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diff --git a/toolkit/components/extensions/ConduitsChild.sys.mjs b/toolkit/components/extensions/ConduitsChild.sys.mjs
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+++ b/toolkit/components/extensions/ConduitsChild.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This @file implements the child side of Conduits, an abstraction over
+ * Fission IPC for extension API subject.  See {@link ConduitsParent.jsm}
+ * for more details about the overall design.
+ *
+ * @typedef {object} MessageData
+ * @property {ConduitID} [target]
+ * @property {ConduitID} [sender]
+ * @property {boolean} query
+ * @property {object} arg
+ */
+
+/**
+ * Base class for both child (Point) and parent (Broadcast) side of conduits,
+ * handles setting up send/receive method stubs.
+ */
+export class BaseConduit {
+  /**
+   * @param {object} subject
+   * @param {ConduitAddress} address
+   */
+  constructor(subject, address) {
+    this.subject = subject;
+    this.address = address;
+    this.id = address.id;
+
+    for (let name of address.send || []) {
+      this[`send${name}`] = this._send.bind(this, name, false);
+    }
+    for (let name of address.query || []) {
+      this[`query${name}`] = this._send.bind(this, name, true);
+    }
+
+    this.recv = new Map();
+    for (let name of address.recv || []) {
+      let method = this.subject[`recv${name}`];
+      if (!method) {
+        throw new Error(`recv${name} not found for conduit ${this.id}`);
+      }
+      this.recv.set(name, method.bind(this.subject));
+    }
+  }
+
+  /**
+   * Internal, partially @abstract, uses the actor to send the message/query.
+   *
+   * @param {string} method
+   * @param {boolean} query Flag indicating a response is expected.
+   * @param {JSWindowActor} actor
+   * @param {MessageData} data
+   * @returns {Promise?}
+   */
+  _send(method, query, actor, data) {
+    if (query) {
+      return actor.sendQuery(method, data);
+    }
+    actor.sendAsyncMessage(method, data);
+  }
+
+  /**
+   * Internal, calls the specific recvX method based on the message.
+   *
+   * @param {string} name Message/method name.
+   * @param {object} arg  Message data, the one and only method argument.
+   * @param {object} meta Metadata about the method call.
+   */
+  async _recv(name, arg, meta) {
+    let method = this.recv.get(name);
+    if (!method) {
+      throw new Error(`recv${name} not found for conduit ${this.id}`);
+    }
+    try {
+      return await method(arg, meta);
+    } catch (e) {
+      if (meta.query) {
+        return Promise.reject(e);
+      }
+      Cu.reportError(e);
+    }
+  }
+}
+
+/**
+ * Child side conduit, can only send/receive point-to-point messages via the
+ * one specific ConduitsChild actor.
+ */
+class PointConduit extends BaseConduit {
+  constructor(subject, address, actor) {
+    super(subject, address);
+    this.actor = actor;
+    this.actor.sendAsyncMessage("ConduitOpened", { arg: address });
+  }
+
+  /**
+   * Internal, sends messages via the actor, used by sendX stubs.
+   *
+   * @param {string} method
+   * @param {boolean} query
+   * @param {object?} arg
+   * @returns {Promise?}
+   */
+  _send(method, query, arg = {}) {
+    if (!this.actor) {
+      throw new Error(`send${method} on closed conduit ${this.id}`);
+    }
+    let sender = this.id;
+    return super._send(method, query, this.actor, { arg, query, sender });
+  }
+
+  /**
+   * Closes the conduit from further IPC, notifies the parent side by default.
+   *
+   * @param {boolean} silent
+   */
+  close(silent = false) {
+    let { actor } = this;
+    if (actor) {
+      this.actor = null;
+      actor.conduits.delete(this.id);
+      if (!silent) {
+        // Catch any exceptions that can occur if the conduit is closed while
+        // the actor is being destroyed due to the containing browser being closed.
+        // This should be treated as if the silent flag was passed.
+        try {
+          actor.sendAsyncMessage("ConduitClosed", { sender: this.id });
+        } catch (ex) {}
+      }
+    }
+    this.closeCallback?.();
+    this.closeCallback = null;
+  }
+
+  /**
+   * Set the callback to be called when the conduit is closed.
+   *
+   * @param {Function} callback
+   */
+  setCloseCallback(callback) {
+    this.closeCallback = callback;
+  }
+}
+
+/**
+ * Implements the child side of the Conduits actor, manages conduit lifetimes.
+ */
+export class ConduitsChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    this.conduits = new Map();
+  }
+
+  /**
+   * Public entry point a child-side subject uses to open a conduit.
+   *
+   * @param {object} subject
+   * @param {ConduitAddress} address
+   * @returns {PointConduit}
+   */
+  openConduit(subject, address) {
+    let conduit = new PointConduit(subject, address, this);
+    this.conduits.set(conduit.id, conduit);
+    return conduit;
+  }
+
+  /**
+   * JSWindowActor method, routes the message to the target subject.
+   *
+   * @param {object} options
+   * @param {string} options.name
+   * @param {MessageData | MessageData[]} options.data
+   * @returns {Promise?}
+   */
+  receiveMessage({ name, data }) {
+    // Batch of webRequest events, run each and return results, ignoring errors.
+    if (Array.isArray(data)) {
+      let run = data => this.receiveMessage({ name, data });
+      return Promise.all(data.map(data => run(data).catch(Cu.reportError)));
+    }
+
+    let { target, arg, query, sender } = data;
+    let conduit = this.conduits.get(target);
+    if (!conduit) {
+      throw new Error(`${name} for closed conduit ${target}: ${uneval(arg)}`);
+    }
+    return conduit._recv(name, arg, { sender, query, actor: this });
+  }
+
+  /**
+   * JSWindowActor method, ensure cleanup.
+   */
+  didDestroy() {
+    for (let conduit of this.conduits.values()) {
+      conduit.close(true);
+    }
+    this.conduits.clear();
+  }
+}
+
+/**
+ * Child side of the Conduits process actor.  Same code as JSWindowActor.
+ */
+export class ProcessConduitsChild extends JSProcessActorChild {
+  constructor() {
+    super();
+    this.conduits = new Map();
+  }
+
+  openConduit = ConduitsChild.prototype.openConduit;
+  receiveMessage = ConduitsChild.prototype.receiveMessage;
+  willDestroy = ConduitsChild.prototype.willDestroy;
+  didDestroy = ConduitsChild.prototype.didDestroy;
+}