Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

60 files changed, 18086 insertions, 0 deletions

diff --git a/remote/marionette/.eslintrc.js b/remote/marionette/.eslintrc.js
new file mode 100644
index 0000000000..64a8883c43
--- /dev/null
+++ b/remote/marionette/.eslintrc.js
@@ -0,0 +1,14 @@
+/* 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";
+
+// inherits from ../../tools/lint/eslint/eslint-plugin-mozilla/lib/configs/recommended.js
+
+module.exports = {
+  rules: {
+    camelcase: ["error", { properties: "never" }],
+    "no-var": "error",
+  },
+};
diff --git a/remote/marionette/README b/remote/marionette/README
new file mode 100644
index 0000000000..d077a5136c
--- /dev/null
+++ b/remote/marionette/README
@@ -0,0 +1,20 @@
+Marionette [ ˌmarɪəˈnɛt] is
+
+  * a puppet worked by strings: the bird bobs up and down like
+    a marionette;
+
+  * a person who is easily manipulated or controlled: many officers
+    dismissed him as the mayor’s marionette;
+
+  * the remote protocol that lets out-of-process programs communicate
+    with, instrument, and control Gecko-based browsers.
+
+Marionette provides interfaces for interacting with both the internal
+JavaScript runtime and UI elements of Gecko-based browsers, such
+as Firefox on desktop and mobile.  It can control both the chrome- and content
+documents, giving a high level of control and ability to replicate,
+or emulate, user interaction.
+
+Head on to the Marionette documentation to find out more:
+
+	https://firefox-source-docs.mozilla.org/testing/marionette/
diff --git a/remote/marionette/accessibility.sys.mjs b/remote/marionette/accessibility.sys.mjs
new file mode 100644
index 0000000000..f21eaf5047
--- /dev/null
+++ b/remote/marionette/accessibility.sys.mjs
@@ -0,0 +1,481 @@
+/* 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",
+  waitForObserverTopic: "chrome://remote/content/marionette/sync.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+XPCOMUtils.defineLazyGetter(lazy, "service", () => {
+  try {
+    return Cc["@mozilla.org/accessibilityService;1"].getService(
+      Ci.nsIAccessibilityService
+    );
+  } catch (e) {
+    lazy.logger.warn("Accessibility module is not present");
+    return undefined;
+  }
+});
+
+/** @namespace */
+export const accessibility = {
+  get service() {
+    return lazy.service;
+  },
+};
+
+/**
+ * Accessible states used to check element"s state from the accessiblity API
+ * perspective.
+ *
+ * Note: if gecko is built with --disable-accessibility, the interfaces
+ * are not defined. This is why we use getters instead to be able to use
+ * these statically.
+ */
+accessibility.State = {
+  get Unavailable() {
+    return Ci.nsIAccessibleStates.STATE_UNAVAILABLE;
+  },
+  get Focusable() {
+    return Ci.nsIAccessibleStates.STATE_FOCUSABLE;
+  },
+  get Selectable() {
+    return Ci.nsIAccessibleStates.STATE_SELECTABLE;
+  },
+  get Selected() {
+    return Ci.nsIAccessibleStates.STATE_SELECTED;
+  },
+};
+
+/**
+ * Accessible object roles that support some action.
+ */
+accessibility.ActionableRoles = new Set([
+  "checkbutton",
+  "check menu item",
+  "check rich option",
+  "combobox",
+  "combobox option",
+  "entry",
+  "key",
+  "link",
+  "listbox option",
+  "listbox rich option",
+  "menuitem",
+  "option",
+  "outlineitem",
+  "pagetab",
+  "pushbutton",
+  "radiobutton",
+  "radio menu item",
+  "rowheader",
+  "slider",
+  "spinbutton",
+  "switch",
+]);
+
+/**
+ * Factory function that constructs a new {@code accessibility.Checks}
+ * object with enforced strictness or not.
+ */
+accessibility.get = function (strict = false) {
+  return new accessibility.Checks(!!strict);
+};
+
+/**
+ * Wait for the document accessibility state to be different from STATE_BUSY.
+ *
+ * @param {Document} doc
+ *     The document to wait for.
+ * @returns {Promise}
+ *     A promise which resolves when the document's accessibility state is no
+ *     longer busy.
+ */
+function waitForDocumentAccessibility(doc) {
+  const documentAccessible = accessibility.service.getAccessibleFor(doc);
+  const state = {};
+  documentAccessible.getState(state, {});
+  if ((state.value & Ci.nsIAccessibleStates.STATE_BUSY) == 0) {
+    return Promise.resolve();
+  }
+
+  // Accessibility for the doc is busy, so wait for the state to change.
+  return lazy.waitForObserverTopic("accessible-event", {
+    checkFn: subject => {
+      // If event type does not match expected type, skip the event.
+      // If event's accessible does not match expected accessible,
+      // skip the event.
+      const event = subject.QueryInterface(Ci.nsIAccessibleEvent);
+      return (
+        event.eventType === Ci.nsIAccessibleEvent.EVENT_STATE_CHANGE &&
+        event.accessible === documentAccessible
+      );
+    },
+  });
+}
+
+/**
+ * Retrieve the Accessible for the provided element.
+ *
+ * @param {Element} element
+ *     The element for which we need to retrieve the accessible.
+ *
+ * @returns {nsIAccessible|null}
+ *     The Accessible object corresponding to the provided element or null if
+ *     the accessibility service is not available.
+ */
+accessibility.getAccessible = async function (element) {
+  if (!accessibility.service) {
+    return null;
+  }
+
+  // First, wait for accessibility to be ready for the element's document.
+  await waitForDocumentAccessibility(element.ownerDocument);
+
+  const acc = accessibility.service.getAccessibleFor(element);
+  if (acc) {
+    return acc;
+  }
+
+  // The Accessible doesn't exist yet. This can happen because a11y tree
+  // mutations happen during refresh driver ticks. Stop the refresh driver from
+  // doing its regular ticks and force two refresh driver ticks: the first to
+  // let layout update and notify a11y, and the second to let a11y process
+  // updates.
+  const windowUtils = element.ownerGlobal.windowUtils;
+  windowUtils.advanceTimeAndRefresh(0);
+  windowUtils.advanceTimeAndRefresh(0);
+  // Go back to normal refresh driver ticks.
+  windowUtils.restoreNormalRefresh();
+  return accessibility.service.getAccessibleFor(element);
+};
+
+/**
+ * Component responsible for interacting with platform accessibility
+ * API.
+ *
+ * Its methods serve as wrappers for testing content and chrome
+ * accessibility as well as accessibility of user interactions.
+ */
+accessibility.Checks = class {
+  /**
+   * @param {boolean} strict
+   *     Flag indicating whether the accessibility issue should be logged
+   *     or cause an error to be thrown.  Default is to log to stdout.
+   */
+  constructor(strict) {
+    this.strict = strict;
+  }
+
+  /**
+   * Assert that the element has a corresponding accessible object, and retrieve
+   * this accessible. Note that if the accessibility.Checks component was
+   * created in non-strict mode, this helper will not attempt to resolve the
+   * accessible at all and will simply return null.
+   *
+   * @param {DOMElement|XULElement} element
+   *     Element to get the accessible object for.
+   * @param {boolean=} mustHaveAccessible
+   *     Flag indicating that the element must have an accessible object.
+   *     Defaults to not require this.
+   *
+   * @returns {Promise.<nsIAccessible>}
+   *     Promise with an accessibility object for the given element.
+   */
+  async assertAccessible(element, mustHaveAccessible = false) {
+    if (!this.strict) {
+      return null;
+    }
+
+    const accessible = await accessibility.getAccessible(element);
+    if (!accessible && mustHaveAccessible) {
+      this.error("Element does not have an accessible object", element);
+    }
+
+    return accessible;
+  }
+
+  /**
+   * Test if the accessible has a role that supports some arbitrary
+   * action.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   *
+   * @returns {boolean}
+   *     True if an actionable role is found on the accessible, false
+   *     otherwise.
+   */
+  isActionableRole(accessible) {
+    return accessibility.ActionableRoles.has(
+      accessibility.service.getStringRole(accessible.role)
+    );
+  }
+
+  /**
+   * Test if an accessible has at least one action that it supports.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   *
+   * @returns {boolean}
+   *     True if the accessible has at least one supported action,
+   *     false otherwise.
+   */
+  hasActionCount(accessible) {
+    return accessible.actionCount > 0;
+  }
+
+  /**
+   * Test if an accessible has a valid name.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   *
+   * @returns {boolean}
+   *     True if the accessible has a non-empty valid name, or false if
+   *     this is not the case.
+   */
+  hasValidName(accessible) {
+    return accessible.name && accessible.name.trim();
+  }
+
+  /**
+   * Test if an accessible has a {@code hidden} attribute.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   *
+   * @returns {boolean}
+   *     True if the accessible object has a {@code hidden} attribute,
+   *     false otherwise.
+   */
+  hasHiddenAttribute(accessible) {
+    let hidden = false;
+    try {
+      hidden = accessible.attributes.getStringProperty("hidden");
+    } catch (e) {}
+    // if the property is missing, error will be thrown
+    return hidden && hidden === "true";
+  }
+
+  /**
+   * Verify if an accessible has a given state.
+   * Test if an accessible has a given state.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object to test.
+   * @param {number} stateToMatch
+   *     State to match.
+   *
+   * @returns {boolean}
+   *     True if |accessible| has |stateToMatch|, false otherwise.
+   */
+  matchState(accessible, stateToMatch) {
+    let state = {};
+    accessible.getState(state, {});
+    return !!(state.value & stateToMatch);
+  }
+
+  /**
+   * Test if an accessible is hidden from the user.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   *
+   * @returns {boolean}
+   *     True if element is hidden from user, false otherwise.
+   */
+  isHidden(accessible) {
+    if (!accessible) {
+      return true;
+    }
+
+    while (accessible) {
+      if (this.hasHiddenAttribute(accessible)) {
+        return true;
+      }
+      accessible = accessible.parent;
+    }
+    return false;
+  }
+
+  /**
+   * Test if the element's visible state corresponds to its accessibility
+   * API visibility.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   * @param {DOMElement|XULElement} element
+   *     Element associated with |accessible|.
+   * @param {boolean} visible
+   *     Visibility state of |element|.
+   *
+   * @throws ElementNotAccessibleError
+   *     If |element|'s visibility state does not correspond to
+   *     |accessible|'s.
+   */
+  assertVisible(accessible, element, visible) {
+    let hiddenAccessibility = this.isHidden(accessible);
+
+    let message;
+    if (visible && hiddenAccessibility) {
+      message =
+        "Element is not currently visible via the accessibility API " +
+        "and may not be manipulated by it";
+    } else if (!visible && !hiddenAccessibility) {
+      message =
+        "Element is currently only visible via the accessibility API " +
+        "and can be manipulated by it";
+    }
+    this.error(message, element);
+  }
+
+  /**
+   * Test if the element's unavailable accessibility state matches the
+   * enabled state.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   * @param {DOMElement|XULElement} element
+   *     Element associated with |accessible|.
+   * @param {boolean} enabled
+   *     Enabled state of |element|.
+   *
+   * @throws ElementNotAccessibleError
+   *     If |element|'s enabled state does not match |accessible|'s.
+   */
+  assertEnabled(accessible, element, enabled) {
+    if (!accessible) {
+      return;
+    }
+
+    let win = element.ownerGlobal;
+    let disabledAccessibility = this.matchState(
+      accessible,
+      accessibility.State.Unavailable
+    );
+    let explorable =
+      win.getComputedStyle(element).getPropertyValue("pointer-events") !==
+      "none";
+
+    let message;
+    if (!explorable && !disabledAccessibility) {
+      message =
+        "Element is enabled but is not explorable via the " +
+        "accessibility API";
+    } else if (enabled && disabledAccessibility) {
+      message = "Element is enabled but disabled via the accessibility API";
+    } else if (!enabled && !disabledAccessibility) {
+      message = "Element is disabled but enabled via the accessibility API";
+    }
+    this.error(message, element);
+  }
+
+  /**
+   * Test if it is possible to activate an element with the accessibility
+   * API.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   * @param {DOMElement|XULElement} element
+   *     Element associated with |accessible|.
+   *
+   * @throws ElementNotAccessibleError
+   *     If it is impossible to activate |element| with |accessible|.
+   */
+  assertActionable(accessible, element) {
+    if (!accessible) {
+      return;
+    }
+
+    let message;
+    if (!this.hasActionCount(accessible)) {
+      message = "Element does not support any accessible actions";
+    } else if (!this.isActionableRole(accessible)) {
+      message =
+        "Element does not have a correct accessibility role " +
+        "and may not be manipulated via the accessibility API";
+    } else if (!this.hasValidName(accessible)) {
+      message = "Element is missing an accessible name";
+    } else if (!this.matchState(accessible, accessibility.State.Focusable)) {
+      message = "Element is not focusable via the accessibility API";
+    }
+
+    this.error(message, element);
+  }
+
+  /**
+   * Test that an element's selected state corresponds to its
+   * accessibility API selected state.
+   *
+   * @param {nsIAccessible} accessible
+   *     Accessible object.
+   * @param {DOMElement|XULElement} element
+   *     Element associated with |accessible|.
+   * @param {boolean} selected
+   *     The |element|s selected state.
+   *
+   * @throws ElementNotAccessibleError
+   *     If |element|'s selected state does not correspond to
+   *     |accessible|'s.
+   */
+  assertSelected(accessible, element, selected) {
+    if (!accessible) {
+      return;
+    }
+
+    // element is not selectable via the accessibility API
+    if (!this.matchState(accessible, accessibility.State.Selectable)) {
+      return;
+    }
+
+    let selectedAccessibility = this.matchState(
+      accessible,
+      accessibility.State.Selected
+    );
+
+    let message;
+    if (selected && !selectedAccessibility) {
+      message =
+        "Element is selected but not selected via the accessibility API";
+    } else if (!selected && selectedAccessibility) {
+      message =
+        "Element is not selected but selected via the accessibility API";
+    }
+    this.error(message, element);
+  }
+
+  /**
+   * Throw an error if strict accessibility checks are enforced and log
+   * the error to the log.
+   *
+   * @param {string} message
+   * @param {DOMElement|XULElement} element
+   *     Element that caused an error.
+   *
+   * @throws ElementNotAccessibleError
+   *     If |strict| is true.
+   */
+  error(message, element) {
+    if (!message || !this.strict) {
+      return;
+    }
+    if (element) {
+      let { id, tagName, className } = element;
+      message += `: id: ${id}, tagName: ${tagName}, className: ${className}`;
+    }
+
+    throw new lazy.error.ElementNotAccessibleError(message);
+  }
+};
diff --git a/remote/marionette/actors/MarionetteCommandsChild.sys.mjs b/remote/marionette/actors/MarionetteCommandsChild.sys.mjs
new file mode 100644
index 0000000000..b51e758cae
--- /dev/null
+++ b/remote/marionette/actors/MarionetteCommandsChild.sys.mjs
@@ -0,0 +1,611 @@
+/* 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/. */
+
+/* eslint-disable no-restricted-globals */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  accessibility: "chrome://remote/content/marionette/accessibility.sys.mjs",
+  action: "chrome://remote/content/shared/webdriver/Actions.sys.mjs",
+  atom: "chrome://remote/content/marionette/atom.sys.mjs",
+  element: "chrome://remote/content/marionette/element.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  evaluate: "chrome://remote/content/marionette/evaluate.sys.mjs",
+  interaction: "chrome://remote/content/marionette/interaction.sys.mjs",
+  json: "chrome://remote/content/marionette/json.sys.mjs",
+  legacyaction: "chrome://remote/content/marionette/legacyaction.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  sandbox: "chrome://remote/content/marionette/evaluate.sys.mjs",
+  Sandboxes: "chrome://remote/content/marionette/evaluate.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+export class MarionetteCommandsChild extends JSWindowActorChild {
+  #processActor;
+
+  constructor() {
+    super();
+
+    this.#processActor = ChromeUtils.domProcessChild.getActor(
+      "WebDriverProcessData"
+    );
+
+    // sandbox storage and name of the current sandbox
+    this.sandboxes = new lazy.Sandboxes(() => this.document.defaultView);
+    // State of the input actions. This is specific to contexts and sessions
+    this.actionState = null;
+  }
+
+  get innerWindowId() {
+    return this.manager.innerWindowId;
+  }
+
+  /**
+   * Lazy getter to create a legacyaction Chain instance for touch events.
+   */
+  get legacyactions() {
+    if (!this._legacyactions) {
+      this._legacyactions = new lazy.legacyaction.Chain();
+    }
+
+    return this._legacyactions;
+  }
+
+  actorCreated() {
+    lazy.logger.trace(
+      `[${this.browsingContext.id}] MarionetteCommands actor created ` +
+        `for window id ${this.innerWindowId}`
+    );
+  }
+
+  didDestroy() {
+    lazy.logger.trace(
+      `[${this.browsingContext.id}] MarionetteCommands actor destroyed ` +
+        `for window id ${this.innerWindowId}`
+    );
+  }
+
+  async receiveMessage(msg) {
+    if (!this.contentWindow) {
+      throw new DOMException("Actor is no longer active", "InactiveActor");
+    }
+
+    try {
+      let result;
+      let waitForNextTick = false;
+
+      const { name, data: serializedData } = msg;
+      const data = lazy.json.deserialize(
+        serializedData,
+        this.#processActor.getNodeCache(),
+        this.contentWindow
+      );
+
+      switch (name) {
+        case "MarionetteCommandsParent:clearElement":
+          this.clearElement(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:clickElement":
+          result = await this.clickElement(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:executeScript":
+          result = await this.executeScript(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:findElement":
+          result = await this.findElement(data);
+          break;
+        case "MarionetteCommandsParent:findElements":
+          result = await this.findElements(data);
+          break;
+        case "MarionetteCommandsParent:getActiveElement":
+          result = await this.getActiveElement();
+          break;
+        case "MarionetteCommandsParent:getComputedLabel":
+          result = await this.getComputedLabel(data);
+          break;
+        case "MarionetteCommandsParent:getComputedRole":
+          result = await this.getComputedRole(data);
+          break;
+        case "MarionetteCommandsParent:getElementAttribute":
+          result = await this.getElementAttribute(data);
+          break;
+        case "MarionetteCommandsParent:getElementProperty":
+          result = await this.getElementProperty(data);
+          break;
+        case "MarionetteCommandsParent:getElementRect":
+          result = await this.getElementRect(data);
+          break;
+        case "MarionetteCommandsParent:getElementTagName":
+          result = await this.getElementTagName(data);
+          break;
+        case "MarionetteCommandsParent:getElementText":
+          result = await this.getElementText(data);
+          break;
+        case "MarionetteCommandsParent:getElementValueOfCssProperty":
+          result = await this.getElementValueOfCssProperty(data);
+          break;
+        case "MarionetteCommandsParent:getPageSource":
+          result = await this.getPageSource();
+          break;
+        case "MarionetteCommandsParent:getScreenshotRect":
+          result = await this.getScreenshotRect(data);
+          break;
+        case "MarionetteCommandsParent:getShadowRoot":
+          result = await this.getShadowRoot(data);
+          break;
+        case "MarionetteCommandsParent:isElementDisplayed":
+          result = await this.isElementDisplayed(data);
+          break;
+        case "MarionetteCommandsParent:isElementEnabled":
+          result = await this.isElementEnabled(data);
+          break;
+        case "MarionetteCommandsParent:isElementSelected":
+          result = await this.isElementSelected(data);
+          break;
+        case "MarionetteCommandsParent:performActions":
+          result = await this.performActions(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:releaseActions":
+          result = await this.releaseActions();
+          break;
+        case "MarionetteCommandsParent:sendKeysToElement":
+          result = await this.sendKeysToElement(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:singleTap":
+          result = await this.singleTap(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:switchToFrame":
+          result = await this.switchToFrame(data);
+          waitForNextTick = true;
+          break;
+        case "MarionetteCommandsParent:switchToParentFrame":
+          result = await this.switchToParentFrame();
+          waitForNextTick = true;
+          break;
+      }
+
+      // Inform the content process that the command has completed. It allows
+      // it to process async follow-up tasks before the reply is sent.
+      if (waitForNextTick) {
+        await new Promise(resolve => Services.tm.dispatchToMainThread(resolve));
+      }
+
+      return {
+        data: lazy.json.clone(result, this.#processActor.getNodeCache()),
+      };
+    } catch (e) {
+      // Always wrap errors as WebDriverError
+      return { error: lazy.error.wrap(e).toJSON() };
+    }
+  }
+
+  // Implementation of WebDriver commands
+
+  /** Clear the text of an element.
+   *
+   * @param {object} options
+   * @param {Element} options.elem
+   */
+  clearElement(options = {}) {
+    const { elem } = options;
+
+    lazy.interaction.clearElement(elem);
+  }
+
+  /**
+   * Click an element.
+   */
+  async clickElement(options = {}) {
+    const { capabilities, elem } = options;
+
+    return lazy.interaction.clickElement(
+      elem,
+      capabilities["moz:accessibilityChecks"],
+      capabilities["moz:webdriverClick"]
+    );
+  }
+
+  /**
+   * Executes a JavaScript function.
+   */
+  async executeScript(options = {}) {
+    const { args, opts = {}, script } = options;
+
+    let sb;
+    if (opts.sandboxName) {
+      sb = this.sandboxes.get(opts.sandboxName, opts.newSandbox);
+    } else {
+      sb = lazy.sandbox.createMutable(this.document.defaultView);
+    }
+
+    return lazy.evaluate.sandbox(sb, script, args, opts);
+  }
+
+  /**
+   * Find an element in the current browsing context's document using the
+   * given search strategy.
+   *
+   * @param {object=} options
+   * @param {string} options.strategy
+   * @param {string} options.selector
+   * @param {object} options.opts
+   * @param {Element} options.opts.startNode
+   *
+   */
+  async findElement(options = {}) {
+    const { strategy, selector, opts } = options;
+
+    opts.all = false;
+
+    const container = { frame: this.document.defaultView };
+    return lazy.element.find(container, strategy, selector, opts);
+  }
+
+  /**
+   * Find elements in the current browsing context's document using the
+   * given search strategy.
+   *
+   * @param {object=} options
+   * @param {string} options.strategy
+   * @param {string} options.selector
+   * @param {object} options.opts
+   * @param {Element} options.opts.startNode
+   *
+   */
+  async findElements(options = {}) {
+    const { strategy, selector, opts } = options;
+
+    opts.all = true;
+
+    const container = { frame: this.document.defaultView };
+    return lazy.element.find(container, strategy, selector, opts);
+  }
+
+  /**
+   * Return the active element in the document.
+   */
+  async getActiveElement() {
+    let elem = this.document.activeElement;
+    if (!elem) {
+      throw new lazy.error.NoSuchElementError();
+    }
+
+    return elem;
+  }
+
+  /**
+   * Return the accessible label for a given element.
+   */
+  async getComputedLabel(options = {}) {
+    const { elem } = options;
+
+    const accessible = await lazy.accessibility.getAccessible(elem);
+    if (!accessible) {
+      return null;
+    }
+
+    return accessible.name;
+  }
+
+  /**
+   * Return the accessible role for a given element.
+   */
+  async getComputedRole(options = {}) {
+    const { elem } = options;
+
+    const accessible = await lazy.accessibility.getAccessible(elem);
+    if (!accessible) {
+      // If it's not in the a11y tree, it's probably presentational.
+      return "none";
+    }
+
+    return accessible.computedARIARole;
+  }
+
+  /**
+   * Get the value of an attribute for the given element.
+   */
+  async getElementAttribute(options = {}) {
+    const { name, elem } = options;
+
+    if (lazy.element.isBooleanAttribute(elem, name)) {
+      if (elem.hasAttribute(name)) {
+        return "true";
+      }
+      return null;
+    }
+    return elem.getAttribute(name);
+  }
+
+  /**
+   * Get the value of a property for the given element.
+   */
+  async getElementProperty(options = {}) {
+    const { name, elem } = options;
+
+    // Waive Xrays to get unfiltered access to the untrusted element.
+    const el = Cu.waiveXrays(elem);
+    return typeof el[name] != "undefined" ? el[name] : null;
+  }
+
+  /**
+   * Get the position and dimensions of the element.
+   */
+  async getElementRect(options = {}) {
+    const { elem } = options;
+
+    const rect = elem.getBoundingClientRect();
+    return {
+      x: rect.x + this.document.defaultView.pageXOffset,
+      y: rect.y + this.document.defaultView.pageYOffset,
+      width: rect.width,
+      height: rect.height,
+    };
+  }
+
+  /**
+   * Get the tagName for the given element.
+   */
+  async getElementTagName(options = {}) {
+    const { elem } = options;
+
+    return elem.tagName.toLowerCase();
+  }
+
+  /**
+   * Get the text content for the given element.
+   */
+  async getElementText(options = {}) {
+    const { elem } = options;
+
+    try {
+      return lazy.atom.getElementText(elem, this.document.defaultView);
+    } catch (e) {
+      lazy.logger.warn(`Atom getElementText failed: "${e.message}"`);
+
+      // Fallback in case the atom implementation is broken.
+      // As known so far this only happens for XML documents (bug 1794099).
+      return elem.textContent;
+    }
+  }
+
+  /**
+   * Get the value of a css property for the given element.
+   */
+  async getElementValueOfCssProperty(options = {}) {
+    const { name, elem } = options;
+
+    const style = this.document.defaultView.getComputedStyle(elem);
+    return style.getPropertyValue(name);
+  }
+
+  /**
+   * Get the source of the current browsing context's document.
+   */
+  async getPageSource() {
+    return this.document.documentElement.outerHTML;
+  }
+
+  /**
+   * Returns the rect of the element to screenshot.
+   *
+   * Because the screen capture takes place in the parent process the dimensions
+   * for the screenshot have to be determined in the appropriate child process.
+   *
+   * Also it takes care of scrolling an element into view if requested.
+   *
+   * @param {object} options
+   * @param {Element} options.elem
+   *     Optional element to take a screenshot of.
+   * @param {boolean=} options.full
+   *     True to take a screenshot of the entire document element.
+   *     Defaults to true.
+   * @param {boolean=} options.scroll
+   *     When <var>elem</var> is given, scroll it into view.
+   *     Defaults to true.
+   *
+   * @returns {DOMRect}
+   *     The area to take a snapshot from.
