retryOnAbort and when the
+          // JSWindowActor pair gets destroyed.
+          throw e;
+        }
+
+        if (++attempts > MAX_RETRY_ATTEMPTS) {
+          lazy.logger.trace(
+            `RootTransport reached the limit of retry attempts (${MAX_RETRY_ATTEMPTS})` +
+              ` for command ${name} and browsing context ${webProgress.browsingContext.id}.`
+          );
+          throw e;
+        }
+
+        lazy.logger.trace(
+          `RootTransport retrying command ${name} for ` +
+            `browsing context ${webProgress.browsingContext.id}, attempt: ${attempts}.`
+        );
+        await new Promise(resolve => Services.tm.dispatchToMainThread(resolve));
+      }
+    }
+  }
+
+  toString() {
+    return `[object ${this.constructor.name} ${this._messageHandler.name}]`;
+  }
+
+  _getBrowsingContextsForDescriptor(contextDescriptor) {
+    const { id, type } = contextDescriptor;
+
+    if (type === lazy.ContextDescriptorType.All) {
+      return this._getBrowsingContexts();
+    }
+
+    if (type === lazy.ContextDescriptorType.TopBrowsingContext) {
+      return this._getBrowsingContexts({ browserId: id });
+    }
+
+    // TODO: Handle other types of context descriptors.
+    throw new Error(
+      `Unsupported contextDescriptor type for broadcasting: ${type}`
+    );
+  }
+
+  /**
+   * Get all browsing contexts, optionally matching the provided options.
+   *
+   * @param {object} options
+   * @param {string=} options.browserId
+   *    The id of the browser to filter the browsing contexts by (optional).
+   * @returns {Array<BrowsingContext>}
+   *    The browsing contexts matching the provided options or all browsing contexts
+   *    if no options are provided.
+   */
+  _getBrowsingContexts(options = {}) {
+    // extract browserId from options
+    const { browserId } = options;
+    let browsingContexts = [];
+
+    // Fetch all tab related browsing contexts for top-level windows.
+    for (const { browsingContext } of lazy.TabManager.browsers) {
+      if (lazy.isBrowsingContextCompatible(browsingContext, { browserId })) {
+        browsingContexts = browsingContexts.concat(
+          browsingContext.getAllBrowsingContextsInSubtree()
+        );
+      }
+    }
+
+    return browsingContexts;
+  }
+}
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs
new file mode 100644
index 0000000000..e516e92614
--- /dev/null
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs
@@ -0,0 +1,53 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ActorManagerParent: "resource://gre/modules/ActorManagerParent.sys.mjs",
+
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
+
+const FRAME_ACTOR_CONFIG = {
+  parent: {
+    esModuleURI:
+      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs",
+  },
+  child: {
+    esModuleURI:
+      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs",
+    events: {
+      DOMWindowCreated: {},
+      pagehide: {},
+      pageshow: {},
+    },
+  },
+  allFrames: true,
+  messageManagerGroups: ["browsers"],
+};
+
+/**
+ * MessageHandlerFrameActor exposes a simple registration helper to lazily
+ * register MessageHandlerFrame JSWindow actors.
+ */
+export const MessageHandlerFrameActor = {
+  registered: false,
+
+  register() {
+    if (this.registered) {
+      return;
+    }
+
+    lazy.ActorManagerParent.addJSWindowActors({
+      MessageHandlerFrame: FRAME_ACTOR_CONFIG,
+    });
+    this.registered = true;
+    lazy.logger.trace("Registered MessageHandlerFrame actors");
+  },
+};
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs
new file mode 100644
index 0000000000..52a8fdc4c9
--- /dev/null
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs
@@ -0,0 +1,111 @@
+/* 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/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  isBrowsingContextCompatible:
+    "chrome://remote/content/shared/messagehandler/transports/BrowsingContextUtils.sys.mjs",
+  MessageHandlerRegistry:
+    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+/**
+ * Map from MessageHandlerRegistry to MessageHandlerFrameChild actor. This will
+ * allow a WindowGlobalMessageHandler to find the JSWindowActorChild instance to
+ * use to send commands.
+ */
+const registryToActor = new WeakMap();
+
+/**
+ * Retrieve the MessageHandlerFrameChild which is linked to the provided
+ * WindowGlobalMessageHandler instance.
+ *
+ * @param {WindowGlobalMessageHandler} messageHandler
+ *     The WindowGlobalMessageHandler for which to get the JSWindowActor.
+ * @returns {MessageHandlerFrameChild}
+ *     The corresponding MessageHandlerFrameChild instance.
+ */
+export function getMessageHandlerFrameChildActor(messageHandler) {
+  return registryToActor.get(messageHandler.registry);
+}
+
+/**
+ * Child actor for the MessageHandlerFrame JSWindowActor. The
+ * MessageHandlerFrame actor is used by RootTransport to communicate between
+ * ROOT MessageHandlers and WINDOW_GLOBAL MessageHandlers.
+ */
+export class MessageHandlerFrameChild extends JSWindowActorChild {
+  actorCreated() {
+    this.type = lazy.WindowGlobalMessageHandler.type;
+    this.context = this.manager.browsingContext;
+
+    this._registry = new lazy.MessageHandlerRegistry(this.type, this.context);
+    registryToActor.set(this._registry, this);
+
+    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.