+   */
+  async getScreenshotRect(options = {}) {
+    const { elem, full = true, scroll = true } = options;
+    const win = elem
+      ? this.document.defaultView
+      : this.browsingContext.top.window;
+
+    let rect;
+
+    if (elem) {
+      if (scroll) {
+        lazy.element.scrollIntoView(elem);
+      }
+      rect = this.getElementRect({ elem });
+    } else if (full) {
+      const docEl = win.document.documentElement;
+      rect = new DOMRect(0, 0, docEl.scrollWidth, docEl.scrollHeight);
+    } else {
+      // viewport
+      rect = new DOMRect(
+        win.pageXOffset,
+        win.pageYOffset,
+        win.innerWidth,
+        win.innerHeight
+      );
+    }
+
+    return rect;
+  }
+
+  /**
+   * Return the shadowRoot attached to an element
+   */
+  async getShadowRoot(options = {}) {
+    const { elem } = options;
+
+    return lazy.element.getShadowRoot(elem);
+  }
+
+  /**
+   * Determine the element displayedness of the given web element.
+   */
+  async isElementDisplayed(options = {}) {
+    const { capabilities, elem } = options;
+
+    return lazy.interaction.isElementDisplayed(
+      elem,
+      capabilities["moz:accessibilityChecks"]
+    );
+  }
+
+  /**
+   * Check if element is enabled.
+   */
+  async isElementEnabled(options = {}) {
+    const { capabilities, elem } = options;
+
+    return lazy.interaction.isElementEnabled(
+      elem,
+      capabilities["moz:accessibilityChecks"]
+    );
+  }
+
+  /**
+   * Determine whether the referenced element is selected or not.
+   */
+  async isElementSelected(options = {}) {
+    const { capabilities, elem } = options;
+
+    return lazy.interaction.isElementSelected(
+      elem,
+      capabilities["moz:accessibilityChecks"]
+    );
+  }
+
+  /**
+   * Perform a series of grouped actions at the specified points in time.
+   *
+   * @param {object} options
+   * @param {object} options.actions
+   *     Array of objects with each representing an action sequence.
+   * @param {object} options.capabilities
+   *     Object with a list of WebDriver session capabilities.
+   */
+  async performActions(options = {}) {
+    const { actions, capabilities } = options;
+    if (this.actionState === null) {
+      this.actionState = new lazy.action.State({
+        specCompatPointerOrigin:
+          !capabilities["moz:useNonSpecCompliantPointerOrigin"],
+      });
+    }
+    let actionChain = lazy.action.Chain.fromJSON(this.actionState, actions);
+
+    await actionChain.dispatch(this.actionState, this.document.defaultView);
+    // Terminate the current wheel transaction if there is one. Wheel
+    // transactions should not live longer than a single action chain.
+    ChromeUtils.endWheelTransaction();
+  }
+
+  /**
+   * The release actions command is used to release all the keys and pointer
+   * buttons that are currently depressed. This causes events to be fired
+   * as if the state was released by an explicit series of actions. It also
+   * clears all the internal state of the virtual devices.
+   */
+  async releaseActions() {
+    if (this.actionState === null) {
+      return;
+    }
+    await this.actionState.release(this.document.defaultView);
+    this.actionState = null;
+  }
+
+  /*
+   * Send key presses to element after focusing on it.
+   */
+  async sendKeysToElement(options = {}) {
+    const { capabilities, elem, text } = options;
+
+    const opts = {
+      strictFileInteractability: capabilities.strictFileInteractability,
+      accessibilityChecks: capabilities["moz:accessibilityChecks"],
+      webdriverClick: capabilities["moz:webdriverClick"],
+    };
+
+    return lazy.interaction.sendKeysToElement(elem, text, opts);
+  }
+
+  /**
+   * Perform a single tap.
+   */
+  async singleTap(options = {}) {
+    const { capabilities, elem, x, y } = options;
+    return this.legacyactions.singleTap(elem, x, y, capabilities);
+  }
+
+  /**
+   * Switch to the specified frame.
+   *
+   * @param {object=} options
+   * @param {(number|Element)=} options.id
+   *     If it's a number treat it as the index for all the existing frames.
+   *     If it's an Element switch to this specific frame.
+   *     If not specified or `null` switch to the top-level browsing context.
+   */
+  async switchToFrame(options = {}) {
+    const { id } = options;
+
+    const childContexts = this.browsingContext.children;
+    let browsingContext;
+
+    if (id == null) {
+      browsingContext = this.browsingContext.top;
+    } else if (typeof id == "number") {
+      if (id < 0 || id >= childContexts.length) {
+        throw new lazy.error.NoSuchFrameError(
+          `Unable to locate frame with index: ${id}`
+        );
+      }
+      browsingContext = childContexts[id];
+    } else {
+      const context = childContexts.find(context => {
+        return context.embedderElement === id;
+      });
+      if (!context) {
+        throw new lazy.error.NoSuchFrameError(
+          `Unable to locate frame for element: ${id}`
+        );
+      }
+      browsingContext = context;
+    }
+
+    // For in-process iframes the window global is lazy-loaded for optimization
+    // reasons. As such force the currentWindowGlobal to be created so we always
+    // have a window (bug 1691348).
+    browsingContext.window;
+
+    return { browsingContextId: browsingContext.id };
+  }
+
+  /**
+   * Switch to the parent frame.
+   */
+  async switchToParentFrame() {
+    const browsingContext = this.browsingContext.parent || this.browsingContext;
+
+    return { browsingContextId: browsingContext.id };
+  }
+}
diff --git a/remote/marionette/actors/MarionetteCommandsParent.sys.mjs b/remote/marionette/actors/MarionetteCommandsParent.sys.mjs
new file mode 100644
index 0000000000..2ab202acc0
--- /dev/null
+++ b/remote/marionette/actors/MarionetteCommandsParent.sys.mjs
@@ -0,0 +1,383 @@
+/* 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, {
+  capture: "chrome://remote/content/shared/Capture.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+export class MarionetteCommandsParent extends JSWindowActorParent {
+  actorCreated() {
+    this._resolveDialogOpened = null;
+  }
+
+  dialogOpenedPromise() {
+    return new Promise(resolve => {
+      this._resolveDialogOpened = resolve;
+    });
+  }
+
+  async sendQuery(name, data) {
+    // return early if a dialog is opened
+    const result = await Promise.race([
+      super.sendQuery(name, data),
+      this.dialogOpenedPromise(),
+    ]).finally(() => {
+      this._resolveDialogOpened = null;
+    });
+
+    if ("error" in result) {
+      throw lazy.error.WebDriverError.fromJSON(result.error);
+    } else {
+      return result.data;
+    }
+  }
+
+  notifyDialogOpened() {
+    if (this._resolveDialogOpened) {
+      this._resolveDialogOpened({ data: null });
+    }
+  }
+
+  // Proxying methods for WebDriver commands
+
+  clearElement(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:clearElement", {
+      elem: webEl,
+    });
+  }
+
+  clickElement(webEl, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:clickElement", {
+      elem: webEl,
+      capabilities: capabilities.toJSON(),
+    });
+  }
+
+  async executeScript(script, args, opts) {
+    return this.sendQuery("MarionetteCommandsParent:executeScript", {
+      script,
+      args,
+      opts,
+    });
+  }
+
+  findElement(strategy, selector, opts) {
+    return this.sendQuery("MarionetteCommandsParent:findElement", {
+      strategy,
+      selector,
+      opts,
+    });
+  }
+
+  findElements(strategy, selector, opts) {
+    return this.sendQuery("MarionetteCommandsParent:findElements", {
+      strategy,
+      selector,
+      opts,
+    });
+  }
+
+  async getShadowRoot(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getShadowRoot", {
+      elem: webEl,
+    });
+  }
+
+  async getActiveElement() {
+    return this.sendQuery("MarionetteCommandsParent:getActiveElement");
+  }
+
+  async getComputedLabel(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getComputedLabel", {
+      elem: webEl,
+    });
+  }
+
+  async getComputedRole(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getComputedRole", {
+      elem: webEl,
+    });
+  }
+
+  async getElementAttribute(webEl, name) {
+    return this.sendQuery("MarionetteCommandsParent:getElementAttribute", {
+      elem: webEl,
+      name,
+    });
+  }
+
+  async getElementProperty(webEl, name) {
+    return this.sendQuery("MarionetteCommandsParent:getElementProperty", {
+      elem: webEl,
+      name,
+    });
+  }
+
+  async getElementRect(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getElementRect", {
+      elem: webEl,
+    });
+  }
+
+  async getElementTagName(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getElementTagName", {
+      elem: webEl,
+    });
+  }
+
+  async getElementText(webEl) {
+    return this.sendQuery("MarionetteCommandsParent:getElementText", {
+      elem: webEl,
+    });
+  }
+
+  async getElementValueOfCssProperty(webEl, name) {
+    return this.sendQuery(
+      "MarionetteCommandsParent:getElementValueOfCssProperty",
+      {
+        elem: webEl,
+        name,
+      }
+    );
+  }
+
+  async getPageSource() {
+    return this.sendQuery("MarionetteCommandsParent:getPageSource");
+  }
+
+  async isElementDisplayed(webEl, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:isElementDisplayed", {
+      capabilities: capabilities.toJSON(),
+      elem: webEl,
+    });
+  }
+
+  async isElementEnabled(webEl, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:isElementEnabled", {
+      capabilities: capabilities.toJSON(),
+      elem: webEl,
+    });
+  }
+
+  async isElementSelected(webEl, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:isElementSelected", {
+      capabilities: capabilities.toJSON(),
+      elem: webEl,
+    });
+  }
+
+  async sendKeysToElement(webEl, text, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:sendKeysToElement", {
+      capabilities: capabilities.toJSON(),
+      elem: webEl,
+      text,
+    });
+  }
+
+  async performActions(actions, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:performActions", {
+      actions,
+      capabilities: capabilities.toJSON(),
+    });
+  }
+
+  async releaseActions() {
+    return this.sendQuery("MarionetteCommandsParent:releaseActions");
+  }
+
+  async singleTap(webEl, x, y, capabilities) {
+    return this.sendQuery("MarionetteCommandsParent:singleTap", {
+      capabilities: capabilities.toJSON(),
+      elem: webEl,
+      x,
+      y,
+    });
+  }
+
+  async switchToFrame(id) {
+    const { browsingContextId } = await this.sendQuery(
+      "MarionetteCommandsParent:switchToFrame",
+      { id }
+    );
+
+    return {
+      browsingContext: BrowsingContext.get(browsingContextId),
+    };
+  }
+
+  async switchToParentFrame() {
+    const { browsingContextId } = await this.sendQuery(
+      "MarionetteCommandsParent:switchToParentFrame"
+    );
+
+    return {
+      browsingContext: BrowsingContext.get(browsingContextId),
+    };
+  }
+
+  async takeScreenshot(webEl, format, full, scroll) {
+    const rect = await this.sendQuery(
+      "MarionetteCommandsParent:getScreenshotRect",
+      {
+        elem: webEl,
+        full,
+        scroll,
+      }
+    );
+
+    // If no element has been specified use the top-level browsing context.
+    // Otherwise use the browsing context from the currently selected frame.
+    const browsingContext = webEl
+      ? this.browsingContext
+      : this.browsingContext.top;
+
+    let canvas = await lazy.capture.canvas(
+      browsingContext.topChromeWindow,
+      browsingContext,
+      rect.x,
+      rect.y,
+      rect.width,
+      rect.height
+    );
+
+    switch (format) {
+      case lazy.capture.Format.Hash:
+        return lazy.capture.toHash(canvas);
+
+      case lazy.capture.Format.Base64:
+        return lazy.capture.toBase64(canvas);
+
+      default:
+        throw new TypeError(`Invalid capture format: ${format}`);
+    }
+  }
+}
+
+/**
+ * Proxy that will dynamically create MarionetteCommands actors for a dynamically
+ * provided browsing context until the method can be fully executed by the
+ * JSWindowActor pair.
+ *
+ * @param {function(): BrowsingContext} browsingContextFn
+ *     A function that returns the reference to the browsing context for which
+ *     the query should run.
+ */
+export function getMarionetteCommandsActorProxy(browsingContextFn) {
+  const MAX_ATTEMPTS = 10;
+
+  /**
+   * Methods which modify the content page cannot be retried safely.
+   * See Bug 1673345.
+   */
+  const NO_RETRY_METHODS = [
+    "clickElement",
+    "executeScript",
+    "performActions",
+    "releaseActions",
+    "sendKeysToElement",
+    "singleTap",
+  ];
+
+  return new Proxy(
+    {},
+    {
+      get(target, methodName) {
+        return async (...args) => {
+          let attempts = 0;
+          while (true) {
+            try {
+              const browsingContext = browsingContextFn();
+              if (!browsingContext) {
+                throw new DOMException(
+                  "No BrowsingContext found",
+                  "NoBrowsingContext"
+                );
+              }
+
+              // TODO: Scenarios where the window/tab got closed and
+              // currentWindowGlobal is null will be handled in Bug 1662808.
+              const actor =
+                browsingContext.currentWindowGlobal.getActor(
+                  "MarionetteCommands"
+                );
+
+              const result = await actor[methodName](...args);
+              return result;
+            } catch (e) {
+              if (!["AbortError", "InactiveActor"].includes(e.name)) {
+                // Only retry when the JSWindowActor pair gets destroyed, or
+                // gets inactive eg. when the page is moved into bfcache.
+                throw e;
+              }
+
+              if (NO_RETRY_METHODS.includes(methodName)) {
+                const browsingContextId = browsingContextFn()?.id;
+                lazy.logger.trace(
+                  `[${browsingContextId}] Querying "${methodName}" failed with` +
+                    ` ${e.name}, returning "null" as fallback`
+                );
+                return null;
+              }
+
+              if (++attempts > MAX_ATTEMPTS) {
+                const browsingContextId = browsingContextFn()?.id;
+                lazy.logger.trace(
+                  `[${browsingContextId}] Querying "${methodName} "` +
+                    `reached the limit of retry attempts (${MAX_ATTEMPTS})`
+                );
+                throw e;
+              }
+
+              lazy.logger.trace(
+                `Retrying "${methodName}", attempt: ${attempts}`
+              );
+            }
+          }
+        };
+      },
+    }
+  );
+}
+
+/**
+ * Register the MarionetteCommands actor that holds all the commands.
+ */
+export function registerCommandsActor() {
+  try {
+    ChromeUtils.registerWindowActor("MarionetteCommands", {
+      kind: "JSWindowActor",
+      parent: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteCommandsParent.sys.mjs",
+      },
+      child: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteCommandsChild.sys.mjs",
+      },
+
+      allFrames: true,
+      includeChrome: true,
+    });
+  } catch (e) {
+    if (e.name === "NotSupportedError") {
+      lazy.logger.warn(`MarionetteCommands actor is already registered!`);
+    } else {
+      throw e;
+    }
+  }
+}
+
+export function unregisterCommandsActor() {
+  ChromeUtils.unregisterWindowActor("MarionetteCommands");
+}
diff --git a/remote/marionette/actors/MarionetteEventsChild.sys.mjs b/remote/marionette/actors/MarionetteEventsChild.sys.mjs
new file mode 100644
index 0000000000..031215e5fc
--- /dev/null
+++ b/remote/marionette/actors/MarionetteEventsChild.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/. */
+
+/* eslint-disable no-restricted-globals */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  event: "chrome://remote/content/marionette/event.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+export class MarionetteEventsChild extends JSWindowActorChild {
+  get innerWindowId() {
+    return this.manager.innerWindowId;
+  }
+
+  actorCreated() {
+    // Prevent the logger from being created if the current log level
+    // isn't set to 'trace'. This is important for a faster content process
+    // creation when Marionette is running.
+    if (lazy.Log.isTraceLevelOrOrMore) {
+      lazy.logger.trace(
+        `[${this.browsingContext.id}] MarionetteEvents actor created ` +
+          `for window id ${this.innerWindowId}`
+      );
+    }
+  }
+
+  handleEvent({ target, type }) {
+    if (!Services.cpmm.sharedData.get("MARIONETTE_EVENTS_ENABLED")) {
+      // The parent process will set MARIONETTE_EVENTS_ENABLED to false when
+      // the Marionette session ends to avoid unnecessary inter process
+      // communications
+      return;
+    }
+
+    // Ignore invalid combinations of load events and document's readyState.
+    if (
+      (type === "DOMContentLoaded" && target.readyState != "interactive") ||
+      (type === "pageshow" && target.readyState != "complete")
+    ) {
+      lazy.logger.warn(
+        `Ignoring event '${type}' because document has an invalid ` +
+          `readyState of '${target.readyState}'.`
+      );
+      return;
+    }
+
+    switch (type) {
+      case "beforeunload":
+      case "DOMContentLoaded":
+      case "hashchange":
+      case "pagehide":
+      case "pageshow":
+      case "popstate":
+        this.sendAsyncMessage("MarionetteEventsChild:PageLoadEvent", {
+          browsingContext: this.browsingContext,
+          documentURI: target.documentURI,
+          readyState: target.readyState,
+          type,
+          windowId: this.innerWindowId,
+        });
+        break;
+
+      // Listen for click event to indicate one click has happened, so actions
+      // code can send dblclick event
+      case "click":
+        lazy.event.DoubleClickTracker.setClick();
+        break;
+      case "dblclick":
+      case "unload":
+        lazy.event.DoubleClickTracker.resetClick();
+        break;
+    }
+  }
+}
diff --git a/remote/marionette/actors/MarionetteEventsParent.sys.mjs b/remote/marionette/actors/MarionetteEventsParent.sys.mjs
new file mode 100644
index 0000000000..4211f99e59
--- /dev/null
+++ b/remote/marionette/actors/MarionetteEventsParent.sys.mjs
@@ -0,0 +1,115 @@
+/* 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, {
+  EventEmitter: "resource://gre/modules/EventEmitter.sys.mjs",
+
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+// Singleton to allow forwarding events to registered listeners.
+export const EventDispatcher = {
+  init() {
+    lazy.EventEmitter.decorate(this);
+  },
+};
+
+EventDispatcher.init();
+
+export class MarionetteEventsParent extends JSWindowActorParent {
+  async receiveMessage(msg) {
+    const { name, data } = msg;
+
+    let rv;
+    switch (name) {
+      case "MarionetteEventsChild:PageLoadEvent":
+        EventDispatcher.emit("page-load", data);
+        break;
+    }
+
+    return rv;
+  }
+}
+
+// Flag to check if the MarionetteEvents actors have already been registed.
+let eventsActorRegistered = false;
+
+/**
+ * Register Events actors to listen for page load events via EventDispatcher.
+ */
+function registerEventsActor() {
+  if (eventsActorRegistered) {
+    return;
+  }
+
+  try {
+    // Register the JSWindowActor pair for events as used by Marionette
+    ChromeUtils.registerWindowActor("MarionetteEvents", {
+      kind: "JSWindowActor",
+      parent: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteEventsParent.sys.mjs",
+      },
+      child: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteEventsChild.sys.mjs",
+        events: {
+          beforeunload: { capture: true },
+          DOMContentLoaded: { mozSystemGroup: true },
+          hashchange: { mozSystemGroup: true },
+          pagehide: { mozSystemGroup: true },
+          pageshow: { mozSystemGroup: true },
+          // popstate doesn't bubble, as such use capturing phase
+          popstate: { capture: true, mozSystemGroup: true },
+
+          click: {},
+          dblclick: {},
+          unload: { capture: true, createActor: false },
+        },
+      },
+
+      allFrames: true,
+      includeChrome: true,
+    });
+
+    eventsActorRegistered = true;
+  } catch (e) {
+    if (e.name === "NotSupportedError") {
+      lazy.logger.warn(`MarionetteEvents actor is already registered!`);
+    } else {
+      throw e;
+    }
+  }
+}
+
+/**
+ * Enable MarionetteEvents actors to start forwarding page load events from the
+ * child actor to the parent actor. Register the MarionetteEvents actor if necessary.
+ */
+export function enableEventsActor() {
+  // sharedData is replicated across processes and will be checked by
+  // MarionetteEventsChild before forward events to the parent actor.
+  Services.ppmm.sharedData.set("MARIONETTE_EVENTS_ENABLED", true);
+  // Request to immediately flush the data to the content processes to avoid races.
+  Services.ppmm.sharedData.flush();
+
+  registerEventsActor();
+}
+
+/**
+ * Disable MarionetteEvents actors to stop forwarding page load events from the
+ * child actor to the parent actor.
+ */
+export function disableEventsActor() {
+  Services.ppmm.sharedData.set("MARIONETTE_EVENTS_ENABLED", false);
+  Services.ppmm.sharedData.flush();
+}
diff --git a/remote/marionette/actors/MarionetteReftestChild.sys.mjs b/remote/marionette/actors/MarionetteReftestChild.sys.mjs
new file mode 100644
index 0000000000..262ddb2f22
--- /dev/null
+++ b/remote/marionette/actors/MarionetteReftestChild.sys.mjs
@@ -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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+/**
+ * Child JSWindowActor to handle navigation for reftests relying on marionette.
+ */
+export class MarionetteReftestChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    // This promise will resolve with the URL recorded in the "load" event
+    // handler. This URL will not be impacted by any hash modification that
+    // might be performed by the test script.
+    // The harness should be loaded before loading any test page, so the actors
+    // should be registered before the "load" event is received for a test page.
+    this._loadedURLPromise = new Promise(
+      r => (this._resolveLoadedURLPromise = r)
+    );
+  }
+
+  handleEvent(event) {
+    if (event.type == "load") {
+      const url = event.target.location.href;
+      lazy.logger.debug(`Handle load event with URL ${url}`);
+      this._resolveLoadedURLPromise(url);
+    }
+  }
+
+  actorCreated() {
+    lazy.logger.trace(
+      `[${this.browsingContext.id}] Reftest actor created ` +
+        `for window id ${this.manager.innerWindowId}`
+    );
+  }
+
+  async receiveMessage(msg) {
+    const { name, data } = msg;
+
+    let result;
+    switch (name) {
+      case "MarionetteReftestParent:flushRendering":
+        result = await this.flushRendering(data);
+        break;
+      case "MarionetteReftestParent:reftestWait":
+        result = await this.reftestWait(data);
+        break;
+    }
+    return result;
+  }
+
+  /**
+   * Wait for a reftest page to be ready for screenshots:
+   * - wait for the loadedURL to be available (see handleEvent)
+   * - check if the URL matches the expected URL
+   * - if present, wait for the "reftest-wait" classname to be removed from the
+   *   document element
+   *
+   * @param {object} options
+   * @param {string} options.url
+   *        The expected test page URL
+   * @param {boolean} options.useRemote
+   *        True when using e10s
+   * @returns {boolean}
+   *         Returns true when the correct page is loaded and ready for
+   *         screenshots. Returns false if the page loaded bug does not have the
+   *         expected URL.
+   */
+  async reftestWait(options = {}) {
+    const { url, useRemote } = options;
+    const loadedURL = await this._loadedURLPromise;
+    if (loadedURL !== url) {
+      lazy.logger.debug(
+        `Window URL does not match the expected URL "${loadedURL}" !== "${url}"`
+      );
+      return false;
+    }
+
+    const documentElement = this.document.documentElement;
+    const hasReftestWait = documentElement.classList.contains("reftest-wait");
+
+    lazy.logger.debug("Waiting for event loop to spin");
+    await new Promise(resolve => lazy.setTimeout(resolve, 0));
+
+    await this.paintComplete({ useRemote, ignoreThrottledAnimations: true });
+
+    if (hasReftestWait) {
+      const event = new this.document.defaultView.Event("TestRendered", {
+        bubbles: true,
+      });
+      documentElement.dispatchEvent(event);
+      lazy.logger.info("Emitted TestRendered event");
+      await this.reftestWaitRemoved();
+      await this.paintComplete({ useRemote, ignoreThrottledAnimations: false });
+    }
+    if (
+      this.document.defaultView.innerWidth < documentElement.scrollWidth ||
+      this.document.defaultView.innerHeight < documentElement.scrollHeight
+    ) {
+      lazy.logger.warn(
+        `${url} overflows viewport (width: ${documentElement.scrollWidth}, height: ${documentElement.scrollHeight})`
+      );
+    }
+    return true;
+  }
+
+  paintComplete({ useRemote, ignoreThrottledAnimations }) {
+    lazy.logger.debug("Waiting for rendering");
+    let windowUtils = this.document.defaultView.windowUtils;
+    return new Promise(resolve => {
+      let maybeResolve = () => {
+        this.flushRendering({ ignoreThrottledAnimations });
+        if (useRemote) {
+          // Flush display (paint)
+          lazy.logger.debug("Force update of layer tree");
+          windowUtils.updateLayerTree();
+        }
+
+        if (windowUtils.isMozAfterPaintPending) {
+          lazy.logger.debug("isMozAfterPaintPending: true");
+          this.document.defaultView.addEventListener(
+            "MozAfterPaint",
+            maybeResolve,
+            {
+              once: true,
+            }
+          );
+        } else {
+          // resolve at the start of the next frame in case of leftover paints
+          lazy.logger.debug("isMozAfterPaintPending: false");
+          this.document.defaultView.requestAnimationFrame(() => {
+            this.document.defaultView.requestAnimationFrame(resolve);
+          });
+        }
+      };
+      maybeResolve();
+    });
+  }
+
+  reftestWaitRemoved() {
+    lazy.logger.debug("Waiting for reftest-wait removal");
+    return new Promise(resolve => {
+      const documentElement = this.document.documentElement;
+      let observer = new this.document.defaultView.MutationObserver(() => {
+        if (!documentElement.classList.contains("reftest-wait")) {
+          observer.disconnect();
+          lazy.logger.debug("reftest-wait removed");
+          lazy.setTimeout(resolve, 0);
+        }
+      });
+      if (documentElement.classList.contains("reftest-wait")) {
+        observer.observe(documentElement, { attributes: true });
+      } else {
+        lazy.setTimeout(resolve, 0);
+      }
+    });
+  }
+
+  /**
+   * Ensure layout is flushed in each frame
+   *
+   * @param {object} options
+   * @param {boolean} options.ignoreThrottledAnimations Don't flush
+   *        the layout of throttled animations. We can end up in a
+   *        situation where flushing a throttled animation causes
+   *        mozAfterPaint events even when all rendering we care about
+   *        should have ceased. See
+   *        https://searchfox.org/mozilla-central/rev/d58860eb739af613774c942c3bb61754123e449b/layout/tools/reftest/reftest-content.js#723-729
+   *        for more detail.
+   */
+  flushRendering(options = {}) {
+    let { ignoreThrottledAnimations } = options;
+    lazy.logger.debug(
+      `flushRendering ignoreThrottledAnimations:${ignoreThrottledAnimations}`
+    );
+    let anyPendingPaintsGeneratedInDescendants = false;
+
+    let windowUtils = this.document.defaultView.windowUtils;
+
+    function flushWindow(win) {
+      let utils = win.windowUtils;
+      let afterPaintWasPending = utils.isMozAfterPaintPending;
+
+      let root = win.document.documentElement;
+      if (root) {
+        try {
+          if (ignoreThrottledAnimations) {
+            utils.flushLayoutWithoutThrottledAnimations();
+          } else {
+            root.getBoundingClientRect();
+          }
+        } catch (e) {
+          lazy.logger.error("flushWindow failed", e);
+        }
+      }
+
+      if (!afterPaintWasPending && utils.isMozAfterPaintPending) {
+        anyPendingPaintsGeneratedInDescendants = true;
+      }
+
+      for (let i = 0; i < win.frames.length; ++i) {
+        // Skip remote frames, flushRendering will be called on their individual
+        // MarionetteReftest actor via _recursiveFlushRendering performed from
+        // the topmost MarionetteReftest actor.
+        if (!Cu.isRemoteProxy(win.frames[i])) {
+          flushWindow(win.frames[i]);
+        }
+      }
+    }
+    flushWindow(this.document.defaultView);
+
+    if (
+      anyPendingPaintsGeneratedInDescendants &&
+      !windowUtils.isMozAfterPaintPending
+    ) {
+      lazy.logger.error(
+        "Descendant frame generated a MozAfterPaint event, " +
+          "but the root document doesn't have one!"
+      );
+    }
+  }
+}
diff --git a/remote/marionette/actors/MarionetteReftestParent.sys.mjs b/remote/marionette/actors/MarionetteReftestParent.sys.mjs
new file mode 100644
index 0000000000..86d40aa650
--- /dev/null
+++ b/remote/marionette/actors/MarionetteReftestParent.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/. */
+
+/**
+ * Parent JSWindowActor to handle navigation for reftests relying on marionette.
+ */
+export class MarionetteReftestParent extends JSWindowActorParent {
+  /**
+   * Wait for the expected URL to be loaded.
+   *
+   * @param {string} url
+   *        The expected url.
+   * @param {boolean} useRemote
+   *        True if tests are running with e10s.
+   * @returns {boolean} true if the page is fully loaded with the expected url,
+   *         false otherwise.
+   */
+  async reftestWait(url, useRemote) {
+    try {
+      const isCorrectUrl = await this.sendQuery(
+        "MarionetteReftestParent:reftestWait",
+        {
+          url,
+          useRemote,
+        }
+      );
+
+      if (isCorrectUrl) {
+        // Trigger flush rendering for all remote frames.
+        await this._flushRenderingInSubtree({
+          ignoreThrottledAnimations: false,
+        });
+      }
+
+      return isCorrectUrl;
+    } catch (e) {
+      if (e.name === "AbortError") {
+        // If the query is aborted, the window global is being destroyed, most
+        // likely because a navigation happened.
+        return false;
+      }
+
+      // Other errors should not be swallowed.
+      throw e;
+    }
+  }
+
+  /**
+   * Call flushRendering on all browsing contexts in the subtree.
+   * Each actor will flush rendering in all the same process frames.
+   */
+  async _flushRenderingInSubtree({ ignoreThrottledAnimations }) {
+    const browsingContext = this.manager.browsingContext;
+    const contexts = browsingContext.getAllBrowsingContextsInSubtree();
+
+    await Promise.all(
+      contexts.map(async context => {
+        if (context === browsingContext) {
+          // Skip the top browsing context, for which flushRendering is
+          // already performed via the initial reftestWait call.
+          return;
+        }
+
+        const windowGlobal = context.currentWindowGlobal;
+        if (!windowGlobal) {
+          // Bail out if there is no window attached to the current context.
+          return;
+        }
+
+        if (!windowGlobal.isProcessRoot) {
+          // Bail out if this window global is not a process root.
+          // MarionetteReftestChild::flushRendering will flush all same process
+          // frames, so we only need to call flushRendering on process roots.
+          return;
+        }
+
+        const reftestActor = windowGlobal.getActor("MarionetteReftest");
+        await reftestActor.sendQuery("MarionetteReftestParent:flushRendering", {
+          ignoreThrottledAnimations,
+        });
+      })
+    );
+  }
+}
diff --git a/remote/marionette/addon.sys.mjs b/remote/marionette/addon.sys.mjs
new file mode 100644
index 0000000000..d1234a0880
--- /dev/null
+++ b/remote/marionette/addon.sys.mjs
@@ -0,0 +1,133 @@
+/* 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, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+});
+
+// from https://developer.mozilla.org/en-US/Add-ons/Add-on_Manager/AddonManager#AddonInstall_errors
+const ERRORS = {
+  [-1]: "ERROR_NETWORK_FAILURE: A network error occured.",
+  [-2]: "ERROR_INCORECT_HASH: The downloaded file did not match the expected hash.",
+  [-3]: "ERROR_CORRUPT_FILE: The file appears to be corrupt.",
+  [-4]: "ERROR_FILE_ACCESS: There was an error accessing the filesystem.",
+  [-5]: "ERROR_SIGNEDSTATE_REQUIRED: The addon must be signed and isn't.",
+};
+
+async function installAddon(file) {
+  let install = await lazy.AddonManager.getInstallForFile(file, null, {
+    source: "internal",
+  });
+
+  if (install.error) {
+    throw new lazy.error.UnknownError(ERRORS[install.error]);
+  }
+
+  return install.install().catch(err => {
+    throw new lazy.error.UnknownError(ERRORS[install.error]);
+  });
+}
+
+/** Installs addons by path and uninstalls by ID. */
+export class Addon {
+  /**
+   * Install a Firefox addon.
+   *
+   * If the addon is restartless, it can be used right away.  Otherwise a
+   * restart is required.
+   *
+   * Temporary addons will automatically be uninstalled on shutdown and
+   * do not need to be signed, though they must be restartless.
+   *
+   * @param {string} path
+   *     Full path to the extension package archive.
+   * @param {boolean=} temporary
+   *     True to install the addon temporarily, false (default) otherwise.
+   *
+   * @returns {Promise.<string>}
+   *     Addon ID.
+   *
+   * @throws {UnknownError}
+   *     If there is a problem installing the addon.
+   */
+  static async install(path, temporary = false) {
+    let addon;
+    let file;
+
+    try {
+      file = new lazy.FileUtils.File(path);
+    } catch (e) {
+      throw new lazy.error.UnknownError(`Expected absolute path: ${e}`, e);
+    }
+
+    if (!file.exists()) {
+      throw new lazy.error.UnknownError(`No such file or directory: ${path}`);
+    }
+
+    try {
+      if (temporary) {
+        addon = await lazy.AddonManager.installTemporaryAddon(file);
+      } else {
+        addon = await installAddon(file);
+      }
+    } catch (e) {
+      throw new lazy.error.UnknownError(
+        `Could not install add-on: ${path}: ${e.message}`,
+        e
+      );
+    }
+
+    return addon.id;
+  }
+
+  /**
+   * Uninstall a Firefox addon.
+   *
+   * If the addon is restartless it will be uninstalled right away.
+   * Otherwise, Firefox must be restarted for the change to take effect.
+   *
+   * @param {string} id
+   *     ID of the addon to uninstall.
+   *
+   * @returns {Promise}
+   *
+   * @throws {UnknownError}
+   *     If there is a problem uninstalling the addon.
+   */
+  static async uninstall(id) {
+    let candidate = await lazy.AddonManager.getAddonByID(id);
+    if (candidate === null) {
+      // `AddonManager.getAddonByID` never rejects but instead
+      // returns `null` if the requested addon cannot be found.
+      throw new lazy.error.UnknownError(`Addon ${id} is not installed`);
+    }
+
+    return new Promise(resolve => {
+      let listener = {
+        onOperationCancelled: addon => {
+          if (addon.id === candidate.id) {
+            lazy.AddonManager.removeAddonListener(listener);
+            throw new lazy.error.UnknownError(
+              `Uninstall of ${candidate.id} has been canceled`
+            );
+          }
+        },
+
+        onUninstalled: addon => {
+          if (addon.id === candidate.id) {
+            lazy.AddonManager.removeAddonListener(listener);
+            resolve();
+          }
+        },
+      };
+
+      lazy.AddonManager.addAddonListener(listener);
+      candidate.uninstall();
+    });
+  }
+}
diff --git a/remote/marionette/atom.sys.mjs b/remote/marionette/atom.sys.mjs
new file mode 100644
index 0000000000..70aa53b91c
--- /dev/null
+++ b/remote/marionette/atom.sys.mjs
@@ -0,0 +1,305 @@
+// Copyright 2011-2017 Software Freedom Conservancy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/** @namespace */
+export const atom = {};
+
+// Follow the instructions to export all the atoms:
+// https://firefox-source-docs.mozilla.org/testing/marionette/SeleniumAtoms.html
+//
+// Built from SHA1: a6b161a159c3d581b130f03a2e6e35f577f38dec
+
+atom.getElementText = function(element, window){return (function(){var k=this||self;function aa(a){return"string"==typeof a}function ba(a,b){a=a.split(".");var c=k;a[0]in c||"undefined"==typeof c.execScript||c.execScript("var "+a[0]);for(var d;a.length&&(d=a.shift());)a.length||void 0===b?c[d]&&c[d]!==Object.prototype[d]?c=c[d]:c=c[d]={}:c[d]=b}
+function ca(a){var b=typeof a;if("object"==b)if(a){if(a instanceof Array)return"array";if(a instanceof Object)return b;var c=Object.prototype.toString.call(a);if("[object Window]"==c)return"object";if("[object Array]"==c||"number"==typeof a.length&&"undefined"!=typeof a.splice&&"undefined"!=typeof a.propertyIsEnumerable&&!a.propertyIsEnumerable("splice"))return"array";if("[object Function]"==c||"undefined"!=typeof a.call&&"undefined"!=typeof a.propertyIsEnumerable&&!a.propertyIsEnumerable("call"))return"function"}else return"null";
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+
+ The MIT License
+
+ Copyright (c) 2007 Cybozu Labs, Inc.
+ Copyright (c) 2012 Google Inc.
+
+ Permission is hereby granted, free of charge, to any person obtaining a copy
+ of this software and associated documentation files (the "Software"), to
+ deal in the Software without restriction, including without limitation the
+ rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ sell copies of the Software, and to permit persons to whom the Software is
+ furnished to do so, subject to the following conditions:
+
+ The above copyright notice and this permission notice shall be included in
+ all copies or substantial portions of the Software.
+
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ IN THE SOFTWARE.
+*/
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+
+ The MIT License
+
+ Copyright (c) 2007 Cybozu Labs, Inc.
+ Copyright (c) 2012 Google Inc.
+
+ Permission is hereby granted, free of charge, to any person obtaining a copy
+ of this software and associated documentation files (the "Software"), to
+ deal in the Software without restriction, including without limitation the
+ rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ sell copies of the Software, and to permit persons to whom the Software is
+ furnished to do so, subject to the following conditions:
+
+ The above copyright notice and this permission notice shall be included in
+ all copies or substantial portions of the Software.
+
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ IN THE SOFTWARE.
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+
+ The MIT License
+
+ Copyright (c) 2007 Cybozu Labs, Inc.
+ Copyright (c) 2012 Google Inc.
+
+ Permission is hereby granted, free of charge, to any person obtaining a copy
+ of this software and associated documentation files (the "Software"), to
+ deal in the Software without restriction, including without limitation the
+ rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ sell copies of the Software, and to permit persons to whom the Software is
+ furnished to do so, subject to the following conditions:
+
+ The above copyright notice and this permission notice shall be included in
+ all copies or substantial portions of the Software.
+
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ IN THE SOFTWARE.
+*/
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+function Y(a,b){b=xa(b);if("float"==b||"cssFloat"==b||"styleFloat"==b)b=Ac?"styleFloat":"cssFloat";a:{var c=b;var d=A(a);if(d.defaultView&&d.defaultView.getComputedStyle&&(d=d.defaultView.getComputedStyle(a,null))){c=d[c]||d.getPropertyValue(c)||"";break a}c=""}a=c||Hc(a,b);if(null===a)a=null;else if(0<=ja(mc,b)){b:{var e=a.match(pc);if(e&&(b=Number(e[1]),c=Number(e[2]),d=Number(e[3]),e=Number(e[4]),0<=b&&255>=b&&0<=c&&255>=c&&0<=d&&255>=d&&0<=e&&1>=e)){b=[b,c,d,e];break b}b=null}if(!b)b:{if(d=a.match(qc))if(b=
+Number(d[1]),c=Number(d[2]),d=Number(d[3]),0<=b&&255>=b&&0<=c&&255>=c&&0<=d&&255>=d){b=[b,c,d,1];break b}b=null}if(!b)b:{b=a.toLowerCase();c=lc[b.toLowerCase()];if(!c&&(c="#"==b.charAt(0)?b:"#"+b,4==c.length&&(c=c.replace(nc,"#$1$1$2$2$3$3")),!oc.test(c))){b=null;break b}b=[parseInt(c.substr(1,2),16),parseInt(c.substr(3,2),16),parseInt(c.substr(5,2),16),1]}a=b?"rgba("+b.join(", ")+")":a}return a}
+function Hc(a,b){var c=a.currentStyle||a.style,d=c[b];void 0===d&&"function"==ca(c.getPropertyValue)&&(d=c.getPropertyValue(b));return"inherit"!=d?void 0!==d?d:null:(a=Gc(a))?Hc(a,b):null}
+function Ic(a,b,c){function d(g){var h=Jc(g);return 0<h.height&&0<h.width?!0:W(g,"PATH")&&(0<h.height||0<h.width)?(g=Y(g,"stroke-width"),!!g&&0<parseInt(g,10)):"hidden"!=Y(g,"overflow")&&ma(g.childNodes,function(m){return 3==m.nodeType||W(m)&&d(m)})}function e(g){return Kc(g)==Z&&na(g.childNodes,function(h){return!W(h)||e(h)||!d(h)})}if(!W(a))throw Error("Argument to isShown must be of type Element");if(W(a,"BODY"))return!0;if(W(a,"OPTION")||W(a,"OPTGROUP"))return a=$a(a,function(g){return W(g,"SELECT")}),
+!!a&&Ic(a,!0,c);var f=Lc(a);if(f)return!!f.image&&0<f.rect.width&&0<f.rect.height&&Ic(f.image,b,c);if(W(a,"INPUT")&&"hidden"==a.type.toLowerCase()||W(a,"NOSCRIPT"))return!1;f=Y(a,"visibility");return"collapse"!=f&&"hidden"!=f&&c(a)&&(b||0!=Mc(a))&&d(a)?!e(a):!1}var Z="hidden";
+function Kc(a){function b(p){function q(fb){if(fb==g)return!0;var Wb=Y(fb,"display");return 0==Wb.lastIndexOf("inline",0)||"contents"==Wb||"absolute"==Xb&&"static"==Y(fb,"position")?!1:!0}var Xb=Y(p,"position");if("fixed"==Xb)return w=!0,p==g?null:g;for(p=Gc(p);p&&!q(p);)p=Gc(p);return p}function c(p){var q=p;if("visible"==m)if(p==g&&h)q=h;else if(p==h)return{x:"visible",y:"visible"};q={x:Y(q,"overflow-x"),y:Y(q,"overflow-y")};p==g&&(q.x="visible"==q.x?"auto":q.x,q.y="visible"==q.y?"auto":q.y);return q}
+function d(p){if(p==g){var q=(new ab(f)).a;p=q.scrollingElement?q.scrollingElement:Ea||"CSS1Compat"!=q.compatMode?q.body||q.documentElement:q.documentElement;q=q.parentWindow||q.defaultView;p=v&&Ka("10")&&q.pageYOffset!=p.scrollTop?new Ua(p.scrollLeft,p.scrollTop):new Ua(q.pageXOffset||p.scrollLeft,q.pageYOffset||p.scrollTop)}else p=new Ua(p.scrollLeft,p.scrollTop);return p}var e=Nc(a),f=A(a),g=f.documentElement,h=f.body,m=Y(g,"overflow"),w;for(a=b(a);a;a=b(a)){var r=c(a);if("visible"!=r.x||"visible"!=
+r.y){var D=Jc(a);if(0==D.width||0==D.height)return Z;var L=e.a<D.a,M=e.b<D.b;if(L&&"hidden"==r.x||M&&"hidden"==r.y)return Z;if(L&&"visible"!=r.x||M&&"visible"!=r.y){L=d(a);M=e.b<D.b-L.y;if(e.a<D.a-L.x&&"visible"!=r.x||M&&"visible"!=r.x)return Z;e=Kc(a);return e==Z?Z:"scroll"}L=e.f>=D.a+D.width;D=e.c>=D.b+D.height;if(L&&"hidden"==r.x||D&&"hidden"==r.y)return Z;if(L&&"visible"!=r.x||D&&"visible"!=r.y){if(w&&(r=d(a),e.f>=g.scrollWidth-r.x||e.a>=g.scrollHeight-r.y))return Z;e=Kc(a);return e==Z?Z:"scroll"}}}return"none"}
+function Jc(a){var b=Lc(a);if(b)return b.rect;if(W(a,"HTML"))return a=A(a),a=((a?a.parentWindow||a.defaultView:window)||window).document,a="CSS1Compat"==a.compatMode?a.documentElement:a.body,a=new Va(a.clientWidth,a.clientHeight),new X(0,0,a.width,a.height);try{var c=a.getBoundingClientRect()}catch(d){return new X(0,0,0,0)}b=new X(c.left,c.top,c.right-c.left,c.bottom-c.top);v&&a.ownerDocument.body&&(a=A(a),b.a-=a.documentElement.clientLeft+a.body.clientLeft,b.b-=a.documentElement.clientTop+a.body.clientTop);
+return b}function Lc(a){var b=W(a,"MAP");if(!b&&!W(a,"AREA"))return null;var c=b?a:W(a.parentNode,"MAP")?a.parentNode:null,d=null,e=null;c&&c.name&&(d=Dc('/descendant::*[@usemap = "#'+c.name+'"]',A(c)))&&(e=Jc(d),b||"default"==a.shape.toLowerCase()||(a=Oc(a),b=Math.min(Math.max(a.a,0),e.width),c=Math.min(Math.max(a.b,0),e.height),e=new X(b+e.a,c+e.b,Math.min(a.width,e.width-b),Math.min(a.height,e.height-c))));return{image:d,rect:e||new X(0,0,0,0)}}
+function Oc(a){var b=a.shape.toLowerCase();a=a.coords.split(",");if("rect"==b&&4==a.length){b=a[0];var c=a[1];return new X(b,c,a[2]-b,a[3]-c)}if("circle"==b&&3==a.length)return b=a[2],new X(a[0]-b,a[1]-b,2*b,2*b);if("poly"==b&&2<a.length){b=a[0];c=a[1];for(var d=b,e=c,f=2;f+1<a.length;f+=2)b=Math.min(b,a[f]),d=Math.max(d,a[f]),c=Math.min(c,a[f+1]),e=Math.max(e,a[f+1]);return new X(b,c,d-b,e-c)}return new X(0,0,0,0)}function Nc(a){a=Jc(a);return new Ec(a.b,a.a+a.width,a.b+a.height,a.a)}
+function Mc(a){if(Ac){if("relative"==Y(a,"position"))return 1;a=Y(a,"filter");return(a=a.match(/^alpha\(opacity=(\d*)\)/)||a.match(/^progid:DXImageTransform.Microsoft.Alpha\(Opacity=(\d*)\)/))?Number(a[1])/100:1}return Pc(a)}function Pc(a){var b=1,c=Y(a,"opacity");c&&(b=Number(c));(a=Gc(a))&&(b*=Pc(a));return b};ba("_",function(a,b){function c(d){if(W(d)&&"none"==Y(d,"display"))return!1;var e;if((e=d.parentNode)&&e.shadowRoot&&void 0!==d.assignedSlot)e=d.assignedSlot?d.assignedSlot.parentNode:null;else if(d.getDestinationInsertionPoints){var f=d.getDestinationInsertionPoints();0<f.length&&(e=f[f.length-1])}if(Fc&&e instanceof ShadowRoot){if(e.host.shadowRoot!==e)return!1;e=e.host}return!e||9!=e.nodeType&&11!=e.nodeType?e&&W(e,"DETAILS")&&!e.open&&!W(d,"SUMMARY")?!1:!!e&&c(e):!0}return Ic(a,!!b,c)});; return this._.apply(null,arguments);}).apply({navigator:typeof window!='undefined'?window.navigator:null,document:typeof window!='undefined'?window.document:null}, arguments);}
diff --git a/remote/marionette/browser.sys.mjs b/remote/marionette/browser.sys.mjs
new file mode 100644
index 0000000000..8823caff82
--- /dev/null
+++ b/remote/marionette/browser.sys.mjs
@@ -0,0 +1,385 @@
+/* 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, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  MessageManagerDestroyedPromise:
+    "chrome://remote/content/marionette/sync.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
+/** @namespace */
+export const browser = {};
+
+/**
+ * Variations of Marionette contexts.
+ *
+ * Choosing a context through the <tt>Marionette:SetContext</tt>
+ * command directs all subsequent browsing context scoped commands
+ * to that context.
+ *
+ * @class Marionette.Context
+ */
+export class Context {
+  /**
+   * Gets the correct context from a string.
+   *
+   * @param {string} s
+   *     Context string serialisation.
+   *
+   * @returns {Context}
+   *     Context.
+   *
+   * @throws {TypeError}
+   *     If <var>s</var> is not a context.
+   */
+  static fromString(s) {
+    switch (s) {
+      case "chrome":
+        return Context.Chrome;
+
+      case "content":
+        return Context.Content;
+
+      default:
+        throw new TypeError(`Unknown context: ${s}`);
+    }
+  }
+}
+
+Context.Chrome = "chrome";
+Context.Content = "content";
+
+/**
+ * Creates a browsing context wrapper.
+ *
+ * Browsing contexts handle interactions with the browser, according to
+ * the current environment.
+ */
+browser.Context = class {
+  /**
+   * @param {ChromeWindow} window
+   *     ChromeWindow that contains the top-level browsing context.
+   * @param {GeckoDriver} driver
+   *     Reference to driver instance.
+   */
+  constructor(window, driver) {
+    this.window = window;
+    this.driver = driver;
+
+    // In Firefox this is <xul:tabbrowser> (not <xul:browser>!)
+    // and MobileTabBrowser in GeckoView.
+    this.tabBrowser = lazy.TabManager.getTabBrowser(this.window);
+
+    // Used to set curFrameId upon new session
+    this.newSession = true;
+
+    // A reference to the tab corresponding to the current window handle,
+    // if any.  Specifically, this.tab refers to the last tab that Marionette
+    // switched to in this browser window. Note that this may not equal the
+    // currently selected tab.  For example, if Marionette switches to tab
+    // A, and then clicks on a button that opens a new tab B in the same
+    // browser window, this.tab will still point to tab A, despite tab B
+    // being the currently selected tab.
+    this.tab = null;
+  }
+
+  /**
+   * Returns the content browser for the currently selected tab.
+   * If there is no tab selected, null will be returned.
+   */
+  get contentBrowser() {
+    if (this.tab) {
+      return lazy.TabManager.getBrowserForTab(this.tab);
+    } else if (
+      this.tabBrowser &&
+      this.driver.isReftestBrowser(this.tabBrowser)
+    ) {
+      return this.tabBrowser;
+    }
+
+    return null;
+  }
+
+  get messageManager() {
+    if (this.contentBrowser) {
+      return this.contentBrowser.messageManager;
+    }
+
+    return null;
+  }
+
+  /**
+   * Checks if the browsing context has been discarded.
+   *
+   * The browsing context will have been discarded if the content
+   * browser, represented by the <code>&lt;xul:browser&gt;</code>,
+   * has been detached.
+   *
+   * @returns {boolean}
+   *     True if browsing context has been discarded, false otherwise.
+   */
+  get closed() {
+    return this.contentBrowser === null;
+  }
+
+  /**
+   * Gets the position and dimensions of the top-level browsing context.
+   *
+   * @returns {Map.<string, number>}
+   *     Object with |x|, |y|, |width|, and |height| properties.
+   */
+  get rect() {
+    return {
+      x: this.window.screenX,
+      y: this.window.screenY,
+      width: this.window.outerWidth,
+      height: this.window.outerHeight,
+    };
+  }
+
+  /**
+   * Retrieves the current tabmodal UI object.  According to the browser
+   * associated with the currently selected tab.
+   */
+  getTabModal() {
+    let br = this.contentBrowser;
+    if (!br.hasAttribute("tabmodalPromptShowing")) {
+      return null;
+    }
+
+    // The modal is a direct sibling of the browser element.
+    // See tabbrowser.xml's getTabModalPromptBox.
+    let modalElements = br.parentNode.getElementsByTagName("tabmodalprompt");
+
+    return br.tabModalPromptBox.getPrompt(modalElements[0]);
+  }
+
+  /**
+   * Close the current window.
+   *
+   * @returns {Promise}
+   *     A promise which is resolved when the current window has been closed.
+   */
+  async closeWindow() {
+    return lazy.windowManager.closeWindow(this.window);
+  }
+
+  /**
+   * Focus the current window.
+   *
+   * @returns {Promise}
+   *     A promise which is resolved when the current window has been focused.
+   */
+  async focusWindow() {
+    await lazy.windowManager.focusWindow(this.window);
+
+    // Also focus the currently selected tab if present.
+    this.contentBrowser?.focus();
+  }
+
+  /**
+   * Open a new browser window.
+   *
+   * @returns {Promise}
+   *     A promise resolving to the newly created chrome window.
+   */
+  openBrowserWindow(focus = false, isPrivate = false) {
+    return lazy.windowManager.openBrowserWindow({
+      openerWindow: this.window,
+      focus,
+      isPrivate,
+    });
+  }
+
+  /**
+   * Close the current tab.
+   *
+   * @returns {Promise}
+   *     A promise which is resolved when the current tab has been closed.
+   *
+   * @throws UnsupportedOperationError
+   *     If tab handling for the current application isn't supported.
+   */
+  async closeTab() {
+    // If the current window is not a browser then close it directly. Do the
+    // same if only one remaining tab is open, or no tab selected at all.
+    //
+    // Note: For GeckoView there will always be a single tab only. But for
+    // consistency with other platforms a specific condition has been added
+    // below as well even it's not really used.
+    if (
+      !this.tabBrowser ||
+      !this.tabBrowser.tabs ||
+      this.tabBrowser.tabs.length === 1 ||
+      !this.tab
+    ) {
+      return this.closeWindow();
+    }
+
+    let destroyed = new lazy.MessageManagerDestroyedPromise(
+      this.messageManager
+    );
+    let tabClosed;
+
+    if (lazy.AppInfo.isAndroid) {
+      await lazy.TabManager.removeTab(this.tab);
+    } else if (lazy.AppInfo.isFirefox) {
+      tabClosed = new lazy.EventPromise(this.tab, "TabClose");
+      await this.tabBrowser.removeTab(this.tab);
+    } else {
+      throw new lazy.error.UnsupportedOperationError(
+        `closeTab() not supported for ${lazy.AppInfo.name}`
+      );
+    }
+
+    return Promise.all([destroyed, tabClosed]);
+  }
+
+  /**
+   * Open a new tab in the currently selected chrome window.