+    this._registry.on("message-handler-registry-event", this._onRegistryEvent);
+  }
+
+  handleEvent({ persisted, type }) {
+    if (type == "DOMWindowCreated" || (type == "pageshow" && persisted)) {
+      // When the window is created or is retrieved from BFCache, instantiate
+      // a MessageHandler for all sessions which might need it.
+      if (lazy.isBrowsingContextCompatible(this.manager.browsingContext)) {
+        this._registry.createAllMessageHandlers();
+      }
+    } else if (type == "pagehide" && persisted) {
+      // When the page is moved to BFCache, all the currently created message
+      // handlers should be destroyed.
+      this._registry.destroy();
+    }
+  }
+
+  async receiveMessage(message) {
+    if (message.name === "MessageHandlerFrameParent:sendCommand") {
+      const { sessionId, command } = message.data;
+      const messageHandler =
+        this._registry.getOrCreateMessageHandler(sessionId);
+      try {
+        return await messageHandler.handleCommand(command);
+      } catch (e) {
+        if (e?.isRemoteError) {
+          return {
+            error: e.toJSON(),
+            isMessageHandlerError: e.isMessageHandlerError,
+          };
+        }
+        throw e;
+      }
+    }
+
+    return null;
+  }
+
+  sendCommand(command, sessionId) {
+    return this.sendQuery("MessageHandlerFrameChild:sendCommand", {
+      command,
+      sessionId,
+    });
+  }
+
+  _onRegistryEvent(eventName, wrappedEvent) {
+    this.sendAsyncMessage(
+      "MessageHandlerFrameChild:messageHandlerEvent",
+      wrappedEvent
+    );
+  }
+
+  didDestroy() {
+    this._registry.off("message-handler-registry-event", this._onRegistryEvent);
+    this._registry.destroy();
+  }
+}
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs
new file mode 100644
index 0000000000..f845b2fa01
--- /dev/null
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs
@@ -0,0 +1,120 @@
+/* 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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/messagehandler/Errors.sys.mjs",
+  RootMessageHandlerRegistry:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "WebDriverError", () => {
+  return ChromeUtils.importESModule(
+    "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
+  ).error.WebDriverError;
+});
+
+/**
+ * Parent actor for the MessageHandlerFrame JSWindowActor. The
+ * MessageHandlerFrame actor is used by RootTransport to communicate between
+ * ROOT MessageHandlers and WINDOW_GLOBAL MessageHandlers.
+ */
+export class MessageHandlerFrameParent extends JSWindowActorParent {
+  async receiveMessage(message) {
+    switch (message.name) {
+      case "MessageHandlerFrameChild:sendCommand": {
+        return this.#handleSendCommandMessage(message.data);
+      }
+      case "MessageHandlerFrameChild:messageHandlerEvent": {
+        return this.#handleMessageHandlerEventMessage(message.data);
+      }
+      default:
+        throw new Error("Unsupported message:" + message.name);
+    }
+  }
+
+  /**
+   * Send a command to the corresponding MessageHandlerFrameChild actor via a
+   * JSWindowActor query.
+   *
+   * @param {Command} command
+   *     The command to forward. See type definition in MessageHandler.js
+   * @param {string} sessionId
+   *     ID of the session that sent the command.
+   * @returns {Promise}
+   *     Promise that will resolve with the result of query sent to the
+   *     MessageHandlerFrameChild actor.
+   */
+  async sendCommand(command, sessionId) {
+    const result = await this.sendQuery(
+      "MessageHandlerFrameParent:sendCommand",
+      {
+        command,
+        sessionId,
+      }
+    );
+
+    if (result?.error) {
+      if (result.isMessageHandlerError) {
+        throw lazy.error.MessageHandlerError.fromJSON(result.error);
+      }
+
+      // TODO: Do not assume WebDriver is the session protocol, see Bug 1779026.
+      throw lazy.WebDriverError.fromJSON(result.error);
+    }
+
+    return result;
+  }
+
+  async #handleMessageHandlerEventMessage(messageData) {
+    const { name, contextInfo, data, sessionId } = messageData;
+    const [moduleName] = name.split(".");
+
+    // Re-emit the event on the RootMessageHandler.
+    const messageHandler =
+      lazy.RootMessageHandlerRegistry.getExistingMessageHandler(sessionId);
+    // TODO: getModuleInstance expects a CommandDestination in theory,
+    // but only uses the MessageHandler type in practice, see Bug 1776389.
+    const module = messageHandler.moduleCache.getModuleInstance(moduleName, {
+      type: lazy.WindowGlobalMessageHandler.type,
+    });
+    let eventPayload = data;
+
+    // Modify an event payload if there is a special method in the targeted module.
+    // If present it can be found in windowglobal-in-root module.
+    if (module?.interceptEvent) {
+      eventPayload = await module.interceptEvent(name, data);
+
+      // Make sure that an event payload is returned.
+      if (!eventPayload) {
+        throw new Error(
+          `${moduleName}.interceptEvent doesn't return the event payload`
+        );
+      }
+    }
+    messageHandler.emitEvent(name, eventPayload, contextInfo);
+  }
+
+  async #handleSendCommandMessage(messageData) {
+    const { sessionId, command } = messageData;
+    const messageHandler =
+      lazy.RootMessageHandlerRegistry.getExistingMessageHandler(sessionId);
+    try {
+      return await messageHandler.handleCommand(command);
+    } catch (e) {
+      if (e?.isRemoteError) {
+        return {
+          error: e.toJSON(),
+          isMessageHandlerError: e.isMessageHandlerError,
+        };
+      }
+      throw e;
+    }
+  }
+}