+   */
+  async openTab(focus = false) {
+    let tab = null;
+
+    // Bug 1795841 - For Firefox the TabManager cannot be used yet. As such
+    // handle opening a tab differently for Android.
+    if (lazy.AppInfo.isAndroid) {
+      tab = await lazy.TabManager.addTab({ focus, window: this.window });
+    } else if (lazy.AppInfo.isFirefox) {
+      const opened = new lazy.EventPromise(this.window, "TabOpen");
+      this.window.BrowserOpenTab({ url: "about:blank" });
+      await opened;
+
+      tab = this.tabBrowser.selectedTab;
+
+      // The new tab is always selected by default. If focus is not wanted,
+      // the previously tab needs to be selected again.
+      if (!focus) {
+        await lazy.TabManager.selectTab(this.tab);
+      }
+    } else {
+      throw new lazy.error.UnsupportedOperationError(
+        `openTab() not supported for ${lazy.AppInfo.name}`
+      );
+    }
+
+    return tab;
+  }
+
+  /**
+   * Set the current tab.
+   *
+   * @param {number=} index
+   *     Tab index to switch to. If the parameter is undefined,
+   *     the currently selected tab will be used.
+   * @param {ChromeWindow=} window
+   *     Switch to this window before selecting the tab.
+   * @param {boolean=} focus
+   *      A boolean value which determins whether to focus
+   *      the window. Defaults to true.
+   *
+   * @returns {Tab}
+   *     The selected tab.
+   *
+   * @throws UnsupportedOperationError
+   *     If tab handling for the current application isn't supported.
+   */
+  async switchToTab(index, window = undefined, focus = true) {
+    if (window) {
+      this.window = window;
+      this.tabBrowser = lazy.TabManager.getTabBrowser(this.window);
+    }
+
+    if (!this.tabBrowser || this.driver.isReftestBrowser(this.tabBrowser)) {
+      return null;
+    }
+
+    if (typeof index == "undefined") {
+      this.tab = this.tabBrowser.selectedTab;
+    } else {
+      this.tab = this.tabBrowser.tabs[index];
+    }
+
+    if (focus) {
+      await lazy.TabManager.selectTab(this.tab);
+    }
+
+    // By accessing the content browser's message manager a new browsing
+    // context is created for browserless tabs, which is needed to successfully
+    // run the WebDriver's is browsing context open step. This is temporary
+    // until we find a better solution on bug 1812258.
+    this.messageManager;
+
+    return this.tab;
+  }
+
+  /**
+   * Registers a new frame, and sets its current frame id to this frame
+   * if it is not already assigned, and if a) we already have a session
+   * or b) we're starting a new session and it is the right start frame.
+   *
+   * @param {XULBrowser} target
+   *     The <xul:browser> that was the target of the originating message.
+   */
+  register(target) {
+    if (!this.tabBrowser) {
+      return;
+    }
+
+    // If we're setting up a new session on Firefox, we only process the
+    // registration for this frame if it belongs to the current tab.
+    if (!this.tab) {
+      this.switchToTab();
+    }
+  }
+};
+
+/**
+ * Marionette representation of the {@link ChromeWindow} window state.
+ *
+ * @enum {string}
+ */
+export const WindowState = {
+  Maximized: "maximized",
+  Minimized: "minimized",
+  Normal: "normal",
+  Fullscreen: "fullscreen",
+
+  /**
+   * Converts {@link nsIDOMChromeWindow.windowState} to WindowState.
+   *
+   * @param {number} windowState
+   *     Attribute from {@link nsIDOMChromeWindow.windowState}.
+   *
+   * @returns {WindowState}
+   *     JSON representation.
+   *
+   * @throws {TypeError}
+   *     If <var>windowState</var> was unknown.
+   */
+  from(windowState) {
+    switch (windowState) {
+      case 1:
+        return WindowState.Maximized;
+
+      case 2:
+        return WindowState.Minimized;
+
+      case 3:
+        return WindowState.Normal;
+
+      case 4:
+        return WindowState.Fullscreen;
+
+      default:
+        throw new TypeError(`Unknown window state: ${windowState}`);
+    }
+  },
+};
diff --git a/remote/marionette/cert.sys.mjs b/remote/marionette/cert.sys.mjs
new file mode 100644
index 0000000000..85f18c5975
--- /dev/null
+++ b/remote/marionette/cert.sys.mjs
@@ -0,0 +1,61 @@
+/* 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, {
+  Preferences: "resource://gre/modules/Preferences.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "sss",
+  "@mozilla.org/ssservice;1",
+  "nsISiteSecurityService"
+);
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "certOverrideService",
+  "@mozilla.org/security/certoverride;1",
+  "nsICertOverrideService"
+);
+
+const CERT_PINNING_ENFORCEMENT_PREF = "security.cert_pinning.enforcement_level";
+const HSTS_PRELOAD_LIST_PREF = "network.stricttransportsecurity.preloadlist";
+
+/** @namespace */
+export const allowAllCerts = {};
+
+/**
+ * Disable all security check and allow all certs.
+ */
+allowAllCerts.enable = function () {
+  // make it possible to register certificate overrides for domains
+  // that use HSTS or HPKP
+  lazy.Preferences.set(HSTS_PRELOAD_LIST_PREF, false);
+  lazy.Preferences.set(CERT_PINNING_ENFORCEMENT_PREF, 0);
+
+  lazy.certOverrideService.setDisableAllSecurityChecksAndLetAttackersInterceptMyData(
+    true
+  );
+};
+
+/**
+ * Enable all security check.
+ */
+allowAllCerts.disable = function () {
+  lazy.certOverrideService.setDisableAllSecurityChecksAndLetAttackersInterceptMyData(
+    false
+  );
+
+  lazy.Preferences.reset(HSTS_PRELOAD_LIST_PREF);
+  lazy.Preferences.reset(CERT_PINNING_ENFORCEMENT_PREF);
+
+  // clear collected HSTS and HPKP state
+  // through the site security service
+  lazy.sss.clearAll();
+};
diff --git a/remote/marionette/chrome/reftest.xhtml b/remote/marionette/chrome/reftest.xhtml
new file mode 100644
index 0000000000..ec4145832d
--- /dev/null
+++ b/remote/marionette/chrome/reftest.xhtml
@@ -0,0 +1,8 @@
+<window
+  id="reftest"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  hidechrome="true"
+  style="background-color: white; overflow: hidden"
+>
+  <script src="reftest-content.js"></script>
+</window>
diff --git a/remote/marionette/chrome/test.xhtml b/remote/marionette/chrome/test.xhtml
new file mode 100644
index 0000000000..c6220a6300
--- /dev/null
+++ b/remote/marionette/chrome/test.xhtml
@@ -0,0 +1,61 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<!DOCTYPE window [ ]>
+<window
+  id="winTest"
+  title="Title Test"
+  windowtype="Test Type"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+>
+  <dialog
+    id="dia"
+    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  >
+    <vbox id="things">
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput2"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput3"
+        class="asdf"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <checkbox id="testBox" label="box" />
+    </vbox>
+
+    <iframe
+      id="iframe"
+      name="iframename"
+      src="chrome://remote/content/marionette/test2.xhtml"
+    />
+    <iframe
+      id="iframe"
+      name="iframename"
+      src="chrome://remote/content/marionette/test_nested_iframe.xhtml"
+    />
+    <hbox id="testXulBox" />
+    <browser
+      id="aBrowser"
+      src="chrome://remote/content/marionette/test2.xhtml"
+    />
+  </dialog>
+</window>
diff --git a/remote/marionette/chrome/test2.xhtml b/remote/marionette/chrome/test2.xhtml
new file mode 100644
index 0000000000..17d528c800
--- /dev/null
+++ b/remote/marionette/chrome/test2.xhtml
@@ -0,0 +1,36 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<!DOCTYPE window [ ]>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <dialog id="dia">
+    <vbox id="things">
+      <checkbox id="testBox" label="box" />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput2"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput3"
+        class="asdf"
+        size="6"
+        value="test"
+        label="input"
+      />
+    </vbox>
+  </dialog>
+</window>
diff --git a/remote/marionette/chrome/test_dialog.dtd b/remote/marionette/chrome/test_dialog.dtd
new file mode 100644
index 0000000000..414cb0ee81
--- /dev/null
+++ b/remote/marionette/chrome/test_dialog.dtd
@@ -0,0 +1,7 @@
+<!-- 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/. -->
+
+<!ENTITY testDialog.title "Test Dialog">
+
+<!ENTITY settings.label "Settings">
diff --git a/remote/marionette/chrome/test_dialog.properties b/remote/marionette/chrome/test_dialog.properties
new file mode 100644
index 0000000000..ade7b6bde3
--- /dev/null
+++ b/remote/marionette/chrome/test_dialog.properties
@@ -0,0 +1,7 @@
+# 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/.
+
+testDialog.title=Test Dialog
+
+settings.label=Settings
diff --git a/remote/marionette/chrome/test_dialog.xhtml b/remote/marionette/chrome/test_dialog.xhtml
new file mode 100644
index 0000000000..0be20b514d
--- /dev/null
+++ b/remote/marionette/chrome/test_dialog.xhtml
@@ -0,0 +1,37 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<!DOCTYPE testdialog [ <!ENTITY % dialogDTD SYSTEM "chrome://remote/content/marionette/test_dialog.dtd">
+%dialogDTD; ]>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  title="&testDialog.title;"
+>
+  <dialog id="testDialog" buttons="accept,cancel">
+    <vbox flex="1" style="min-width: 300px; min-height: 500px">
+      <label>&settings.label;</label>
+      <separator class="thin" />
+      <richlistbox id="test-list" flex="1">
+        <richlistitem id="item-choose" orient="horizontal" selected="true">
+          <label id="choose-label" value="First Entry" flex="1" />
+          <button id="choose-button" oncommand="" label="Choose..." />
+        </richlistitem>
+      </richlistbox>
+      <separator class="thin" />
+      <checkbox id="check-box" label="Test Mode 2" />
+      <hbox align="center">
+        <label id="text-box-label" control="text-box">Name:</label>
+        <input
+          xmlns="http://www.w3.org/1999/xhtml"
+          id="text-box"
+          style="-moz-box-flex: 1"
+        />
+      </hbox>
+    </vbox>
+  </dialog>
+</window>
diff --git a/remote/marionette/chrome/test_menupopup.xhtml b/remote/marionette/chrome/test_menupopup.xhtml
new file mode 100644
index 0000000000..23ddde041f
--- /dev/null
+++ b/remote/marionette/chrome/test_menupopup.xhtml
@@ -0,0 +1,33 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<!DOCTYPE window [ ]>
+<window
+  id="test-window"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+>
+  <popupset id="options-popupset">
+    <menupopup id="options-menupopup" position="before_end">
+      <menuitem id="option-enabled" type="checkbox" label="enabled" />
+      <menuitem
+        id="option-hidden"
+        type="checkbox"
+        label="hidden"
+        hidden="true"
+      />
+      <menuitem
+        id="option-disabled"
+        type="checkbox"
+        label="disabled"
+        disabled="true"
+      />
+    </menupopup>
+  </popupset>
+  <hbox align="center" style="height: 300px">
+    <button id="options-button" popup="options-menupopup" label="button" />
+  </hbox>
+</window>
diff --git a/remote/marionette/chrome/test_nested_iframe.xhtml b/remote/marionette/chrome/test_nested_iframe.xhtml
new file mode 100644
index 0000000000..5c45fa54c9
--- /dev/null
+++ b/remote/marionette/chrome/test_nested_iframe.xhtml
@@ -0,0 +1,8 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<!DOCTYPE window [ ]>
+
+<iframe id="iframe" name="iframename" src="test2.xhtml" />
diff --git a/remote/marionette/chrome/test_no_xul.xhtml b/remote/marionette/chrome/test_no_xul.xhtml
new file mode 100644
index 0000000000..e7cd10f602
--- /dev/null
+++ b/remote/marionette/chrome/test_no_xul.xhtml
@@ -0,0 +1,58 @@
+<?xml version="1.0"?>
+<!-- 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 file for a non XUL window by using a XHTML document instead.  -->
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<html
+  id="winTest"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns="http://www.w3.org/1999/xhtml"
+>
+  <head>
+    <title>Title Test</title>
+  </head>
+
+  <body xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+    <vbox id="things">
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput2"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input
+        xmlns="http://www.w3.org/1999/xhtml"
+        id="textInput3"
+        class="asdf"
+        size="6"
+        value="test"
+        label="input"
+      />
+      <input type="checkbox" id="testBox" label="box" />
+    </vbox>
+
+    <html:iframe
+      id="iframe"
+      name="iframename"
+      src="chrome://remote/content/marionette/test2.xhtml"
+    />
+    <html:iframe
+      id="iframe"
+      name="iframename"
+      src="chrome://remote/content/marionette/test_nested_iframe.xhtml"
+    />
+  </body>
+</html>
diff --git a/remote/marionette/cookie.sys.mjs b/remote/marionette/cookie.sys.mjs
new file mode 100644
index 0000000000..117ccc33ed
--- /dev/null
+++ b/remote/marionette/cookie.sys.mjs
@@ -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 lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
+const IPV4_PORT_EXPR = /:\d+$/;
+
+const SAMESITE_MAP = new Map([
+  ["None", Ci.nsICookie.SAMESITE_NONE],
+  ["Lax", Ci.nsICookie.SAMESITE_LAX],
+  ["Strict", Ci.nsICookie.SAMESITE_STRICT],
+]);
+
+/** @namespace */
+export const cookie = {
+  manager: Services.cookies,
+};
+
+/**
+ * @name Cookie
+ *
+ * @returns {Object<string, (number|boolean|string)>}
+ */
+
+/**
+ * Unmarshal a JSON Object to a cookie representation.
+ *
+ * Effectively this will run validation checks on ``json``, which
+ * will produce the errors expected by WebDriver if the input is
+ * not valid.
+ *
+ * @param {Object<string, (number | boolean | string)>} json
+ *     Cookie to be deserialised. ``name`` and ``value`` are required
+ *     fields which must be strings.  The ``path`` and ``domain`` fields
+ *     are optional, but must be a string if provided.  The ``secure``,
+ *     and ``httpOnly`` are similarly optional, but must be booleans.
+ *     Likewise, the ``expiry`` field is optional but must be
+ *     unsigned integer.
+ *
+ * @returns {Cookie}
+ *     Valid cookie object.
+ *
+ * @throws {InvalidArgumentError}
+ *     If any of the properties are invalid.
+ */
+cookie.fromJSON = function (json) {
+  let newCookie = {};
+
+  lazy.assert.object(json, lazy.pprint`Expected cookie object, got ${json}`);
+
+  newCookie.name = lazy.assert.string(json.name, "Cookie name must be string");
+  newCookie.value = lazy.assert.string(
+    json.value,
+    "Cookie value must be string"
+  );
+
+  if (typeof json.path != "undefined") {
+    newCookie.path = lazy.assert.string(
+      json.path,
+      "Cookie path must be string"
+    );
+  }
+  if (typeof json.domain != "undefined") {
+    newCookie.domain = lazy.assert.string(
+      json.domain,
+      "Cookie domain must be string"
+    );
+  }
+  if (typeof json.secure != "undefined") {
+    newCookie.secure = lazy.assert.boolean(
+      json.secure,
+      "Cookie secure flag must be boolean"
+    );
+  }
+  if (typeof json.httpOnly != "undefined") {
+    newCookie.httpOnly = lazy.assert.boolean(
+      json.httpOnly,
+      "Cookie httpOnly flag must be boolean"
+    );
+  }
+  if (typeof json.expiry != "undefined") {
+    newCookie.expiry = lazy.assert.positiveInteger(
+      json.expiry,
+      "Cookie expiry must be a positive integer"
+    );
+  }
+  if (typeof json.sameSite != "undefined") {
+    newCookie.sameSite = lazy.assert.in(
+      json.sameSite,
+      Array.from(SAMESITE_MAP.keys()),
+      "Cookie SameSite flag must be one of None, Lax, or Strict"
+    );
+  }
+
+  return newCookie;
+};
+
+/**
+ * Insert cookie to the cookie store.
+ *
+ * @param {Cookie} newCookie
+ *     Cookie to add.
+ * @param {object} options
+ * @param {string=} options.restrictToHost
+ *     Perform test that ``newCookie``'s domain matches this.
+ * @param {string=} options.protocol
+ *     The protocol of the caller. It can be `http:` or `https:`.
+ *
+ * @throws {TypeError}
+ *     If ``name``, ``value``, or ``domain`` are not present and
+ *     of the correct type.
+ * @throws {InvalidCookieDomainError}
+ *     If ``restrictToHost`` is set and ``newCookie``'s domain does
+ *     not match.
+ * @throws {UnableToSetCookieError}
+ *     If an error occurred while trying to save the cookie.
+ */
+cookie.add = function (
+  newCookie,
+  { restrictToHost = null, protocol = null } = {}
+) {
+  lazy.assert.string(newCookie.name, "Cookie name must be string");
+  lazy.assert.string(newCookie.value, "Cookie value must be string");
+
+  if (typeof newCookie.path == "undefined") {
+    newCookie.path = "/";
+  }
+
+  let hostOnly = false;
+  if (typeof newCookie.domain == "undefined") {
+    hostOnly = true;
+    newCookie.domain = restrictToHost;
+  }
+  lazy.assert.string(newCookie.domain, "Cookie domain must be string");
+  if (newCookie.domain.substring(0, 1) === ".") {
+    newCookie.domain = newCookie.domain.substring(1);
+  }
+
+  if (typeof newCookie.secure == "undefined") {
+    newCookie.secure = false;
+  }
+  if (typeof newCookie.httpOnly == "undefined") {
+    newCookie.httpOnly = false;
+  }
+  if (typeof newCookie.expiry == "undefined") {
+    // The XPCOM interface requires the expiry field even for session cookies.
+    newCookie.expiry = Number.MAX_SAFE_INTEGER;
+    newCookie.session = true;
+  } else {
+    newCookie.session = false;
+  }
+  newCookie.sameSite = SAMESITE_MAP.get(newCookie.sameSite || "None");
+
+  let isIpAddress = false;
+  try {
+    Services.eTLD.getPublicSuffixFromHost(newCookie.domain);
+  } catch (e) {
+    switch (e.result) {
+      case Cr.NS_ERROR_HOST_IS_IP_ADDRESS:
+        isIpAddress = true;
+        break;
+      default:
+        throw new lazy.error.InvalidCookieDomainError(newCookie.domain);
+    }
+  }
+
+  if (!hostOnly && !isIpAddress) {
+    // only store this as a domain cookie if the domain was specified in the
+    // request and it wasn't an IP address.
+    newCookie.domain = "." + newCookie.domain;
+  }
+
+  if (restrictToHost) {
+    if (
+      !restrictToHost.endsWith(newCookie.domain) &&
+      "." + restrictToHost !== newCookie.domain &&
+      restrictToHost !== newCookie.domain
+    ) {
+      throw new lazy.error.InvalidCookieDomainError(
+        `Cookies may only be set ` +
+          `for the current domain (${restrictToHost})`
+      );
+    }
+  }
+
+  let schemeType = Ci.nsICookie.SCHEME_UNSET;
+  switch (protocol) {
+    case "http:":
+      schemeType = Ci.nsICookie.SCHEME_HTTP;
+      break;
+    case "https:":
+      schemeType = Ci.nsICookie.SCHEME_HTTPS;
+      break;
+    default:
+      // Any other protocol that is supported by the cookie service.
+      break;
+  }
+
+  // remove port from domain, if present.
+  // unfortunately this catches IPv6 addresses by mistake
+  // TODO: Bug 814416
+  newCookie.domain = newCookie.domain.replace(IPV4_PORT_EXPR, "");
+
+  try {
+    cookie.manager.add(
+      newCookie.domain,
+      newCookie.path,
+      newCookie.name,
+      newCookie.value,
+      newCookie.secure,
+      newCookie.httpOnly,
+      newCookie.session,
+      newCookie.expiry,
+      {} /* origin attributes */,
+      newCookie.sameSite,
+      schemeType
+    );
+  } catch (e) {
+    throw new lazy.error.UnableToSetCookieError(e);
+  }
+};
+
+/**
+ * Remove cookie from the cookie store.
+ *
+ * @param {Cookie} toDelete
+ *     Cookie to remove.
+ */
+cookie.remove = function (toDelete) {
+  cookie.manager.remove(
+    toDelete.domain,
+    toDelete.name,
+    toDelete.path,
+    {} /* originAttributes */
+  );
+};
+
+/**
+ * Iterates over the cookies for the current ``host``.  You may
+ * optionally filter for specific paths on that ``host`` by specifying
+ * a path in ``currentPath``.
+ *
+ * @param {string} host
+ *     Hostname to retrieve cookies for.
+ * @param {string=} [currentPath="/"] currentPath
+ *     Optionally filter the cookies for ``host`` for the specific path.
+ *     Defaults to ``/``, meaning all cookies for ``host`` are included.
+ *
+ * @returns {Iterable.<Cookie>}
+ *     Iterator.
+ */
+cookie.iter = function* (host, currentPath = "/") {
+  lazy.assert.string(host, "host must be string");
+  lazy.assert.string(currentPath, "currentPath must be string");
+
+  const isForCurrentPath = path => currentPath.includes(path);
+
+  let cookies = cookie.manager.getCookiesFromHost(host, {});
+  for (let cookie of cookies) {
+    // take the hostname and progressively shorten
+    let hostname = host;
+    do {
+      if (
+        (cookie.host == "." + hostname || cookie.host == hostname) &&
+        isForCurrentPath(cookie.path)
+      ) {
+        let data = {
+          name: cookie.name,
+          value: cookie.value,
+          path: cookie.path,
+          domain: cookie.host,
+          secure: cookie.isSecure,
+          httpOnly: cookie.isHttpOnly,
+        };
+
+        if (!cookie.isSession) {
+          data.expiry = cookie.expiry;
+        }
+
+        data.sameSite = [...SAMESITE_MAP].find(
+          ([, value]) => cookie.sameSite === value
+        )[0];
+
+        yield data;
+      }
+      hostname = hostname.replace(/^.*?\./, "");
+    } while (hostname.includes("."));
+  }
+};
diff --git a/remote/marionette/driver.sys.mjs b/remote/marionette/driver.sys.mjs
new file mode 100644
index 0000000000..69fc4078cd
--- /dev/null
+++ b/remote/marionette/driver.sys.mjs
@@ -0,0 +1,3418 @@
+/* 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 {
+  element,
+  ShadowRoot,
+  WebElement,
+} from "chrome://remote/content/marionette/element.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Addon: "chrome://remote/content/marionette/addon.sys.mjs",
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  atom: "chrome://remote/content/marionette/atom.sys.mjs",
+  browser: "chrome://remote/content/marionette/browser.sys.mjs",
+  capture: "chrome://remote/content/shared/Capture.sys.mjs",
+  Context: "chrome://remote/content/marionette/browser.sys.mjs",
+  cookie: "chrome://remote/content/marionette/cookie.sys.mjs",
+  DebounceCallback: "chrome://remote/content/marionette/sync.sys.mjs",
+  disableEventsActor:
+    "chrome://remote/content/marionette/actors/MarionetteEventsParent.sys.mjs",
+  enableEventsActor:
+    "chrome://remote/content/marionette/actors/MarionetteEventsParent.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  getMarionetteCommandsActorProxy:
+    "chrome://remote/content/marionette/actors/MarionetteCommandsParent.sys.mjs",
+  IdlePromise: "chrome://remote/content/marionette/sync.sys.mjs",
+  l10n: "chrome://remote/content/marionette/l10n.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  Marionette: "chrome://remote/content/components/Marionette.sys.mjs",
+  MarionettePrefs: "chrome://remote/content/marionette/prefs.sys.mjs",
+  modal: "chrome://remote/content/marionette/modal.sys.mjs",
+  navigate: "chrome://remote/content/marionette/navigate.sys.mjs",
+  permissions: "chrome://remote/content/marionette/permissions.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+  print: "chrome://remote/content/shared/PDF.sys.mjs",
+  reftest: "chrome://remote/content/marionette/reftest.sys.mjs",
+  registerCommandsActor:
+    "chrome://remote/content/marionette/actors/MarionetteCommandsParent.sys.mjs",
+  RemoteAgent: "chrome://remote/content/components/RemoteAgent.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  TimedPromise: "chrome://remote/content/marionette/sync.sys.mjs",
+  Timeouts: "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  UnhandledPromptBehavior:
+    "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  unregisterCommandsActor:
+    "chrome://remote/content/marionette/actors/MarionetteCommandsParent.sys.mjs",
+  waitForInitialNavigationCompleted:
+    "chrome://remote/content/shared/Navigate.sys.mjs",
+  waitForObserverTopic: "chrome://remote/content/marionette/sync.sys.mjs",
+  WebDriverSession: "chrome://remote/content/shared/webdriver/Session.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+  WindowState: "chrome://remote/content/marionette/browser.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+const SUPPORTED_STRATEGIES = new Set([
+  element.Strategy.ClassName,
+  element.Strategy.Selector,
+  element.Strategy.ID,
+  element.Strategy.Name,
+  element.Strategy.LinkText,
+  element.Strategy.PartialLinkText,
+  element.Strategy.TagName,
+  element.Strategy.XPath,
+]);
+
+// Timeout used to abort fullscreen, maximize, and minimize
+// commands if no window manager is present.
+const TIMEOUT_NO_WINDOW_MANAGER = 5000;
+
+// Observer topic to wait for until the browser window is ready.
+const TOPIC_BROWSER_READY = "browser-delayed-startup-finished";
+
+/**
+ * The Marionette WebDriver services provides a standard conforming
+ * implementation of the W3C WebDriver specification.
+ *
+ * @see {@link https://w3c.github.io/webdriver/webdriver-spec.html}
+ * @namespace driver
+ */
+
+/**
+ * Implements (parts of) the W3C WebDriver protocol.  GeckoDriver lives
+ * in chrome space and mediates calls to the current browsing context's actor.
+ *
+ * Throughout this prototype, functions with the argument <var>cmd</var>'s
+ * documentation refers to the contents of the <code>cmd.parameter</code>
+ * object.
+ *
+ * @class GeckoDriver
+ *
+ * @param {MarionetteServer} server
+ *     The instance of Marionette server.
+ */
+export function GeckoDriver(server) {
+  this._server = server;
+
+  // WebDriver Session
+  this._currentSession = null;
+
+  this.browsers = {};
+
+  // points to current browser
+  this.curBrowser = null;
+  // top-most chrome window
+  this.mainFrame = null;
+
+  // Use content context by default
+  this.context = lazy.Context.Content;
+
+  // used for modal dialogs or tab modal alerts
+  this.dialog = null;
+  this.dialogObserver = null;
+}
+
+/**
+ * The current context decides if commands are executed in chrome- or
+ * content space.
+ */
+Object.defineProperty(GeckoDriver.prototype, "context", {
+  get() {
+    return this._context;
+  },
+
+  set(context) {
+    this._context = lazy.Context.fromString(context);
+  },
+});
+
+/**
+ * The current WebDriver Session.
+ */
+Object.defineProperty(GeckoDriver.prototype, "currentSession", {
+  get() {
+    if (lazy.RemoteAgent.webDriverBiDi) {
+      return lazy.RemoteAgent.webDriverBiDi.session;
+    }
+
+    return this._currentSession;
+  },
+});
+
+/**
+ * Returns the current URL of the ChromeWindow or content browser,
+ * depending on context.
+ *
+ * @returns {URL}
+ *     Read-only property containing the currently loaded URL.
+ */
+Object.defineProperty(GeckoDriver.prototype, "currentURL", {
+  get() {
+    const browsingContext = this.getBrowsingContext({ top: true });
+    return new URL(browsingContext.currentWindowGlobal.documentURI.spec);
+  },
+});
+
+/**
+ * Returns the title of the ChromeWindow or content browser,
+ * depending on context.
+ *
+ * @returns {string}
+ *     Read-only property containing the title of the loaded URL.
+ */
+Object.defineProperty(GeckoDriver.prototype, "title", {
+  get() {
+    const browsingContext = this.getBrowsingContext({ top: true });
+    return browsingContext.currentWindowGlobal.documentTitle;
+  },
+});
+
+Object.defineProperty(GeckoDriver.prototype, "windowType", {
+  get() {
+    return this.curBrowser.window.document.documentElement.getAttribute(
+      "windowtype"
+    );
+  },
+});
+
+GeckoDriver.prototype.QueryInterface = ChromeUtils.generateQI([
+  "nsIObserver",
+  "nsISupportsWeakReference",
+]);
+
+/**
+ * Callback used to observe the creation of new modal or tab modal dialogs
+ * during the session's lifetime.
+ */
+GeckoDriver.prototype.handleModalDialog = function (action, dialog) {
+  if (!this.currentSession) {
+    return;
+  }
+
+  if (action === lazy.modal.ACTION_OPENED) {
+    this.dialog = new lazy.modal.Dialog(() => this.curBrowser, dialog);
+    this.getActor().notifyDialogOpened();
+  } else if (action === lazy.modal.ACTION_CLOSED) {
+    this.dialog = null;
+  }
+};
+
+/**
+ * Get the current visible URL.
+ */
+GeckoDriver.prototype._getCurrentURL = function () {
+  const browsingContext = this.getBrowsingContext({ top: true });
+  return new URL(browsingContext.currentURI.spec);
+};
+
+/**
+ * Get the current "MarionetteCommands" parent actor.
+ *
+ * @param {object} options
+ * @param {boolean=} options.top
+ *     If set to true use the window's top-level browsing context for the actor,
+ *     otherwise the one from the currently selected frame. Defaults to false.
+ *
+ * @returns {MarionetteCommandsParent}
+ *     The parent actor.
+ */
+GeckoDriver.prototype.getActor = function (options = {}) {
+  return lazy.getMarionetteCommandsActorProxy(() =>
+    this.getBrowsingContext(options)
+  );
+};
+
+/**
+ * Get the selected BrowsingContext for the current context.
+ *
+ * @param {object} options
+ * @param {Context=} options.context
+ *     Context (content or chrome) for which to retrieve the browsing context.
+ *     Defaults to the current one.
+ * @param {boolean=} options.parent
+ *     If set to true return the window's parent browsing context,
+ *     otherwise the one from the currently selected frame. Defaults to false.
+ * @param {boolean=} options.top
+ *     If set to true return the window's top-level browsing context,
+ *     otherwise the one from the currently selected frame. Defaults to false.
+ *
+ * @returns {BrowsingContext}
+ *     The browsing context, or `null` if none is available
+ */
+GeckoDriver.prototype.getBrowsingContext = function (options = {}) {
+  const { context = this.context, parent = false, top = false } = options;
+
+  let browsingContext = null;
+  if (context === lazy.Context.Chrome) {
+    browsingContext = this.currentSession?.chromeBrowsingContext;
+  } else {
+    browsingContext = this.currentSession?.contentBrowsingContext;
+  }
+
+  if (browsingContext && parent) {
+    browsingContext = browsingContext.parent;
+  }
+
+  if (browsingContext && top) {
+    browsingContext = browsingContext.top;
+  }
+
+  return browsingContext;
+};
+
+/**
+ * Get the currently selected window.
+ *
+ * It will return the outer {@link ChromeWindow} previously selected by
+ * window handle through {@link #switchToWindow}, or the first window that
+ * was registered.
+ *
+ * @param {object} options
+ * @param {Context=} options.context
+ *     Optional name of the context to use for finding the window.
+ *     It will be required if a command always needs a specific context,
+ *     whether which context is currently set. Defaults to the current
+ *     context.
+ *
+ * @returns {ChromeWindow}
+ *     The current top-level browsing context.
+ */
+GeckoDriver.prototype.getCurrentWindow = function (options = {}) {
+  const { context = this.context } = options;
+
+  let win = null;
+  switch (context) {
+    case lazy.Context.Chrome:
+      if (this.curBrowser) {
+        win = this.curBrowser.window;
+      }
+      break;
+
+    case lazy.Context.Content:
+      if (this.curBrowser && this.curBrowser.contentBrowser) {
+        win = this.curBrowser.window;
+      }
+      break;
+  }
+
+  return win;
+};
+
+GeckoDriver.prototype.isReftestBrowser = function (element) {
+  return (
+    this._reftest &&
+    element &&
+    element.tagName === "xul:browser" &&
+    element.parentElement &&
+    element.parentElement.id === "reftest"
+  );
+};
+
+/**
+ * Create a new browsing context for window and add to known browsers.
+ *
+ * @param {ChromeWindow} win
+ *     Window for which we will create a browsing context.
+ *
+ * @returns {string}
+ *     Returns the unique server-assigned ID of the window.
+ */
+GeckoDriver.prototype.addBrowser = function (win) {
+  let context = new lazy.browser.Context(win, this);
+  let winId = lazy.windowManager.getIdForWindow(win);
+
+  this.browsers[winId] = context;
+  this.curBrowser = this.browsers[winId];
+};
+
+/**
+ * Recursively get all labeled text.
+ *
+ * @param {Element} el
+ *     The parent element.
+ * @param {Array.<string>} lines
+ *      Array that holds the text lines.
+ */
+GeckoDriver.prototype.getVisibleText = function (el, lines) {
+  try {
+    if (lazy.atom.isElementDisplayed(el, this.getCurrentWindow())) {
+      if (el.value) {
+        lines.push(el.value);
+      }
+      for (let child in el.childNodes) {
+        this.getVisibleText(el.childNodes[child], lines);
+      }
+    }
+  } catch (e) {
+    if (el.nodeName == "#text") {
+      lines.push(el.textContent);
+    }
+  }
+};
+
+/**
+ * Handles registration of new content browsers.  Depending on
+ * their type they are either accepted or ignored.
+ *
+ * @param {XULBrowser} browserElement
+ */
+GeckoDriver.prototype.registerBrowser = function (browserElement) {
+  // We want to ignore frames that are XUL browsers that aren't in the "main"
+  // tabbrowser, but accept things on Fennec (which doesn't have a
+  // xul:tabbrowser), and accept HTML iframes (because tests depend on it),
+  // as well as XUL frames. Ideally this should be cleaned up and we should
+  // keep track of browsers a different way.
+  if (
+    !lazy.AppInfo.isFirefox ||
+    browserElement.namespaceURI != XUL_NS ||
+    browserElement.nodeName != "browser" ||
+    browserElement.getTabBrowser()
+  ) {
+    this.curBrowser.register(browserElement);
+  }
+};
+
+/**
+ * Create a new WebDriver session.
+ *
+ * @param {object} cmd
+ * @param {Object<string, *>=} cmd.parameters
+ *     JSON Object containing any of the recognised capabilities as listed
+ *     on the `WebDriverSession` class.
+ *
+ * @returns {object}
+ *     Session ID and capabilities offered by the WebDriver service.
+ *
+ * @throws {SessionNotCreatedError}
+ *     If, for whatever reason, a session could not be created.
+ */
+GeckoDriver.prototype.newSession = async function (cmd) {
+  if (this.currentSession) {
+    throw new lazy.error.SessionNotCreatedError(
+      "Maximum number of active sessions"
+    );
+  }
+
+  const { parameters: capabilities } = cmd;
+
+  try {
+    // If the WebDriver BiDi protocol is active always use the Remote Agent
+    // to handle the WebDriver session. If it's not the case then Marionette
+    // itself needs to handle it, and has to nullify the "webSocketUrl"
+    // capability.
+    if (lazy.RemoteAgent.webDriverBiDi) {
+      await lazy.RemoteAgent.webDriverBiDi.createSession(capabilities);
+    } else {
+      this._currentSession = new lazy.WebDriverSession(capabilities);
+      this._currentSession.capabilities.delete("webSocketUrl");
+    }
+
+    // Don't wait for the initial window when Marionette is in windowless mode
+    if (!this.currentSession.capabilities.get("moz:windowless")) {
+      // Creating a WebDriver session too early can cause issues with
+      // clients in not being able to find any available window handle.
+      // Also when closing the application while it's still starting up can
+      // cause shutdown hangs. As such Marionette will return a new session
+      // once the initial application window has finished initializing.
+      lazy.logger.debug(`Waiting for initial application window`);
+      await lazy.Marionette.browserStartupFinished;
+
+      const appWin =
+        await lazy.windowManager.waitForInitialApplicationWindowLoaded();
+
+      if (lazy.MarionettePrefs.clickToStart) {
+        Services.prompt.alert(
+          appWin,
+          "",
+          "Click to start execution of marionette tests"
+        );
+      }
+
+      this.addBrowser(appWin);
+      this.mainFrame = appWin;
+
+      // Setup observer for modal dialogs
+      this.dialogObserver = new lazy.modal.DialogObserver(
+        () => this.curBrowser
+      );
+      this.dialogObserver.add(this.handleModalDialog.bind(this));
+
+      for (let win of lazy.windowManager.windows) {
+        this.registerWindow(win, { registerBrowsers: true });
+      }
+
+      if (this.mainFrame) {
+        this.currentSession.chromeBrowsingContext =
+          this.mainFrame.browsingContext;
+        this.mainFrame.focus();
+      }
+
+      if (this.curBrowser.tab) {
+        const browsingContext = this.curBrowser.contentBrowser.browsingContext;
+        this.currentSession.contentBrowsingContext = browsingContext;
+
+        // Bug 1838381 - Only use a longer unload timeout for desktop, because
+        // on Android only the initial document is loaded, and loading a
+        // specific page during startup doesn't succeed.
+        const options = {};
+        if (!lazy.AppInfo.isAndroid) {
+          options.unloadTimeout = 5000;
+        }
+
+        await lazy.waitForInitialNavigationCompleted(
+          browsingContext.webProgress,
+          options
+        );
+
+        this.curBrowser.contentBrowser.focus();
+      }
+
+      // Check if there is already an open dialog for the selected browser window.
+      this.dialog = lazy.modal.findModalDialogs(this.curBrowser);
+    }
+
+    lazy.registerCommandsActor();
+    lazy.enableEventsActor();
+
+    Services.obs.addObserver(this, TOPIC_BROWSER_READY);
+  } catch (e) {
+    throw new lazy.error.SessionNotCreatedError(e);
+  }
+
+  return {
+    sessionId: this.currentSession.id,
+    capabilities: this.currentSession.capabilities,
+  };
+};
+
+/**
+ * Start observing the specified window.
+ *
+ * @param {ChromeWindow} win
+ *     Chrome window to register event listeners for.
+ * @param {object=} options
+ * @param {boolean=} options.registerBrowsers
+ *     If true, register all content browsers of found tabs. Defaults to false.
+ */
+GeckoDriver.prototype.registerWindow = function (win, options = {}) {
+  const { registerBrowsers = false } = options;
+  const tabBrowser = lazy.TabManager.getTabBrowser(win);
+
+  if (registerBrowsers && tabBrowser) {
+    for (const tab of tabBrowser.tabs) {
+      const contentBrowser = lazy.TabManager.getBrowserForTab(tab);
+      this.registerBrowser(contentBrowser);
+    }
+  }
+
+  // Listen for any kind of top-level process switch
+  tabBrowser?.addEventListener("XULFrameLoaderCreated", this);
+};
+
+/**
+ * Stop observing the specified window.
+ *
+ * @param {ChromeWindow} win
+ *     Chrome window to unregister event listeners for.
+ */
+GeckoDriver.prototype.stopObservingWindow = function (win) {
+  const tabBrowser = lazy.TabManager.getTabBrowser(win);
+
+  tabBrowser?.removeEventListener("XULFrameLoaderCreated", this);
+};
+
+GeckoDriver.prototype.handleEvent = function ({ target, type }) {
+  switch (type) {
+    case "XULFrameLoaderCreated":
+      if (target === this.curBrowser.contentBrowser) {
+        lazy.logger.trace(
+          "Remoteness change detected. Set new top-level browsing context " +
+            `to ${target.browsingContext.id}`
+        );
+
+        this.currentSession.contentBrowsingContext = target.browsingContext;
+      }
+      break;
+  }
+};
+
+GeckoDriver.prototype.observe = function (subject, topic, data) {
+  switch (topic) {
+    case TOPIC_BROWSER_READY:
+      this.registerWindow(subject);
+      break;
+  }
+};
+
+/**
+ * Send the current session's capabilities to the client.
+ *
+ * Capabilities informs the client of which WebDriver features are
+ * supported by Firefox and Marionette.  They are immutable for the
+ * length of the session.
+ *
+ * The return value is an immutable map of string keys
+ * ("capabilities") to values, which may be of types boolean,
+ * numerical or string.
+ */
+GeckoDriver.prototype.getSessionCapabilities = function () {
+  return { capabilities: this.currentSession.capabilities };
+};
+
+/**
+ * Sets the context of the subsequent commands.
+ *
+ * All subsequent requests to commands that in some way involve
+ * interaction with a browsing context will target the chosen browsing
+ * context.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.value
+ *     Name of the context to be switched to.  Must be one of "chrome" or
+ *     "content".
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>value</var> is not a string.
+ * @throws {WebDriverError}
+ *     If <var>value</var> is not a valid browsing context.
+ */
+GeckoDriver.prototype.setContext = function (cmd) {
+  let value = lazy.assert.string(cmd.parameters.value);
+
+  this.context = value;
+};
+
+/**
+ * Gets the context type that is Marionette's current target for
+ * browsing context scoped commands.
+ *
+ * You may choose a context through the {@link #setContext} command.
+ *
+ * The default browsing context is {@link Context.Content}.
+ *
+ * @returns {Context}
+ *     Current context.
+ */
+GeckoDriver.prototype.getContext = function () {
+  return this.context;
+};
+
+/**
+ * Executes a JavaScript function in the context of the current browsing
+ * context, if in content space, or in chrome space otherwise, and returns
+ * the return value of the function.
+ *
+ * It is important to note that if the <var>sandboxName</var> parameter
+ * is left undefined, the script will be evaluated in a mutable sandbox,
+ * causing any change it makes on the global state of the document to have
+ * lasting side-effects.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.script
+ *     Script to evaluate as a function body.
+ * @param {Array.<(string|boolean|number|object|WebReference)>} cmd.parameters.args
+ *     Arguments exposed to the script in <code>arguments</code>.
+ *     The array items must be serialisable to the WebDriver protocol.
+ * @param {string=} cmd.parameters.sandbox
+ *     Name of the sandbox to evaluate the script in.  The sandbox is
+ *     cached for later re-use on the same Window object if
+ *     <var>newSandbox</var> is false.  If he parameter is undefined,
+ *     the script is evaluated in a mutable sandbox.  If the parameter
+ *     is "system", it will be evaluted in a sandbox with elevated system
+ *     privileges, equivalent to chrome space.
+ * @param {boolean=} cmd.parameters.newSandbox
+ *     Forces the script to be evaluated in a fresh sandbox.  Note that if
+ *     it is undefined, the script will normally be evaluted in a fresh
+ *     sandbox.
+ * @param {string=} cmd.parameters.filename
+ *     Filename of the client's program where this script is evaluated.
+ * @param {number=} cmd.parameters.line
+ *     Line in the client's program where this script is evaluated.
+ *
+ * @returns {(string|boolean|number|object|WebReference)}
+ *     Return value from the script, or null which signifies either the
+ *     JavaScript notion of null or undefined.
+ *
+ * @throws {JavaScriptError}
+ *     If an {@link Error} was thrown whilst evaluating the script.
+ * @throws {NoSuchElementError}
+ *     If an element that was passed as part of <var>args</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {ScriptTimeoutError}
+ *     If the script was interrupted due to reaching the session's
+ *     script timeout.
+ * @throws {StaleElementReferenceError}
+ *     If an element that was passed as part of <var>args</var> or that is
+ *     returned as result has gone stale.
+ */
+GeckoDriver.prototype.executeScript = function (cmd) {
+  let { script, args } = cmd.parameters;
+  let opts = {
+    script: cmd.parameters.script,
+    args: cmd.parameters.args,
+    sandboxName: cmd.parameters.sandbox,
+    newSandbox: cmd.parameters.newSandbox,
+    file: cmd.parameters.filename,
+    line: cmd.parameters.line,
+  };
+
+  return this.execute_(script, args, opts);
+};
+
+/**
+ * Executes a JavaScript function in the context of the current browsing
+ * context, if in content space, or in chrome space otherwise, and returns
+ * the object passed to the callback.
+ *
+ * The callback is always the last argument to the <var>arguments</var>
+ * list passed to the function scope of the script.  It can be retrieved
+ * as such:
+ *
+ * <pre><code>
+ *     let callback = arguments[arguments.length - 1];
+ *     callback("foo");
+ *     // "foo" is returned
+ * </code></pre>
+ *
+ * It is important to note that if the <var>sandboxName</var> parameter
+ * is left undefined, the script will be evaluated in a mutable sandbox,
+ * causing any change it makes on the global state of the document to have
+ * lasting side-effects.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.script
+ *     Script to evaluate as a function body.
+ * @param {Array.<(string|boolean|number|object|WebReference)>} cmd.parameters.args
+ *     Arguments exposed to the script in <code>arguments</code>.
+ *     The array items must be serialisable to the WebDriver protocol.
+ * @param {string=} cmd.parameters.sandbox
+ *     Name of the sandbox to evaluate the script in.  The sandbox is
+ *     cached for later re-use on the same Window object if
+ *     <var>newSandbox</var> is false.  If the parameter is undefined,
+ *     the script is evaluated in a mutable sandbox.  If the parameter
+ *     is "system", it will be evaluted in a sandbox with elevated system
+ *     privileges, equivalent to chrome space.
+ * @param {boolean=} cmd.parameters.newSandbox
+ *     Forces the script to be evaluated in a fresh sandbox.  Note that if
+ *     it is undefined, the script will normally be evaluted in a fresh
+ *     sandbox.
+ * @param {string=} cmd.parameters.filename
+ *     Filename of the client's program where this script is evaluated.
+ * @param {number=} cmd.parameters.line
+ *     Line in the client's program where this script is evaluated.
+ *
+ * @returns {(string|boolean|number|object|WebReference)}
+ *     Return value from the script, or null which signifies either the
+ *     JavaScript notion of null or undefined.
+ *
+ * @throws {JavaScriptError}
+ *     If an Error was thrown whilst evaluating the script.
+ * @throws {NoSuchElementError}
+ *     If an element that was passed as part of <var>args</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {ScriptTimeoutError}
+ *     If the script was interrupted due to reaching the session's
+ *     script timeout.
+ * @throws {StaleElementReferenceError}
+ *     If an element that was passed as part of <var>args</var> or that is
+ *     returned as result has gone stale.
+ */
+GeckoDriver.prototype.executeAsyncScript = function (cmd) {
+  let { script, args } = cmd.parameters;
+  let opts = {
+    script: cmd.parameters.script,
+    args: cmd.parameters.args,
+    sandboxName: cmd.parameters.sandbox,
+    newSandbox: cmd.parameters.newSandbox,
+    file: cmd.parameters.filename,
+    line: cmd.parameters.line,
+    async: true,
+  };
+
+  return this.execute_(script, args, opts);
+};
+
+GeckoDriver.prototype.execute_ = async function (
+  script,
+  args = [],
+  {
+    sandboxName = null,
+    newSandbox = false,
+    file = "",
+    line = 0,
+    async = false,
+  } = {}
+) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  lazy.assert.string(
+    script,
+    lazy.pprint`Expected "script" to be a string: ${script}`
+  );
+  lazy.assert.array(
+    args,
+    lazy.pprint`Expected script args to be an array: ${args}`
+  );
+  if (sandboxName !== null) {
+    lazy.assert.string(
+      sandboxName,
+      lazy.pprint`Expected sandbox name to be a string: ${sandboxName}`
+    );
+  }
+  lazy.assert.boolean(
+    newSandbox,
+    lazy.pprint`Expected newSandbox to be boolean: ${newSandbox}`
+  );
+  lazy.assert.string(file, lazy.pprint`Expected file to be a string: ${file}`);
+  lazy.assert.number(line, lazy.pprint`Expected line to be a number: ${line}`);
+
+  let opts = {
+    timeout: this.currentSession.timeouts.script,
+    sandboxName,
+    newSandbox,
+    file,
+    line,
+    async,
+  };
+
+  return this.getActor().executeScript(script, args, opts);
+};
+
+/**
+ * Navigate to given URL.
+ *
+ * Navigates the current browsing context to the given URL and waits for
+ * the document to load or the session's page timeout duration to elapse
+ * before returning.
+ *
+ * The command will return with a failure if there is an error loading
+ * the document or the URL is blocked.  This can occur if it fails to
+ * reach host, the URL is malformed, or if there is a certificate issue
+ * to name some examples.
+ *
+ * The document is considered successfully loaded when the
+ * DOMContentLoaded event on the frame element associated with the
+ * current window triggers and document.readyState is "complete".
+ *
+ * In chrome context it will change the current window's location to
+ * the supplied URL and wait until document.readyState equals "complete"
+ * or the page timeout duration has elapsed.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.url
+ *     URL to navigate to.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.navigateTo = async function (cmd) {
+  lazy.assert.content(this.context);
+  const browsingContext = lazy.assert.open(
+    this.getBrowsingContext({ top: true })
+  );
+  await this._handleUserPrompts();
+
+  let validURL;
+  try {
+    validURL = new URL(cmd.parameters.url);
+  } catch (e) {
+    throw new lazy.error.InvalidArgumentError(`Malformed URL: ${e.message}`);
+  }
+
+  // Switch to the top-level browsing context before navigating
+  this.currentSession.contentBrowsingContext = browsingContext;
+
+  const loadEventExpected = lazy.navigate.isLoadEventExpected(
+    this._getCurrentURL(),
+    {
+      future: validURL,
+    }
+  );
+
+  await lazy.navigate.waitForNavigationCompleted(
+    this,
+    () => {
+      lazy.navigate.navigateTo(browsingContext, validURL);
+    },
+    { loadEventExpected }
+  );
+
+  this.curBrowser.contentBrowser.focus();
+};
+
+/**
+ * Get a string representing the current URL.
+ *
+ * On Desktop this returns a string representation of the URL of the
+ * current top level browsing context.  This is equivalent to
+ * document.location.href.
+ *
+ * When in the context of the chrome, this returns the canonical URL
+ * of the current resource.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getCurrentUrl = async function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  return this._getCurrentURL().href;
+};
+
+/**
+ * Gets the current title of the window.
+ *
+ * @returns {string}
+ *     Document title of the top-level browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getTitle = async function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  return this.title;
+};
+
+/**
+ * Gets the current type of the window.
+ *
+ * @returns {string}
+ *     Type of window
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.getWindowType = function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+
+  return this.windowType;
+};
+
+/**
+ * Gets the page source of the content document.
+ *
+ * @returns {string}
+ *     String serialisation of the DOM of the current browsing context's
+ *     active document.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getPageSource = async function () {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  return this.getActor().getPageSource();
+};
+
+/**
+ * Cause the browser to traverse one step backward in the joint history
+ * of the current browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.goBack = async function () {
+  lazy.assert.content(this.context);
+  const browsingContext = lazy.assert.open(
+    this.getBrowsingContext({ top: true })
+  );
+  await this._handleUserPrompts();
+
+  // If there is no history, just return
+  if (!browsingContext.embedderElement?.canGoBack) {
+    return;
+  }
+
+  await lazy.navigate.waitForNavigationCompleted(this, () => {
+    browsingContext.goBack();
+  });
+};
+
+/**
+ * Cause the browser to traverse one step forward in the joint history
+ * of the current browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.goForward = async function () {
+  lazy.assert.content(this.context);
+  const browsingContext = lazy.assert.open(
+    this.getBrowsingContext({ top: true })
+  );
+  await this._handleUserPrompts();
+
+  // If there is no history, just return
+  if (!browsingContext.embedderElement?.canGoForward) {
+    return;
+  }
+
+  await lazy.navigate.waitForNavigationCompleted(this, () => {
+    browsingContext.goForward();
+  });
+};
+
+/**
+ * Causes the browser to reload the page in current top-level browsing
+ * context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.refresh = async function () {
+  lazy.assert.content(this.context);
+  const browsingContext = lazy.assert.open(
+    this.getBrowsingContext({ top: true })
+  );
+  await this._handleUserPrompts();
+
+  // Switch to the top-level browsing context before navigating
+  this.currentSession.contentBrowsingContext = browsingContext;
+
+  await lazy.navigate.waitForNavigationCompleted(this, () => {
+    lazy.navigate.refresh(browsingContext);
+  });
+};
+
+/**
+ * Get the current window's handle. On desktop this typically corresponds
+ * to the currently selected tab.
+ *
+ * For chrome scope it returns the window identifier for the current chrome
+ * window for tests interested in managing the chrome window and tab separately.
+ *
+ * Return an opaque server-assigned identifier to this window that
+ * uniquely identifies it within this Marionette instance.  This can
+ * be used to switch to this window at a later point.
+ *
+ * @returns {string}
+ *     Unique window handle.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.getWindowHandle = function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+
+  if (this.context == lazy.Context.Chrome) {
+    return lazy.windowManager.getIdForWindow(this.curBrowser.window);
+  }
+  return lazy.TabManager.getIdForBrowser(this.curBrowser.contentBrowser);
+};
+
+/**
+ * Get a list of top-level browsing contexts. On desktop this typically
+ * corresponds to the set of open tabs for browser windows, or the window
+ * itself for non-browser chrome windows.
+ *
+ * For chrome scope it returns identifiers for each open chrome window for
+ * tests interested in managing a set of chrome windows and tabs separately.
+ *
+ * Each window handle is assigned by the server and is guaranteed unique,
+ * however the return array does not have a specified ordering.
+ *
+ * @returns {Array.<string>}
+ *     Unique window handles.
+ */
+GeckoDriver.prototype.getWindowHandles = function () {
+  if (this.context == lazy.Context.Chrome) {
+    return lazy.windowManager.chromeWindowHandles.map(String);
+  }
+  return lazy.TabManager.allBrowserUniqueIds.map(String);
+};
+
+/**
+ * Get the current position and size of the browser window currently in focus.
+ *
+ * Will return the current browser window size in pixels. Refers to
+ * window outerWidth and outerHeight values, which include scroll bars,
+ * title bars, etc.
+ *
+ * @returns {Object<string, number>}
+ *     Object with |x| and |y| coordinates, and |width| and |height|
+ *     of browser window.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getWindowRect = async function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  return this.curBrowser.rect;
+};
+
+/**
+ * Set the window position and size of the browser on the operating
+ * system window manager.
+ *
+ * The supplied `width` and `height` values refer to the window `outerWidth`
+ * and `outerHeight` values, which include browser chrome and OS-level
+ * window borders.
+ *
+ * @param {object} cmd
+ * @param {number} cmd.parameters.x
+ *     X coordinate of the top/left of the window that it will be
+ *     moved to.
+ * @param {number} cmd.parameters.y
+ *     Y coordinate of the top/left of the window that it will be
+ *     moved to.
+ * @param {number} cmd.parameters.width
+ *     Width to resize the window to.
+ * @param {number} cmd.parameters.height
+ *     Height to resize the window to.
+ *
+ * @returns {Object<string, number>}
+ *     Object with `x` and `y` coordinates and `width` and `height`
+ *     dimensions.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not applicable to application.
+ */
+GeckoDriver.prototype.setWindowRect = async function (cmd) {
+  lazy.assert.desktop();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  const { x = null, y = null, width = null, height = null } = cmd.parameters;
+  if (x !== null) {
+    lazy.assert.integer(x);
+  }
+  if (y !== null) {
+    lazy.assert.integer(y);
+  }
+  if (height !== null) {
+    lazy.assert.positiveInteger(height);
+  }
+  if (width !== null) {
+    lazy.assert.positiveInteger(width);
+  }
+
+  const win = this.getCurrentWindow();
+  switch (lazy.WindowState.from(win.windowState)) {
+    case lazy.WindowState.Fullscreen:
+      await exitFullscreen(win);
+      break;
+
+    case lazy.WindowState.Maximized:
+    case lazy.WindowState.Minimized:
+      await restoreWindow(win);
+      break;
+  }
+
+  function geometryMatches() {
+    if (
+      width !== null &&
+      height !== null &&
+      (win.outerWidth !== width || win.outerHeight !== height)
+    ) {
+      return false;
+    }
+    if (x !== null && y !== null && (win.screenX !== x || win.screenY !== y)) {
+      return false;
+    }
+    lazy.logger.trace(`Requested window geometry matches`);
+    return true;
+  }
+
+  if (!geometryMatches()) {
+    // There might be more than one resize or MozUpdateWindowPos event due
+    // to previous geometry changes, such as from restoreWindow(), so
+    // wait longer if window geometry does not match.
+    const options = { checkFn: geometryMatches, timeout: 500 };
+    const promises = [];
+    if (width !== null && height !== null) {
+      promises.push(new lazy.EventPromise(win, "resize", options));
+      win.resizeTo(width, height);
+    }
+    if (x !== null && y !== null) {
+      promises.push(
+        new lazy.EventPromise(win.windowRoot, "MozUpdateWindowPos", options)
+      );
+      win.moveTo(x, y);
+    }
+    try {
+      await Promise.race(promises);
+    } catch (e) {
+      if (e instanceof lazy.error.TimeoutError) {
+        // The operating system might not honor the move or resize, in which
+        // case assume that geometry will have been adjusted "as close as
+        // possible" to that requested.  There may be no event received if the
+        // geometry is already as close as possible.
+      } else {
+        throw e;
+      }
+    }
+  }
+
+  return this.curBrowser.rect;
+};
+
+/**
+ * Switch current top-level browsing context by name or server-assigned
+ * ID.  Searches for windows by name, then ID.  Content windows take
+ * precedence.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.handle
+ *     Handle of the window to switch to.
+ * @param {boolean=} cmd.parameters.focus
+ *     A boolean value which determines whether to focus
+ *     the window. Defaults to true.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>handle</var> is not a string or <var>focus</var> not a boolean.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.switchToWindow = async function (cmd) {
+  const { focus = true, handle } = cmd.parameters;
+
+  lazy.assert.string(
+    handle,
+    lazy.pprint`Expected "handle" to be a string, got ${handle}`
+  );
+  lazy.assert.boolean(
+    focus,
+    lazy.pprint`Expected "focus" to be a boolean, got ${focus}`
+  );
+
+  const found = lazy.windowManager.findWindowByHandle(handle);
+
+  let selected = false;
+  if (found) {
+    try {
+      await this.setWindowHandle(found, focus);
+      selected = true;
+    } catch (e) {
+      lazy.logger.error(e);
+    }
+  }
+
+  if (!selected) {
+    throw new lazy.error.NoSuchWindowError(
+      `Unable to locate window: ${handle}`
+    );
+  }
+};
+
+/**
+ * Switch the marionette window to a given window. If the browser in
+ * the window is unregistered, register that browser and wait for
+ * the registration is complete. If |focus| is true then set the focus
+ * on the window.
+ *
+ * @param {object} winProperties
+ *     Object containing window properties such as returned from
+ *     :js:func:`GeckoDriver#getWindowProperties`
+ * @param {boolean=} focus
+ *     A boolean value which determines whether to focus the window.
+ *     Defaults to true.
+ */
+GeckoDriver.prototype.setWindowHandle = async function (
+  winProperties,
+  focus = true
+) {
+  if (!(winProperties.id in this.browsers)) {
+    // Initialise Marionette if the current chrome window has not been seen
+    // before. Also register the initial tab, if one exists.
+    this.addBrowser(winProperties.win);
+    this.mainFrame = winProperties.win;
+
+    this.currentSession.chromeBrowsingContext = this.mainFrame.browsingContext;
+
+    if (!winProperties.hasTabBrowser) {
+      this.currentSession.contentBrowsingContext = null;
+    } else {
+      const tabBrowser = lazy.TabManager.getTabBrowser(winProperties.win);
+
+      // For chrome windows such as a reftest window, `getTabBrowser` is not
+      // a tabbrowser, it is the content browser which should be used here.
+      const contentBrowser = tabBrowser.tabs
+        ? tabBrowser.selectedBrowser
+        : tabBrowser;
+
+      this.currentSession.contentBrowsingContext =
+        contentBrowser.browsingContext;
+      this.registerBrowser(contentBrowser);
+    }
+  } else {
+    // Otherwise switch to the known chrome window
+    this.curBrowser = this.browsers[winProperties.id];
+    this.mainFrame = this.curBrowser.window;
+
+    // Activate the tab if it's a content window.
+    let tab = null;
+    if (winProperties.hasTabBrowser) {
+      tab = await this.curBrowser.switchToTab(
+        winProperties.tabIndex,
+        winProperties.win,
+        focus
+      );
+    }
+
+    this.currentSession.chromeBrowsingContext = this.mainFrame.browsingContext;
+    this.currentSession.contentBrowsingContext =
+      tab?.linkedBrowser.browsingContext;
+  }
+
+  // Check for existing dialogs for the new window
+  this.dialog = lazy.modal.findModalDialogs(this.curBrowser);
+
+  // If there is an open window modal dialog the underlying chrome window
+  // cannot be focused.
+  if (focus && !this.dialog?.isWindowModal) {
+    await this.curBrowser.focusWindow();
+  }
+};
+
+/**
+ * Set the current browsing context for future commands to the parent
+ * of the current browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.switchToParentFrame = async function () {
+  let browsingContext = this.getBrowsingContext();
+  if (browsingContext && !browsingContext.parent) {
+    return;
+  }
+
+  browsingContext = lazy.assert.open(browsingContext?.parent);
+
+  this.currentSession.contentBrowsingContext = browsingContext;
+};
+
+/**
+ * Switch to a given frame within the current window.
+ *
+ * @param {object} cmd
+ * @param {(string | object)=} cmd.parameters.element
+ *     A web element reference of the frame or its element id.
+ * @param {number=} cmd.parameters.id
+ *     The index of the frame to switch to.
+ *     If both element and id are not defined, switch to top-level frame.
+ *
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>element</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>element</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.switchToFrame = async function (cmd) {
+  const { element: el, id } = cmd.parameters;
+
+  if (typeof id == "number") {
+    lazy.assert.unsignedShort(
+      id,
+      `Expected id to be unsigned short, got ${id}`
+    );
+  }
+
+  const top = id == null && el == null;
+  lazy.assert.open(this.getBrowsingContext({ top }));
+  await this._handleUserPrompts();
+
+  // Bug 1495063: Elements should be passed as WebReference reference
+  let byFrame;
+  if (typeof el == "string") {
+    byFrame = WebElement.fromUUID(el).toJSON();
+  } else if (el) {
+    byFrame = el;
+  }
+
+  const { browsingContext } = await this.getActor({ top }).switchToFrame(
+    byFrame || id
+  );
+
+  this.currentSession.contentBrowsingContext = browsingContext;
+};
+
+GeckoDriver.prototype.getTimeouts = function () {
+  return this.currentSession.timeouts;
+};
+
+/**
+ * Set timeout for page loading, searching, and scripts.
+ *
+ * @param {object} cmd
+ * @param {Object<string, number>} cmd.parameters
+ *     Dictionary of timeout types and their new value, where all timeout
+ *     types are optional.
+ *
+ * @throws {InvalidArgumentError}
+ *     If timeout type key is unknown, or the value provided with it is
+ *     not an integer.
+ */
+GeckoDriver.prototype.setTimeouts = function (cmd) {
+  // merge with existing timeouts
+  let merged = Object.assign(
+    this.currentSession.timeouts.toJSON(),
+    cmd.parameters
+  );
+
+  this.currentSession.timeouts = lazy.Timeouts.fromJSON(merged);
+};
+
+/** Single tap. */
+GeckoDriver.prototype.singleTap = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+
+  let { id, x, y } = cmd.parameters;
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  await this.getActor().singleTap(
+    webEl,
+    x,
+    y,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * Perform a series of grouped actions at the specified points in time.
+ *
+ * @param {object} cmd
+ * @param {Array<?>} cmd.parameters.actions
+ *     Array of objects that each represent an action sequence.
+ *
+ * @throws {NoSuchElementError}
+ *     If an element that is used as part of the action chain is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If an element that is used as part of the action chain has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not yet available in current context.
+ */
+GeckoDriver.prototype.performActions = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  const actions = cmd.parameters.actions;
+
+  await this.getActor().performActions(
+    actions,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * Release all the keys and pointer buttons that are currently depressed.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.releaseActions = async function () {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  await this.getActor().releaseActions();
+};
+
+/**
+ * Find an element using the indicated search strategy.
+ *
+ * @param {object} cmd
+ * @param {string=} cmd.parameters.element
+ *     Web element reference ID to the element that will be used as start node.
+ * @param {string} cmd.parameters.using
+ *     Indicates which search method to use.
+ * @param {string} cmd.parameters.value
+ *     Value the client is looking for.
+ *
+ * @returns {WebElement}
+ *     Return the found element.
+ *
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>element</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>element</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.findElement = async function (cmd) {
+  const { element: el, using, value } = cmd.parameters;
+
+  if (!SUPPORTED_STRATEGIES.has(using)) {
+    throw new lazy.error.InvalidSelectorError(
+      `Strategy not supported: ${using}`
+    );
+  }
+
+  lazy.assert.defined(value);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let startNode;
+  if (typeof el != "undefined") {
+    startNode = WebElement.fromUUID(el).toJSON();
+  }
+
+  let opts = {
+    startNode,
+    timeout: this.currentSession.timeouts.implicit,
+    all: false,
+  };
+
+  return this.getActor().findElement(using, value, opts);
+};
+
+/**
+ * Find an element within shadow root using the indicated search strategy.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.shadowRoot
+ *     Shadow root reference ID.
+ * @param {string} cmd.parameters.using
+ *     Indicates which search method to use.
+ * @param {string} cmd.parameters.value
+ *     Value the client is looking for.
+ *
+ * @returns {WebElement}
+ *     Return the found element.
+ *
+ * @throws {DetachedShadowRootError}
+ *     If shadow root represented by reference <var>id</var> is
+ *     no longer attached to the DOM.
+ * @throws {NoSuchElementError}
+ *     If the element which is looked for with <var>value</var> was
+ *     not found.
+ * @throws {NoSuchShadowRoot}
+ *     If shadow root represented by reference <var>shadowRoot</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.findElementFromShadowRoot = async function (cmd) {
+  const { shadowRoot, using, value } = cmd.parameters;
+
+  if (!SUPPORTED_STRATEGIES.has(using)) {
+    throw new lazy.error.InvalidSelectorError(
+      `Strategy not supported: ${using}`
+    );
+  }
+
+  lazy.assert.defined(value);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  const opts = {
+    all: false,
+    startNode: ShadowRoot.fromUUID(shadowRoot).toJSON(),
+    timeout: this.currentSession.timeouts.implicit,
+  };
+
+  return this.getActor().findElement(using, value, opts);
+};
+
+/**
+ * Find elements using the indicated search strategy.
+ *
+ * @param {object} cmd
+ * @param {string=} cmd.parameters.element
+ *     Web element reference ID to the element that will be used as start node.
+ * @param {string} cmd.parameters.using
+ *     Indicates which search method to use.
+ * @param {string} cmd.parameters.value
+ *     Value the client is looking for.
+ *
+ * @returns {Array<WebElement>}
+ *     Return the array of found elements.
+ *
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>element</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>element</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.findElements = async function (cmd) {
+  const { element: el, using, value } = cmd.parameters;
+
+  if (!SUPPORTED_STRATEGIES.has(using)) {
+    throw new lazy.error.InvalidSelectorError(
+      `Strategy not supported: ${using}`
+    );
+  }
+
+  lazy.assert.defined(value);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let startNode;
+  if (typeof el != "undefined") {
+    startNode = WebElement.fromUUID(el).toJSON();
+  }
+
+  let opts = {
+    startNode,
+    timeout: this.currentSession.timeouts.implicit,
+    all: true,
+  };
+
+  return this.getActor().findElements(using, value, opts);
+};
+
+/**
+ * Find elements within shadow root using the indicated search strategy.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.shadowRoot
+ *     Shadow root reference ID.
+ * @param {string} cmd.parameters.using
+ *     Indicates which search method to use.
+ * @param {string} cmd.parameters.value
+ *     Value the client is looking for.
+ *
+ * @returns {Array<WebElement>}
+ *     Return the array of found elements.
+ *
+ * @throws {DetachedShadowRootError}
+ *     If shadow root represented by reference <var>id</var> is
+ *     no longer attached to the DOM.
+ * @throws {NoSuchShadowRoot}
+ *     If shadow root represented by reference <var>shadowRoot</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.findElementsFromShadowRoot = async function (cmd) {
+  const { shadowRoot, using, value } = cmd.parameters;
+
+  if (!SUPPORTED_STRATEGIES.has(using)) {
+    throw new lazy.error.InvalidSelectorError(
+      `Strategy not supported: ${using}`
+    );
+  }
+
+  lazy.assert.defined(value);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  const opts = {
+    all: true,
+    startNode: ShadowRoot.fromUUID(shadowRoot).toJSON(),
+    timeout: this.currentSession.timeouts.implicit,
+  };
+
+  return this.getActor().findElements(using, value, opts);
+};
+
+/**
+ * Return the shadow root of an element in the document.
+ *
+ * @param {object} cmd
+ * @param {id} cmd.parameters.id
+ *     A web element id reference.
+ * @returns {ShadowRoot}
+ *     ShadowRoot of the element.
+ *
+ * @throws {InvalidArgumentError}
+ *     If element <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchShadowRoot}
+ *     Element does not have a shadow root attached.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in chrome current context.
+ */
+GeckoDriver.prototype.getShadowRoot = async function (cmd) {
+  // Bug 1743541: Add support for chrome scope.
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(
+    cmd.parameters.id,
+    lazy.pprint`Expected "id" to be a string, got ${cmd.parameters.id}`
+  );
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getShadowRoot(webEl);
+};
+
+/**
+ * Return the active element in the document.
+ *
+ * @returns {WebReference}
+ *     Active element of the current browsing context's document
+ *     element, if the document element is non-null.
+ *
+ * @throws {NoSuchElementError}
+ *     If the document does not have an active element, i.e. if
+ *     its document element has been deleted.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in chrome context.
+ */
+GeckoDriver.prototype.getActiveElement = async function () {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  return this.getActor().getActiveElement();
+};
+
+/**
+ * Send click event to element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be clicked.
+ *
+ * @throws {InvalidArgumentError}
+ *     If element <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.clickElement = async function (cmd) {
+  const browsingContext = lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  const actor = this.getActor();
+
+  const loadEventExpected = lazy.navigate.isLoadEventExpected(
+    this._getCurrentURL(),
+    {
+      browsingContext,
+      target: await actor.getElementAttribute(webEl, "target"),
+    }
+  );
+
+  await lazy.navigate.waitForNavigationCompleted(
+    this,
+    () => actor.clickElement(webEl, this.currentSession.capabilities),
+    {
+      loadEventExpected,
+      // The click might trigger a navigation, so don't count on it.
+      requireBeforeUnload: false,
+    }
+  );
+};
+
+/**
+ * Get a given attribute of an element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Web element reference ID to the element that will be inspected.
+ * @param {string} cmd.parameters.name
+ *     Name of the attribute which value to retrieve.
+ *
+ * @returns {string}
+ *     Value of the attribute.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> or <var>name</var> are not strings.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementAttribute = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  const id = lazy.assert.string(cmd.parameters.id);
+  const name = lazy.assert.string(cmd.parameters.name);
+  const webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementAttribute(webEl, name);
+};
+
+/**
+ * Returns the value of a property associated with given element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Web element reference ID to the element that will be inspected.
+ * @param {string} cmd.parameters.name
+ *     Name of the property which value to retrieve.
+ *
+ * @returns {string}
+ *     Value of the property.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> or <var>name</var> are not strings.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementProperty = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  const id = lazy.assert.string(cmd.parameters.id);
+  const name = lazy.assert.string(cmd.parameters.name);
+  const webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementProperty(webEl, name);
+};
+
+/**
+ * Get the text of an element, if any.  Includes the text of all child
+ * elements.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be inspected.
+ *
+ * @returns {string}
+ *     Element's text "as rendered".
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementText = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementText(webEl);
+};
+
+/**
+ * Get the tag name of the element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be inspected.
+ *
+ * @returns {string}
+ *     Local tag name of element.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementTagName = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementTagName(webEl);
+};
+
+/**
+ * Check if element is displayed.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be inspected.
+ *
+ * @returns {boolean}
+ *     True if displayed, false otherwise.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.isElementDisplayed = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().isElementDisplayed(
+    webEl,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * Return the property of the computed style of an element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be checked.
+ * @param {string} cmd.parameters.propertyName
+ *     CSS rule that is being requested.
+ *
+ * @returns {string}
+ *     Value of |propertyName|.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> or <var>propertyName</var> are not strings.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementValueOfCssProperty = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let prop = lazy.assert.string(cmd.parameters.propertyName);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementValueOfCssProperty(webEl, prop);
+};
+
+/**
+ * Check if element is enabled.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be checked.
+ *
+ * @returns {boolean}
+ *     True if enabled, false if disabled.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.isElementEnabled = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().isElementEnabled(
+    webEl,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * Check if element is selected.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be checked.
+ *
+ * @returns {boolean}
+ *     True if selected, false if unselected.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.isElementSelected = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().isElementSelected(
+    webEl,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementRect = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getElementRect(webEl);
+};
+
+/**
+ * Send key presses to element after focusing on it.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be checked.
+ * @param {string} cmd.parameters.text
+ *     Value to send to the element.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> or <var>text</var> are not strings.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.sendKeysToElement = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let text = lazy.assert.string(cmd.parameters.text);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().sendKeysToElement(
+    webEl,
+    text,
+    this.currentSession.capabilities
+  );
+};
+
+/**
+ * Clear the text of an element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Reference ID to the element that will be cleared.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>id</var> is not a string.
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.clearElement = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  await this.getActor().clearElement(webEl);
+};
+
+/**
+ * Add a single cookie to the cookie store associated with the active
+ * document's address.
+ *
+ * @param {object} cmd
+ * @param {Map.<string, (string|number|boolean)>} cmd.parameters.cookie
+ *     Cookie object.
+ *
+ * @throws {InvalidCookieDomainError}
+ *     If <var>cookie</var> is for a different domain than the active
+ *     document's host.
+ * @throws {NoSuchWindowError}
+ *     Bbrowsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.addCookie = async function (cmd) {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let { protocol, hostname } = this._getCurrentURL();
+
+  const networkSchemes = ["http:", "https:"];
+  if (!networkSchemes.includes(protocol)) {
+    throw new lazy.error.InvalidCookieDomainError("Document is cookie-averse");
+  }
+
+  let newCookie = lazy.cookie.fromJSON(cmd.parameters.cookie);
+
+  lazy.cookie.add(newCookie, { restrictToHost: hostname, protocol });
+};
+
+/**
+ * Get all the cookies for the current domain.
+ *
+ * This is the equivalent of calling <code>document.cookie</code> and
+ * parsing the result.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.getCookies = async function () {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let { hostname, pathname } = this._getCurrentURL();
+  return [...lazy.cookie.iter(hostname, pathname)];
+};
+
+/**
+ * Delete all cookies that are visible to a document.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.deleteAllCookies = async function () {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let { hostname, pathname } = this._getCurrentURL();
+  for (let toDelete of lazy.cookie.iter(hostname, pathname)) {
+    lazy.cookie.remove(toDelete);
+  }
+};
+
+/**
+ * Delete a cookie by name.
+ *
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ */
+GeckoDriver.prototype.deleteCookie = async function (cmd) {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let { hostname, pathname } = this._getCurrentURL();
+  let name = lazy.assert.string(cmd.parameters.name);
+  for (let c of lazy.cookie.iter(hostname, pathname)) {
+    if (c.name === name) {
+      lazy.cookie.remove(c);
+    }
+  }
+};
+
+/**
+ * Open a new top-level browsing context.
+ *
+ * @param {object} cmd
+ * @param {string=} cmd.parameters.type
+ *     Optional type of the new top-level browsing context. Can be one of
+ *     `tab` or `window`. Defaults to `tab`.
+ * @param {boolean=} cmd.parameters.focus
+ *     Optional flag if the new top-level browsing context should be opened
+ *     in foreground (focused) or background (not focused). Defaults to false.
+ * @param {boolean=} cmd.parameters.private
+ *     Optional flag, which gets only evaluated for type `window`. True if the
+ *     new top-level browsing context should be a private window.
+ *     Defaults to false.
+ *
+ * @returns {Object<string, string>}
+ *     Handle and type of the new browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.newWindow = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  let focus = false;
+  if (typeof cmd.parameters.focus != "undefined") {
+    focus = lazy.assert.boolean(
+      cmd.parameters.focus,
+      lazy.pprint`Expected "focus" to be a boolean, got ${cmd.parameters.focus}`
+    );
+  }
+
+  let isPrivate = false;
+  if (typeof cmd.parameters.private != "undefined") {
+    isPrivate = lazy.assert.boolean(
+      cmd.parameters.private,
+      lazy.pprint`Expected "private" to be a boolean, got ${cmd.parameters.private}`
+    );
+  }
+
+  let type;
+  if (typeof cmd.parameters.type != "undefined") {
+    type = lazy.assert.string(
+      cmd.parameters.type,
+      lazy.pprint`Expected "type" to be a string, got ${cmd.parameters.type}`
+    );
+  }
+
+  // If an invalid or no type has been specified default to a tab.
+  if (typeof type == "undefined" || !["tab", "window"].includes(type)) {
+    type = "tab";
+  }
+
+  let contentBrowser;
+
+  switch (type) {
+    case "window":
+      let win = await this.curBrowser.openBrowserWindow(focus, isPrivate);
+      contentBrowser = lazy.TabManager.getTabBrowser(win).selectedBrowser;
+      break;
+
+    default:
+      // To not fail if a new type gets added in the future, make opening
+      // a new tab the default action.
+      let tab = await this.curBrowser.openTab(focus);
+      contentBrowser = lazy.TabManager.getBrowserForTab(tab);
+  }
+
+  // Actors need the new window to be loaded to safely execute queries.
+  // Wait until the initial page load has been finished.
+  await lazy.waitForInitialNavigationCompleted(
+    contentBrowser.browsingContext.webProgress,
+    {
+      unloadTimeout: 5000,
+    }
+  );
+
+  const id = lazy.TabManager.getIdForBrowser(contentBrowser);
+
+  return { handle: id.toString(), type };
+};
+
+/**
+ * Close the currently selected tab/window.
+ *
+ * With multiple open tabs present the currently selected tab will
+ * be closed.  Otherwise the window itself will be closed. If it is the
+ * last window currently open, the window will not be closed to prevent
+ * a shutdown of the application. Instead the returned list of window
+ * handles is empty.
+ *
+ * @returns {Array.<string>}
+ *     Unique window handles of remaining windows.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.close = async function () {
+  lazy.assert.open(
+    this.getBrowsingContext({ context: lazy.Context.Content, top: true })
+  );
+  await this._handleUserPrompts();
+
+  // If there is only one window left, do not close unless windowless mode is
+  // enabled. Instead return a faked empty array of window handles.
+  // This will instruct geckodriver to terminate the application.
+  if (
+    lazy.TabManager.getTabCount() === 1 &&
+    !this.currentSession.capabilities.get("moz:windowless")
+  ) {
+    return [];
+  }
+
+  await this.curBrowser.closeTab();
+  this.currentSession.contentBrowsingContext = null;
+
+  return lazy.TabManager.allBrowserUniqueIds.map(String);
+};
+
+/**
+ * Close the currently selected chrome window.
+ *
+ * If it is the last window currently open, the chrome window will not be
+ * closed to prevent a shutdown of the application. Instead the returned
+ * list of chrome window handles is empty.
+ *
+ * @returns {Array.<string>}
+ *     Unique chrome window handles of remaining chrome windows.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.closeChromeWindow = async function () {
+  lazy.assert.desktop();
+  lazy.assert.open(
+    this.getBrowsingContext({ context: lazy.Context.Chrome, top: true })
+  );
+
+  let nwins = 0;
+
+  // eslint-disable-next-line
+  for (let _ of lazy.windowManager.windows) {
+    nwins++;
+  }
+
+  // If there is only one window left, do not close unless windowless mode is
+  // enabled. Instead return a faked empty array of window handles.
+  // This will instruct geckodriver to terminate the application.
+  if (nwins == 1 && !this.currentSession.capabilities.get("moz:windowless")) {
+    return [];
+  }
+
+  await this.curBrowser.closeWindow();
+  this.currentSession.chromeBrowsingContext = null;
+  this.currentSession.contentBrowsingContext = null;
+
+  return lazy.windowManager.chromeWindowHandles.map(String);
+};
+
+/** Delete Marionette session. */
+GeckoDriver.prototype.deleteSession = function () {
+  if (!this.currentSession) {
+    return;
+  }
+
+  for (let win of lazy.windowManager.windows) {
+    this.stopObservingWindow(win);
+  }
+
+  // reset to the top-most frame
+  this.mainFrame = null;
+
+  if (this.dialogObserver) {
+    this.dialogObserver.cleanup();
+    this.dialogObserver = null;
+  }
+
+  try {
+    Services.obs.removeObserver(this, TOPIC_BROWSER_READY);
+  } catch (e) {
+    lazy.logger.debug(`Failed to remove observer "${TOPIC_BROWSER_READY}"`);
+  }
+
+  // Always unregister actors after all other observers
+  // and listeners have been removed.
+  lazy.unregisterCommandsActor();
+  // MarionetteEvents actors are only disabled to avoid IPC errors if there are
+  // in flight events being forwarded from the content process to the parent
+  // process.
+  lazy.disableEventsActor();
+
+  if (lazy.RemoteAgent.webDriverBiDi) {
+    lazy.RemoteAgent.webDriverBiDi.deleteSession();
+  } else {
+    this.currentSession.destroy();
+    this._currentSession = null;
+  }
+};
+
+/**
+ * Takes a screenshot of a web element, current frame, or viewport.
+ *
+ * The screen capture is returned as a lossless PNG image encoded as
+ * a base 64 string.
+ *
+ * If called in the content context, the |id| argument is not null and
+ * refers to a present and visible web element's ID, the capture area will
+ * be limited to the bounding box of that element.  Otherwise, the capture
+ * area will be the bounding box of the current frame.
+ *
+ * If called in the chrome context, the screenshot will always represent
+ * the entire viewport.
+ *
+ * @param {object} cmd
+ * @param {string=} cmd.parameters.id
+ *     Optional web element reference to take a screenshot of.
+ *     If undefined, a screenshot will be taken of the document element.
+ * @param {boolean=} cmd.parameters.full
+ *     True to take a screenshot of the entire document element. Is only
+ *     considered if <var>id</var> is not defined. Defaults to true.
+ * @param {boolean=} cmd.parameters.hash
+ *     True if the user requests a hash of the image data. Defaults to false.
+ * @param {boolean=} cmd.parameters.scroll
+ *     Scroll to element if |id| is provided. Defaults to true.
+ *
+ * @returns {string}
+ *     If <var>hash</var> is false, PNG image encoded as Base64 encoded
+ *     string.  If <var>hash</var> is true, hex digest of the SHA-256
+ *     hash of the Base64 encoded string.
+ *
+ * @throws {NoSuchElementError}
+ *     If element represented by reference <var>id</var> is unknown.
+ * @throws {NoSuchWindowError}
+ *     Browsing context has been discarded.
+ * @throws {StaleElementReferenceError}
+ *     If element represented by reference <var>id</var> has gone stale.
+ */
+GeckoDriver.prototype.takeScreenshot = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  let { id, full, hash, scroll } = cmd.parameters;
+  let format = hash ? lazy.capture.Format.Hash : lazy.capture.Format.Base64;
+
+  full = typeof full == "undefined" ? true : full;
+  scroll = typeof scroll == "undefined" ? true : scroll;
+
+  let webEl = id ? WebElement.fromUUID(id).toJSON() : null;
+
+  // Only consider full screenshot if no element has been specified
+  full = webEl ? false : full;
+
+  return this.getActor().takeScreenshot(webEl, format, full, scroll);
+};
+
+/**
+ * Get the current browser orientation.
+ *
+ * Will return one of the valid primary orientation values
+ * portrait-primary, landscape-primary, portrait-secondary, or
+ * landscape-secondary.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.getScreenOrientation = function () {
+  lazy.assert.mobile();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+
+  const win = this.getCurrentWindow();
+
+  return win.screen.orientation.type;
+};
+
+/**
+ * Set the current browser orientation.
+ *
+ * The supplied orientation should be given as one of the valid
+ * orientation values.  If the orientation is unknown, an error will
+ * be raised.
+ *
+ * Valid orientations are "portrait" and "landscape", which fall
+ * back to "portrait-primary" and "landscape-primary" respectively,
+ * and "portrait-secondary" as well as "landscape-secondary".
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.setScreenOrientation = async function (cmd) {
+  lazy.assert.mobile();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+
+  const ors = [
+    "portrait",
+    "landscape",
+    "portrait-primary",
+    "landscape-primary",
+    "portrait-secondary",
+    "landscape-secondary",
+  ];
+
+  let or = String(cmd.parameters.orientation);
+  lazy.assert.string(or);
+  let mozOr = or.toLowerCase();
+  if (!ors.includes(mozOr)) {
+    throw new lazy.error.InvalidArgumentError(
+      `Unknown screen orientation: ${or}`
+    );
+  }
+
+  const win = this.getCurrentWindow();
+
+  try {
+    await win.screen.orientation.lock(mozOr);
+  } catch (e) {
+    throw new lazy.error.WebDriverError(
+      `Unable to set screen orientation: ${or}`
+    );
+  }
+};
+
+/**
+ * Synchronously minimizes the user agent window as if the user pressed
+ * the minimize button.
+ *
+ * No action is taken if the window is already minimized.
+ *
+ * Not supported on Fennec.
+ *
+ * @returns {Object<string, number>}
+ *     Window rect and window state.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available for current application.
+ */
+GeckoDriver.prototype.minimizeWindow = async function () {
+  lazy.assert.desktop();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  const win = this.getCurrentWindow();
+  switch (lazy.WindowState.from(win.windowState)) {
+    case lazy.WindowState.Fullscreen:
+      await exitFullscreen(win);
+      break;
+
+    case lazy.WindowState.Maximized:
+      await restoreWindow(win);
+      break;
+  }
+
+  if (lazy.WindowState.from(win.windowState) != lazy.WindowState.Minimized) {
+    let cb;
+    // Use a timed promise to abort if no window manager is present
+    await new lazy.TimedPromise(
+      resolve => {
+        cb = new lazy.DebounceCallback(resolve);
+        win.addEventListener("sizemodechange", cb);
+        win.minimize();
+      },
+      { throws: null, timeout: TIMEOUT_NO_WINDOW_MANAGER }
+    );
+    win.removeEventListener("sizemodechange", cb);
+    await new lazy.IdlePromise(win);
+  }
+
+  return this.curBrowser.rect;
+};
+
+/**
+ * Synchronously maximizes the user agent window as if the user pressed
+ * the maximize button.
+ *
+ * No action is taken if the window is already maximized.
+ *
+ * Not supported on Fennec.
+ *
+ * @returns {Object<string, number>}
+ *     Window rect.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available for current application.
+ */
+GeckoDriver.prototype.maximizeWindow = async function () {
+  lazy.assert.desktop();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  const win = this.getCurrentWindow();
+  switch (lazy.WindowState.from(win.windowState)) {
+    case lazy.WindowState.Fullscreen:
+      await exitFullscreen(win);
+      break;
+
+    case lazy.WindowState.Minimized:
+      await restoreWindow(win);
+      break;
+  }
+
+  if (lazy.WindowState.from(win.windowState) != lazy.WindowState.Maximized) {
+    let cb;
+    // Use a timed promise to abort if no window manager is present
+    await new lazy.TimedPromise(
+      resolve => {
+        cb = new lazy.DebounceCallback(resolve);
+        win.addEventListener("sizemodechange", cb);
+        win.maximize();
+      },
+      { throws: null, timeout: TIMEOUT_NO_WINDOW_MANAGER }
+    );
+    win.removeEventListener("sizemodechange", cb);
+    await new lazy.IdlePromise(win);
+  }
+
+  return this.curBrowser.rect;
+};
+
+/**
+ * Synchronously sets the user agent window to full screen as if the user
+ * had done "View > Enter Full Screen".
+ *
+ * No action is taken if the window is already in full screen mode.
+ *
+ * Not supported on Fennec.
+ *
+ * @returns {Map.<string, number>}
+ *     Window rect.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available for current application.
+ */
+GeckoDriver.prototype.fullscreenWindow = async function () {
+  lazy.assert.desktop();
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  const win = this.getCurrentWindow();
+  switch (lazy.WindowState.from(win.windowState)) {
+    case lazy.WindowState.Maximized:
+    case lazy.WindowState.Minimized:
+      await restoreWindow(win);
+      break;
+  }
+
+  if (lazy.WindowState.from(win.windowState) != lazy.WindowState.Fullscreen) {
+    let cb;
+    // Use a timed promise to abort if no window manager is present
+    await new lazy.TimedPromise(
+      resolve => {
+        cb = new lazy.DebounceCallback(resolve);
+        win.addEventListener("sizemodechange", cb);
+        win.fullScreen = true;
+      },
+      { throws: null, timeout: TIMEOUT_NO_WINDOW_MANAGER }
+    );
+    win.removeEventListener("sizemodechange", cb);
+  }
+  await new lazy.IdlePromise(win);
+
+  return this.curBrowser.rect;
+};
+
+/**
+ * Dismisses a currently displayed tab modal, or returns no such alert if
+ * no modal is displayed.
+ *
+ * @throws {NoSuchAlertError}
+ *     If there is no current user prompt.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.dismissDialog = async function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  this._checkIfAlertIsPresent();
+
+  const dialogClosed = this.dialogObserver.dialogClosed();
+  this.dialog.dismiss();
+  await dialogClosed;
+
+  const win = this.getCurrentWindow();
+  await new lazy.IdlePromise(win);
+};
+
+/**
+ * Accepts a currently displayed tab modal, or returns no such alert if
+ * no modal is displayed.
+ *
+ * @throws {NoSuchAlertError}
+ *     If there is no current user prompt.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.acceptDialog = async function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  this._checkIfAlertIsPresent();
+
+  const dialogClosed = this.dialogObserver.dialogClosed();
+  this.dialog.accept();
+  await dialogClosed;
+
+  const win = this.getCurrentWindow();
+  await new lazy.IdlePromise(win);
+};
+
+/**
+ * Returns the message shown in a currently displayed modal, or returns
+ * a no such alert error if no modal is currently displayed.
+ *
+ * @throws {NoSuchAlertError}
+ *     If there is no current user prompt.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ */
+GeckoDriver.prototype.getTextFromDialog = function () {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  this._checkIfAlertIsPresent();
+  return this.dialog.text;
+};
+
+/**
+ * Set the user prompt's value field.
+ *
+ * Sends keys to the input field of a currently displayed modal, or
+ * returns a no such alert error if no modal is currently displayed. If
+ * a tab modal is currently displayed but has no means for text input,
+ * an element not visible error is returned.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.text
+ *     Input to the user prompt's value field.
+ *
+ * @throws {ElementNotInteractableError}
+ *     If the current user prompt is an alert or confirm.
+ * @throws {NoSuchAlertError}
+ *     If there is no current user prompt.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnsupportedOperationError}
+ *     If the current user prompt is something other than an alert,
+ *     confirm, or a prompt.
+ */
+GeckoDriver.prototype.sendKeysToDialog = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  this._checkIfAlertIsPresent();
+
+  let text = lazy.assert.string(cmd.parameters.text);
+  let promptType = this.dialog.args.promptType;
+
+  switch (promptType) {
+    case "alert":
+    case "confirm":
+      throw new lazy.error.ElementNotInteractableError(
+        `User prompt of type ${promptType} is not interactable`
+      );
+    case "prompt":
+      break;
+    default:
+      await this.dismissDialog();
+      throw new lazy.error.UnsupportedOperationError(
+        `User prompt of type ${promptType} is not supported`
+      );
+  }
+  this.dialog.text = text;
+};
+
+GeckoDriver.prototype._checkIfAlertIsPresent = function () {
+  if (!this.dialog || !this.dialog.isOpen) {
+    throw new lazy.error.NoSuchAlertError();
+  }
+};
+
+GeckoDriver.prototype._handleUserPrompts = async function () {
+  if (!this.dialog || !this.dialog.isOpen) {
+    return;
+  }
+
+  let textContent = this.dialog.text;
+
+  const behavior = this.currentSession.unhandledPromptBehavior;
+  switch (behavior) {
+    case lazy.UnhandledPromptBehavior.Accept:
+      await this.acceptDialog();
+      break;
+
+    case lazy.UnhandledPromptBehavior.AcceptAndNotify:
+      await this.acceptDialog();
+      throw new lazy.error.UnexpectedAlertOpenError(
+        `Accepted user prompt dialog: ${textContent}`
+      );
+
+    case lazy.UnhandledPromptBehavior.Dismiss:
+      await this.dismissDialog();
+      break;
+
+    case lazy.UnhandledPromptBehavior.DismissAndNotify:
+      await this.dismissDialog();
+      throw new lazy.error.UnexpectedAlertOpenError(
+        `Dismissed user prompt dialog: ${textContent}`
+      );
+
+    case lazy.UnhandledPromptBehavior.Ignore:
+      throw new lazy.error.UnexpectedAlertOpenError(
+        "Encountered unhandled user prompt dialog"
+      );
+
+    default:
+      throw new TypeError(`Unknown unhandledPromptBehavior "${behavior}"`);
+  }
+};
+
+/**
+ * Enables or disables accepting new socket connections.
+ *
+ * By calling this method with `false` the server will not accept any
+ * further connections, but existing connections will not be forcible
+ * closed. Use `true` to re-enable accepting connections.
+ *
+ * Please note that when closing the connection via the client you can
+ * end-up in a non-recoverable state if it hasn't been enabled before.
+ *
+ * This method is used for custom in application shutdowns via
+ * marionette.quit() or marionette.restart(), like File -> Quit.
+ *
+ * @param {object} cmd
+ * @param {boolean} cmd.parameters.value
+ *     True if the server should accept new socket connections.
+ */
+GeckoDriver.prototype.acceptConnections = async function (cmd) {
+  lazy.assert.boolean(cmd.parameters.value);
+  await this._server.setAcceptConnections(cmd.parameters.value);
+};
+
+/**
+ * Quits the application with the provided flags.
+ *
+ * Marionette will stop accepting new connections before ending the
+ * current session, and finally attempting to quit the application.
+ *
+ * Optional {@link nsIAppStartup} flags may be provided as
+ * an array of masks, and these will be combined by ORing
+ * them with a bitmask.  The available masks are defined in
+ * https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIAppStartup.
+ *
+ * Crucially, only one of the *Quit flags can be specified. The |eRestart|
+ * flag may be bit-wise combined with one of the *Quit flags to cause
+ * the application to restart after it quits.
+ *
+ * @param {object} cmd
+ * @param {Array.<string>=} cmd.parameters.flags
+ *     Constant name of masks to pass to |Services.startup.quit|.
+ *     If empty or undefined, |nsIAppStartup.eAttemptQuit| is used.
+ * @param {boolean=} cmd.parameters.safeMode
+ *     Optional flag to indicate that the application has to
+ *     be restarted in safe mode.
+ *
+ * @returns {Object<string,boolean>}
+ *     Dictionary containing information that explains the shutdown reason.
+ *     The value for `cause` contains the shutdown kind like "shutdown" or
+ *     "restart", while `forced` will indicate if it was a normal or forced
+ *     shutdown of the application. "in_app" is always set to indicate that
+ *     it is a shutdown triggered from within the application.
+ *
+ * @throws {InvalidArgumentError}
+ *     If <var>flags</var> contains unknown or incompatible flags,
+ *     for example multiple Quit flags.
+ */
+GeckoDriver.prototype.quit = async function (cmd) {
+  const { flags = [], safeMode = false } = cmd.parameters;
+  const quits = ["eConsiderQuit", "eAttemptQuit", "eForceQuit"];
+
+  lazy.assert.array(flags, `Expected "flags" to be an array`);
+  lazy.assert.boolean(safeMode, `Expected "safeMode" to be a boolean`);
+
+  if (safeMode && !flags.includes("eRestart")) {
+    throw new lazy.error.InvalidArgumentError(
+      `"safeMode" only works with restart flag`
+    );
+  }
+
+  if (flags.includes("eSilently")) {
+    if (!this.currentSession.capabilities.get("moz:windowless")) {
+      throw new lazy.error.UnsupportedOperationError(
+        `Silent restarts only allowed with "moz:windowless" capability set`
+      );
+    }
+    if (!flags.includes("eRestart")) {
+      throw new lazy.error.InvalidArgumentError(
+        `"silently" only works with restart flag`
+      );
+    }
+  }
+
+  let quitSeen;
+  let mode = 0;
+  if (flags.length) {
+    for (let k of flags) {
+      lazy.assert.in(k, Ci.nsIAppStartup);
+
+      if (quits.includes(k)) {
+        if (quitSeen) {
+          throw new lazy.error.InvalidArgumentError(
+            `${k} cannot be combined with ${quitSeen}`
+          );
+        }
+        quitSeen = k;
+      }
+
+      mode |= Ci.nsIAppStartup[k];
+    }
+  }
+
+  if (!quitSeen) {
+    mode |= Ci.nsIAppStartup.eAttemptQuit;
+  }
+
+  await this._server.setAcceptConnections(false);
+  this.deleteSession();
+
+  // Notify all windows that an application quit has been requested.
+  const cancelQuit = Cc["@mozilla.org/supports-PRBool;1"].createInstance(
+    Ci.nsISupportsPRBool
+  );
+  Services.obs.notifyObservers(cancelQuit, "quit-application-requested");
+
+  // If the shutdown of the application is prevented force quit it instead.
+  if (cancelQuit.data) {
+    mode |= Ci.nsIAppStartup.eForceQuit;
+  }
+
+  // delay response until the application is about to quit
+  let quitApplication = lazy.waitForObserverTopic("quit-application");
+
+  if (safeMode) {
+    Services.startup.restartInSafeMode(mode);
+  } else {
+    Services.startup.quit(mode);
+  }
+
+  return {
+    cause: (await quitApplication).data,
+    forced: cancelQuit.data,
+    in_app: true,
+  };
+};
+
+GeckoDriver.prototype.installAddon = function (cmd) {
+  lazy.assert.desktop();
+
+  let path = cmd.parameters.path;
+  let temp = cmd.parameters.temporary || false;
+  if (
+    typeof path == "undefined" ||
+    typeof path != "string" ||
+    typeof temp != "boolean"
+  ) {
+    throw new lazy.error.InvalidArgumentError();
+  }
+
+  return lazy.Addon.install(path, temp);
+};
+
+GeckoDriver.prototype.uninstallAddon = function (cmd) {
+  lazy.assert.desktop();
+
+  let id = cmd.parameters.id;
+  if (typeof id == "undefined" || typeof id != "string") {
+    throw new lazy.error.InvalidArgumentError();
+  }
+
+  return lazy.Addon.uninstall(id);
+};
+
+/**
+ * Retrieve the localized string for the specified entity id.
+ *
+ * Example:
+ *     localizeEntity(["chrome://branding/locale/brand.dtd"], "brandShortName")
+ *
+ * @param {object} cmd
+ * @param {Array.<string>} cmd.parameters.urls
+ *     Array of .dtd URLs.
+ * @param {string} cmd.parameters.id
+ *     The ID of the entity to retrieve the localized string for.
+ *
+ * @returns {string}
+ *     The localized string for the requested entity.
+ */
+GeckoDriver.prototype.localizeEntity = function (cmd) {
+  let { urls, id } = cmd.parameters;
+
+  if (!Array.isArray(urls)) {
+    throw new lazy.error.InvalidArgumentError(
+      "Value of `urls` should be of type 'Array'"
+    );
+  }
+  if (typeof id != "string") {
+    throw new lazy.error.InvalidArgumentError(
+      "Value of `id` should be of type 'string'"
+    );
+  }
+
+  return lazy.l10n.localizeEntity(urls, id);
+};
+
+/**
+ * Retrieve the localized string for the specified property id.
+ *
+ * Example:
+ *
+ *     localizeProperty(
+ *         ["chrome://global/locale/findbar.properties"], "FastFind");
+ *
+ * @param {object} cmd
+ * @param {Array.<string>} cmd.parameters.urls
+ *     Array of .properties URLs.
+ * @param {string} cmd.parameters.id
+ *     The ID of the property to retrieve the localized string for.
+ *
+ * @returns {string}
+ *     The localized string for the requested property.
+ */
+GeckoDriver.prototype.localizeProperty = function (cmd) {
+  let { urls, id } = cmd.parameters;
+
+  if (!Array.isArray(urls)) {
+    throw new lazy.error.InvalidArgumentError(
+      "Value of `urls` should be of type 'Array'"
+    );
+  }
+  if (typeof id != "string") {
+    throw new lazy.error.InvalidArgumentError(
+      "Value of `id` should be of type 'string'"
+    );
+  }
+
+  return lazy.l10n.localizeProperty(urls, id);
+};
+
+/**
+ * Initialize the reftest mode
+ */
+GeckoDriver.prototype.setupReftest = async function (cmd) {
+  if (this._reftest) {
+    throw new lazy.error.UnsupportedOperationError(
+      "Called reftest:setup with a reftest session already active"
+    );
+  }
+
+  let {
+    urlCount = {},
+    screenshot = "unexpected",
+    isPrint = false,
+  } = cmd.parameters;
+  if (!["always", "fail", "unexpected"].includes(screenshot)) {
+    throw new lazy.error.InvalidArgumentError(
+      "Value of `screenshot` should be 'always', 'fail' or 'unexpected'"
+    );
+  }
+
+  this._reftest = new lazy.reftest.Runner(this);
+  this._reftest.setup(urlCount, screenshot, isPrint);
+};
+
+/** Run a reftest. */
+GeckoDriver.prototype.runReftest = function (cmd) {
+  let { test, references, expected, timeout, width, height, pageRanges } =
+    cmd.parameters;
+
+  if (!this._reftest) {
+    throw new lazy.error.UnsupportedOperationError(
+      "Called reftest:run before reftest:start"
+    );
+  }
+
+  lazy.assert.string(test);
+  lazy.assert.string(expected);
+  lazy.assert.array(references);
+
+  return this._reftest.run(
+    test,
+    references,
+    expected,
+    timeout,
+    pageRanges,
+    width,
+    height
+  );
+};
+
+/**
+ * End a reftest run.
+ *
+ * Closes the reftest window (without changing the current window handle),
+ * and removes cached canvases.
+ */
+GeckoDriver.prototype.teardownReftest = function () {
+  if (!this._reftest) {
+    throw new lazy.error.UnsupportedOperationError(
+      "Called reftest:teardown before reftest:start"
+    );
+  }
+
+  this._reftest.teardown();
+  this._reftest = null;
+};
+
+/**
+ * Print page as PDF.
+ *
+ * @param {object} cmd
+ * @param {boolean=} cmd.parameters.background
+ *     Whether or not to print background colors and images.
+ *     Defaults to false, which prints without background graphics.
+ * @param {number=} cmd.parameters.margin.bottom
+ *     Bottom margin in cm. Defaults to 1cm (~0.4 inches).
+ * @param {number=} cmd.parameters.margin.left
+ *     Left margin in cm. Defaults to 1cm (~0.4 inches).
+ * @param {number=} cmd.parameters.margin.right
+ *     Right margin in cm. Defaults to 1cm (~0.4 inches).
+ * @param {number=} cmd.parameters.margin.top
+ *     Top margin in cm. Defaults to 1cm (~0.4 inches).
+ * @param {('landscape'|'portrait')=} cmd.parameters.options.orientation
+ *     Paper orientation. Defaults to 'portrait'.
+ * @param {Array.<string|number>=} cmd.parameters.pageRanges
+ *     Paper ranges to print, e.g., ['1-5', 8, '11-13'].
+ *     Defaults to the empty array, which means print all pages.
+ * @param {number=} cmd.parameters.page.height
+ *     Paper height in cm. Defaults to US letter height (27.94cm / 11 inches)
+ * @param {number=} cmd.parameters.page.width
+ *     Paper width in cm. Defaults to US letter width (21.59cm / 8.5 inches)
+ * @param {number=} cmd.parameters.scale
+ *     Scale of the webpage rendering. Defaults to 1.0.
+ * @param {boolean=} cmd.parameters.shrinkToFit
+ *     Whether or not to override page size as defined by CSS.
+ *     Defaults to true, in which case the content will be scaled
+ *     to fit the paper size.
+ *
+ * @returns {string}
+ *     Base64 encoded PDF representing printed document
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ * @throws {UnsupportedOperationError}
+ *     Not available in chrome context.
+ */
+GeckoDriver.prototype.print = async function (cmd) {
+  lazy.assert.content(this.context);
+  lazy.assert.open(this.getBrowsingContext({ top: true }));
+  await this._handleUserPrompts();
+
+  const settings = lazy.print.addDefaultSettings(cmd.parameters);
+  for (const prop of ["top", "bottom", "left", "right"]) {
+    lazy.assert.positiveNumber(
+      settings.margin[prop],
+      lazy.pprint`margin.${prop} is not a positive number`
+    );
+  }
+  for (const prop of ["width", "height"]) {
+    lazy.assert.positiveNumber(
+      settings.page[prop],
+      lazy.pprint`page.${prop} is not a positive number`
+    );
+  }
+  lazy.assert.positiveNumber(
+    settings.scale,
+    `scale ${settings.scale} is not a positive number`
+  );
+  lazy.assert.that(
+    s =>
+      s >= lazy.print.minScaleValue &&
+      settings.scale <= lazy.print.maxScaleValue,
+    `scale ${settings.scale} is outside the range ${lazy.print.minScaleValue}-${lazy.print.maxScaleValue}`
+  )(settings.scale);
+  lazy.assert.boolean(settings.shrinkToFit);
+  lazy.assert.that(
+    orientation => lazy.print.defaults.orientationValue.includes(orientation),
+    `orientation ${
+      settings.orientation
+    } doesn't match allowed values "${lazy.print.defaults.orientationValue.join(
+      "/"
+    )}"`
+  )(settings.orientation);
+  lazy.assert.boolean(settings.background);
+  lazy.assert.array(settings.pageRanges);
+
+  const browsingContext = this.curBrowser.tab.linkedBrowser.browsingContext;
+  const printSettings = await lazy.print.getPrintSettings(settings);
+  const binaryString = await lazy.print.printToBinaryString(
+    browsingContext,
+    printSettings
+  );
+
+  return btoa(binaryString);
+};
+
+GeckoDriver.prototype.setPermission = async function (cmd) {
+  const { descriptor, state, oneRealm = false } = cmd.parameters;
+
+  lazy.assert.boolean(oneRealm);
+  lazy.assert.that(
+    state => ["granted", "denied", "prompt"].includes(state),
+    `state is ${state}, expected "granted", "denied", or "prompt"`
+  )(state);
+
+  lazy.permissions.set(descriptor, state, oneRealm);
+};
+
+/**
+ * Determines the Accessibility label for this element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Web element reference ID to the element for which the accessibility label
+ *     will be returned.
+ *
+ * @returns {string}
+ *     The Accessibility label for this element
+ */
+GeckoDriver.prototype.getComputedLabel = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+
+  return this.getActor().getComputedLabel(webEl);
+};
+
+/**
+ * Determines the Accessibility role for this element.
+ *
+ * @param {object} cmd
+ * @param {string} cmd.parameters.id
+ *     Web element reference ID to the element for which the accessibility role
+ *     will be returned.
+ *
+ * @returns {string}
+ *     The Accessibility role for this element
+ */
+GeckoDriver.prototype.getComputedRole = async function (cmd) {
+  lazy.assert.open(this.getBrowsingContext());
+  await this._handleUserPrompts();
+
+  let id = lazy.assert.string(cmd.parameters.id);
+  let webEl = WebElement.fromUUID(id).toJSON();
+  return this.getActor().getComputedRole(webEl);
+};
+
+GeckoDriver.prototype.commands = {
+  // Marionette service
+  "Marionette:AcceptConnections": GeckoDriver.prototype.acceptConnections,
+  "Marionette:GetContext": GeckoDriver.prototype.getContext,
+  "Marionette:GetScreenOrientation": GeckoDriver.prototype.getScreenOrientation,
+  "Marionette:GetWindowType": GeckoDriver.prototype.getWindowType,
+  "Marionette:Quit": GeckoDriver.prototype.quit,
+  "Marionette:SetContext": GeckoDriver.prototype.setContext,
+  "Marionette:SetScreenOrientation": GeckoDriver.prototype.setScreenOrientation,
+  "Marionette:SingleTap": GeckoDriver.prototype.singleTap,
+
+  // Addon service
+  "Addon:Install": GeckoDriver.prototype.installAddon,
+  "Addon:Uninstall": GeckoDriver.prototype.uninstallAddon,
+
+  // L10n service
+  "L10n:LocalizeEntity": GeckoDriver.prototype.localizeEntity,
+  "L10n:LocalizeProperty": GeckoDriver.prototype.localizeProperty,
+
+  // Reftest service
+  "reftest:setup": GeckoDriver.prototype.setupReftest,
+  "reftest:run": GeckoDriver.prototype.runReftest,
+  "reftest:teardown": GeckoDriver.prototype.teardownReftest,
+
+  // WebDriver service
+  "WebDriver:AcceptAlert": GeckoDriver.prototype.acceptDialog,
+  // deprecated, no longer used since the geckodriver 0.30.0 release
+  "WebDriver:AcceptDialog": GeckoDriver.prototype.acceptDialog,
+  "WebDriver:AddCookie": GeckoDriver.prototype.addCookie,
+  "WebDriver:Back": GeckoDriver.prototype.goBack,
+  "WebDriver:CloseChromeWindow": GeckoDriver.prototype.closeChromeWindow,
+  "WebDriver:CloseWindow": GeckoDriver.prototype.close,
+  "WebDriver:DeleteAllCookies": GeckoDriver.prototype.deleteAllCookies,
+  "WebDriver:DeleteCookie": GeckoDriver.prototype.deleteCookie,
+  "WebDriver:DeleteSession": GeckoDriver.prototype.deleteSession,
+  "WebDriver:DismissAlert": GeckoDriver.prototype.dismissDialog,
+  "WebDriver:ElementClear": GeckoDriver.prototype.clearElement,
+  "WebDriver:ElementClick": GeckoDriver.prototype.clickElement,
+  "WebDriver:ElementSendKeys": GeckoDriver.prototype.sendKeysToElement,
+  "WebDriver:ExecuteAsyncScript": GeckoDriver.prototype.executeAsyncScript,
+  "WebDriver:ExecuteScript": GeckoDriver.prototype.executeScript,
+  "WebDriver:FindElement": GeckoDriver.prototype.findElement,
+  "WebDriver:FindElementFromShadowRoot":
+    GeckoDriver.prototype.findElementFromShadowRoot,
+  "WebDriver:FindElements": GeckoDriver.prototype.findElements,
+  "WebDriver:FindElementsFromShadowRoot":
+    GeckoDriver.prototype.findElementsFromShadowRoot,
+  "WebDriver:Forward": GeckoDriver.prototype.goForward,
+  "WebDriver:FullscreenWindow": GeckoDriver.prototype.fullscreenWindow,
+  "WebDriver:GetActiveElement": GeckoDriver.prototype.getActiveElement,
+  "WebDriver:GetAlertText": GeckoDriver.prototype.getTextFromDialog,
+  "WebDriver:GetCapabilities": GeckoDriver.prototype.getSessionCapabilities,
+  "WebDriver:GetComputedLabel": GeckoDriver.prototype.getComputedLabel,
+  "WebDriver:GetComputedRole": GeckoDriver.prototype.getComputedRole,
+  "WebDriver:GetCookies": GeckoDriver.prototype.getCookies,
+  "WebDriver:GetCurrentURL": GeckoDriver.prototype.getCurrentUrl,
+  "WebDriver:GetElementAttribute": GeckoDriver.prototype.getElementAttribute,
+  "WebDriver:GetElementCSSValue":
+    GeckoDriver.prototype.getElementValueOfCssProperty,
+  "WebDriver:GetElementProperty": GeckoDriver.prototype.getElementProperty,
+  "WebDriver:GetElementRect": GeckoDriver.prototype.getElementRect,
+  "WebDriver:GetElementTagName": GeckoDriver.prototype.getElementTagName,
+  "WebDriver:GetElementText": GeckoDriver.prototype.getElementText,
+  "WebDriver:GetPageSource": GeckoDriver.prototype.getPageSource,
+  "WebDriver:GetShadowRoot": GeckoDriver.prototype.getShadowRoot,
+  "WebDriver:GetTimeouts": GeckoDriver.prototype.getTimeouts,
+  "WebDriver:GetTitle": GeckoDriver.prototype.getTitle,
+  "WebDriver:GetWindowHandle": GeckoDriver.prototype.getWindowHandle,
+  "WebDriver:GetWindowHandles": GeckoDriver.prototype.getWindowHandles,
+  "WebDriver:GetWindowRect": GeckoDriver.prototype.getWindowRect,
+  "WebDriver:IsElementDisplayed": GeckoDriver.prototype.isElementDisplayed,
+  "WebDriver:IsElementEnabled": GeckoDriver.prototype.isElementEnabled,
+  "WebDriver:IsElementSelected": GeckoDriver.prototype.isElementSelected,
+  "WebDriver:MinimizeWindow": GeckoDriver.prototype.minimizeWindow,
+  "WebDriver:MaximizeWindow": GeckoDriver.prototype.maximizeWindow,
+  "WebDriver:Navigate": GeckoDriver.prototype.navigateTo,
+  "WebDriver:NewSession": GeckoDriver.prototype.newSession,
+  "WebDriver:NewWindow": GeckoDriver.prototype.newWindow,
+  "WebDriver:PerformActions": GeckoDriver.prototype.performActions,
+  "WebDriver:Print": GeckoDriver.prototype.print,
+  "WebDriver:Refresh": GeckoDriver.prototype.refresh,
+  "WebDriver:ReleaseActions": GeckoDriver.prototype.releaseActions,
+  "WebDriver:SendAlertText": GeckoDriver.prototype.sendKeysToDialog,
+  "WebDriver:SetPermission": GeckoDriver.prototype.setPermission,
+  "WebDriver:SetTimeouts": GeckoDriver.prototype.setTimeouts,
+  "WebDriver:SetWindowRect": GeckoDriver.prototype.setWindowRect,
+  "WebDriver:SwitchToFrame": GeckoDriver.prototype.switchToFrame,
+  "WebDriver:SwitchToParentFrame": GeckoDriver.prototype.switchToParentFrame,
+  "WebDriver:SwitchToWindow": GeckoDriver.prototype.switchToWindow,
+  "WebDriver:TakeScreenshot": GeckoDriver.prototype.takeScreenshot,
+};
+
+async function exitFullscreen(win) {
+  let cb;
+  // Use a timed promise to abort if no window manager is present
+  await new lazy.TimedPromise(
+    resolve => {
+      cb = new lazy.DebounceCallback(resolve);
+      win.addEventListener("sizemodechange", cb);
+      win.fullScreen = false;
+    },
+    { throws: null, timeout: TIMEOUT_NO_WINDOW_MANAGER }
+  );
+  win.removeEventListener("sizemodechange", cb);
+  await new lazy.IdlePromise(win);
+}
+
+async function restoreWindow(win) {
+  let cb;
+  if (lazy.WindowState.from(win.windowState) == lazy.WindowState.Normal) {
+    return;
+  }
+  // Use a timed promise to abort if no window manager is present
+  await new lazy.TimedPromise(
+    resolve => {
+      cb = new lazy.DebounceCallback(resolve);
+      win.addEventListener("sizemodechange", cb);
+      win.restore();
+    },
+    { throws: null, timeout: TIMEOUT_NO_WINDOW_MANAGER }
+  );
+  win.removeEventListener("sizemodechange", cb);
+  await new lazy.IdlePromise(win);
+}
diff --git a/remote/marionette/element.sys.mjs b/remote/marionette/element.sys.mjs
new file mode 100644
index 0000000000..131e69ebcd
--- /dev/null
+++ b/remote/marionette/element.sys.mjs
@@ -0,0 +1,1620 @@
+/* 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, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  atom: "chrome://remote/content/marionette/atom.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  generateUUID: "chrome://remote/content/shared/UUID.sys.mjs",
+  PollPromise: "chrome://remote/content/marionette/sync.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
+const ORDERED_NODE_ITERATOR_TYPE = 5;
+const FIRST_ORDERED_NODE_TYPE = 9;
+
+const DOCUMENT_FRAGMENT_NODE = 11;
+const ELEMENT_NODE = 1;
+
+const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+/** XUL elements that support checked property. */
+const XUL_CHECKED_ELS = new Set(["button", "checkbox", "toolbarbutton"]);
+
+/** XUL elements that support selected property. */
+const XUL_SELECTED_ELS = new Set([
+  "menu",
+  "menuitem",
+  "menuseparator",
+  "radio",
+  "richlistitem",
+  "tab",
+]);
+
+/**
+ * This module provides shared functionality for dealing with DOM-
+ * and web elements in Marionette.
+ *
+ * A web element is an abstraction used to identify an element when it
+ * is transported across the protocol, between remote- and local ends.
+ *
+ * Each element has an associated web element reference (a UUID) that
+ * uniquely identifies the the element across all browsing contexts. The
+ * web element reference for every element representing the same element
+ * is the same.
+ *
+ * @namespace
+ */
+export const element = {};
+
+element.Strategy = {
+  ClassName: "class name",
+  Selector: "css selector",
+  ID: "id",
+  Name: "name",
+  LinkText: "link text",
+  PartialLinkText: "partial link text",
+  TagName: "tag name",
+  XPath: "xpath",
+};
+
+/**
+ * Find a single element or a collection of elements starting at the
+ * document root or a given node.
+ *
+ * If |timeout| is above 0, an implicit search technique is used.
+ * This will wait for the duration of <var>timeout</var> for the
+ * element to appear in the DOM.
+ *
+ * See the {@link element.Strategy} enum for a full list of supported
+ * search strategies that can be passed to <var>strategy</var>.
+ *
+ * @param {Object<string, WindowProxy>} container
+ *     Window object.
+ * @param {string} strategy
+ *     Search strategy whereby to locate the element(s).
+ * @param {string} selector
+ *     Selector search pattern.  The selector must be compatible with
+ *     the chosen search <var>strategy</var>.
+ * @param {object=} options
+ * @param {boolean=} options.all
+ *     If true, a multi-element search selector is used and a sequence of
+ *     elements will be returned, otherwise a single element. Defaults to false.
+ * @param {Element=} options.startNode
+ *     Element to use as the root of the search.
+ * @param {number=} options.timeout
+ *     Duration to wait before timing out the search.  If <code>all</code>
+ *     is false, a {@link NoSuchElementError} is thrown if unable to
+ *     find the element within the timeout duration.
+ *
+ * @returns {Promise.<(Element|Array.<Element>)>}
+ *     Single element or a sequence of elements.
+ *
+ * @throws InvalidSelectorError
+ *     If <var>strategy</var> is unknown.
+ * @throws InvalidSelectorError
+ *     If <var>selector</var> is malformed.
+ * @throws NoSuchElementError
+ *     If a single element is requested, this error will throw if the
+ *     element is not found.
+ */
+element.find = function (container, strategy, selector, options = {}) {
+  const { all = false, startNode, timeout = 0 } = options;
+
+  let searchFn;
+  if (all) {
+    searchFn = findElements.bind(this);
+  } else {
+    searchFn = findElement.bind(this);
+  }
+
+  return new Promise((resolve, reject) => {
+    let findElements = new lazy.PollPromise(
+      (resolve, reject) => {
+        let res = find_(container, strategy, selector, searchFn, {
+          all,
+          startNode,
+        });
+        if (res.length) {
+          resolve(Array.from(res));
+        } else {
+          reject([]);
+        }
+      },
+      { timeout }
+    );
+
+    findElements.then(foundEls => {
+      // the following code ought to be moved into findElement
+      // and findElements when bug 1254486 is addressed
+      if (!all && (!foundEls || !foundEls.length)) {
+        let msg = `Unable to locate element: ${selector}`;
+        reject(new lazy.error.NoSuchElementError(msg));
+      }
+
+      if (all) {
+        resolve(foundEls);
+      }
+      resolve(foundEls[0]);
+    }, reject);
+  });
+};
+
+function find_(
+  container,
+  strategy,
+  selector,
+  searchFn,
+  { startNode = null, all = false } = {}
+) {
+  let rootNode;
+
+  if (element.isShadowRoot(startNode)) {
+    rootNode = startNode.ownerDocument;
+  } else {
+    rootNode = container.frame.document;
+  }
+
+  if (!startNode) {
+    startNode = rootNode;
+  }
+
+  let res;
+  try {
+    res = searchFn(strategy, selector, rootNode, startNode);
+  } catch (e) {
+    throw new lazy.error.InvalidSelectorError(
+      `Given ${strategy} expression "${selector}" is invalid: ${e}`
+    );
+  }
+
+  if (res) {
+    if (all) {
+      return res;
+    }
+    return [res];
+  }
+  return [];
+}
+
+/**
+ * Find a single element by XPath expression.
+ *
+ * @param {Document} document
+ *     Document root.
+ * @param {Element} startNode
+ *     Where in the DOM hiearchy to begin searching.
+ * @param {string} expression
+ *     XPath search expression.
+ *
+ * @returns {Node}
+ *     First element matching <var>expression</var>.
+ */
+element.findByXPath = function (document, startNode, expression) {
+  let iter = document.evaluate(
+    expression,
+    startNode,
+    null,
+    FIRST_ORDERED_NODE_TYPE,
+    null
+  );
+  return iter.singleNodeValue;
+};
+
+/**
+ * Find elements by XPath expression.
+ *
+ * @param {Document} document
+ *     Document root.
+ * @param {Element} startNode
+ *     Where in the DOM hierarchy to begin searching.
+ * @param {string} expression
+ *     XPath search expression.
+ *
+ * @returns {Iterable.<Node>}
+ *     Iterator over nodes matching <var>expression</var>.
+ */
+element.findByXPathAll = function* (document, startNode, expression) {
+  let iter = document.evaluate(
+    expression,
+    startNode,
+    null,
+    ORDERED_NODE_ITERATOR_TYPE,
+    null
+  );
+  let el = iter.iterateNext();
+  while (el) {
+    yield el;
+    el = iter.iterateNext();
+  }
+};
+
+/**
+ * Find all hyperlinks descendant of <var>startNode</var> which
+ * link text is <var>linkText</var>.
+ *
+ * @param {Element} startNode
+ *     Where in the DOM hierarchy to begin searching.
+ * @param {string} linkText
+ *     Link text to search for.
+ *
+ * @returns {Iterable.<HTMLAnchorElement>}
+ *     Sequence of link elements which text is <var>s</var>.
+ */
+element.findByLinkText = function (startNode, linkText) {
+  return filterLinks(
+    startNode,
+    link => lazy.atom.getElementText(link).trim() === linkText
+  );
+};
+
+/**
+ * Find all hyperlinks descendant of <var>startNode</var> which
+ * link text contains <var>linkText</var>.
+ *
+ * @param {Element} startNode
+ *     Where in the DOM hierachy to begin searching.
+ * @param {string} linkText
+ *     Link text to search for.
+ *
+ * @returns {Iterable.<HTMLAnchorElement>}
+ *     Iterator of link elements which text containins
+ *     <var>linkText</var>.
+ */
+element.findByPartialLinkText = function (startNode, linkText) {
+  return filterLinks(startNode, link =>
+    lazy.atom.getElementText(link).includes(linkText)
+  );
+};
+
+/**
+ * Filters all hyperlinks that are descendant of <var>startNode</var>
+ * by <var>predicate</var>.
+ *
+ * @param {Element} startNode
+ *     Where in the DOM hierarchy to begin searching.
+ * @param {function(HTMLAnchorElement): boolean} predicate
+ *     Function that determines if given link should be included in
+ *     return value or filtered away.
+ *
+ * @returns {Iterable.<HTMLAnchorElement>}
+ *     Iterator of link elements matching <var>predicate</var>.
+ */
+function* filterLinks(startNode, predicate) {
+  const links = getLinks(startNode);
+
+  for (const link of links) {
+    if (predicate(link)) {
+      yield link;
+    }
+  }
+}
+
+/**
+ * Finds a single element.
+ *
+ * @param {element.Strategy} strategy
+ *     Selector strategy to use.
+ * @param {string} selector
+ *     Selector expression.
+ * @param {Document} document
+ *     Document root.
+ * @param {Element=} startNode
+ *     Optional Element from which to start searching.
+ *
+ * @returns {Element}
+ *     Found element.
+ *
+ * @throws {InvalidSelectorError}
+ *     If strategy <var>using</var> is not recognised.
+ * @throws {Error}
+ *     If selector expression <var>selector</var> is malformed.
+ */
+function findElement(strategy, selector, document, startNode = undefined) {
+  switch (strategy) {
+    case element.Strategy.ID: {
+      if (startNode.getElementById) {
+        return startNode.getElementById(selector);
+      }
+      let expr = `.//*[@id="${selector}"]`;
+      return element.findByXPath(document, startNode, expr);
+    }
+
+    case element.Strategy.Name: {
+      if (startNode.getElementsByName) {
+        return startNode.getElementsByName(selector)[0];
+      }
+      let expr = `.//*[@name="${selector}"]`;
+      return element.findByXPath(document, startNode, expr);
+    }
+
+    case element.Strategy.ClassName:
+      return startNode.getElementsByClassName(selector)[0];
+
+    case element.Strategy.TagName:
+      return startNode.getElementsByTagName(selector)[0];
+
+    case element.Strategy.XPath:
+      return element.findByXPath(document, startNode, selector);
+
+    case element.Strategy.LinkText: {
+      const links = getLinks(startNode);
+      for (const link of links) {
+        if (lazy.atom.getElementText(link).trim() === selector) {
+          return link;
+        }
+      }
+      return undefined;
+    }
+
+    case element.Strategy.PartialLinkText: {
+      const links = getLinks(startNode);
+      for (const link of links) {
+        if (lazy.atom.getElementText(link).includes(selector)) {
+          return link;
+        }
+      }
+      return undefined;
+    }
+
+    case element.Strategy.Selector:
+      try {
+        return startNode.querySelector(selector);
+      } catch (e) {
+        throw new lazy.error.InvalidSelectorError(
+          `${e.message}: "${selector}"`
+        );
+      }
+  }
+
+  throw new lazy.error.InvalidSelectorError(`No such strategy: ${strategy}`);
+}
+
+/**
+ * Find multiple elements.
+ *
+ * @param {element.Strategy} strategy
+ *     Selector strategy to use.
+ * @param {string} selector
+ *     Selector expression.
+ * @param {Document} document
+ *     Document root.
+ * @param {Element=} startNode
+ *     Optional Element from which to start searching.
+ *
+ * @returns {Array.<Element>}
+ *     Found elements.
+ *
+ * @throws {InvalidSelectorError}
+ *     If strategy <var>strategy</var> is not recognised.
+ * @throws {Error}
+ *     If selector expression <var>selector</var> is malformed.
+ */
+function findElements(strategy, selector, document, startNode = undefined) {
+  switch (strategy) {
+    case element.Strategy.ID:
+      selector = `.//*[@id="${selector}"]`;
+
+    // fall through
+    case element.Strategy.XPath:
+      return [...element.findByXPathAll(document, startNode, selector)];
+
+    case element.Strategy.Name:
+      if (startNode.getElementsByName) {
+        return startNode.getElementsByName(selector);
+      }
+      return [
+        ...element.findByXPathAll(
+          document,
+          startNode,
+          `.//*[@name="${selector}"]`
+        ),
+      ];
+
+    case element.Strategy.ClassName:
+      return startNode.getElementsByClassName(selector);
+
+    case element.Strategy.TagName:
+      return startNode.getElementsByTagName(selector);
+
+    case element.Strategy.LinkText:
+      return [...element.findByLinkText(startNode, selector)];
+
+    case element.Strategy.PartialLinkText:
+      return [...element.findByPartialLinkText(startNode, selector)];
+
+    case element.Strategy.Selector:
+      return startNode.querySelectorAll(selector);
+
+    default:
+      throw new lazy.error.InvalidSelectorError(
+        `No such strategy: ${strategy}`
+      );
+  }
+}
+
+function getLinks(startNode) {
+  // DocumentFragment doesn't have `getElementsByTagName` so using `querySelectorAll`.
+  if (element.isShadowRoot(startNode)) {
+    return startNode.querySelectorAll("a");
+  }
+  return startNode.getElementsByTagName("a");
+}
+
+/**
+ * Finds the closest parent node of <var>startNode</var> matching a CSS
+ * <var>selector</var> expression.
+ *
+ * @param {Node} startNode
+ *     Cycle through <var>startNode</var>'s parent nodes in tree-order
+ *     and return the first match to <var>selector</var>.
+ * @param {string} selector
+ *     CSS selector expression.
+ *
+ * @returns {Node=}
+ *     First match to <var>selector</var>, or null if no match was found.
+ */
+element.findClosest = function (startNode, selector) {
+  let node = startNode;
+  while (node.parentNode && node.parentNode.nodeType == ELEMENT_NODE) {
+    node = node.parentNode;
+    if (node.matches(selector)) {
+      return node;
+    }
+  }
+  return null;
+};
+
+/**
+ * Resolve element from specified web reference identifier.
+ *
+ * @param {BrowsingContext} browsingContext
+ *     The browsing context to retrieve the element from.
+ * @param {string} nodeId
+ *     The WebReference uuid for a DOM element.
+ * @param {NodeCache} nodeCache
+ *     Node cache that holds already seen WebElement and ShadowRoot references.
+ *
+ * @returns {Element}
+ *     The DOM element that the identifier was generated for.
+ *
+ * @throws {NoSuchElementError}
+ *     If the element doesn't exist in the current browsing context.
+ * @throws {StaleElementReferenceError}
+ *     If the element has gone stale, indicating its node document is no
+ *     longer the active document or it is no longer attached to the DOM.
+ */
+element.getKnownElement = function (browsingContext, nodeId, nodeCache) {
+  if (!element.isNodeReferenceKnown(browsingContext, nodeId, nodeCache)) {
+    throw new lazy.error.NoSuchElementError(
+      `The element with the reference ${nodeId} is not known in the current browsing context`
+    );
+  }
+
+  const node = nodeCache.getNode(browsingContext, nodeId);
+
+  // Ensure the node is of the correct Node type.
+  if (node !== null && !element.isElement(node)) {
+    throw new lazy.error.NoSuchElementError(
+      `The element with the reference ${nodeId} is not of type HTMLElement`
+    );
+  }
+
+  // If null, which may be the case if the element has been unwrapped from a
+  // weak reference, it is always considered stale.
+  if (node === null || element.isStale(node)) {
+    throw new lazy.error.StaleElementReferenceError(
+      `The element with the reference ${nodeId} ` +
+        "is stale; either its node document is not the active document, " +
+        "or it is no longer connected to the DOM"
+    );
+  }
+
+  return node;
+};
+
+/**
+ * Resolve ShadowRoot from specified web reference identifier.
+ *
+ * @param {BrowsingContext} browsingContext
+ *     The browsing context to retrieve the shadow root from.
+ * @param {string} nodeId
+ *     The WebReference uuid for a ShadowRoot.
+ * @param {NodeCache} nodeCache
+ *     Node cache that holds already seen WebElement and ShadowRoot references.
+ *
+ * @returns {ShadowRoot}
+ *     The ShadowRoot that the identifier was generated for.
+ *
+ * @throws {NoSuchShadowRootError}
+ *     If the ShadowRoot doesn't exist in the current browsing context.
+ * @throws {DetachedShadowRootError}
+ *     If the ShadowRoot is detached, indicating its node document is no
+ *     longer the active document or it is no longer attached to the DOM.
+ */
+element.getKnownShadowRoot = function (browsingContext, nodeId, nodeCache) {
+  if (!element.isNodeReferenceKnown(browsingContext, nodeId, nodeCache)) {
+    throw new lazy.error.NoSuchShadowRootError(
+      `The shadow root with the reference ${nodeId} is not known in the current browsing context`
+    );
+  }
+
+  const node = nodeCache.getNode(browsingContext, nodeId);
+
+  // Ensure the node is of the correct Node type.
+  if (node !== null && !element.isShadowRoot(node)) {
+    throw new lazy.error.NoSuchShadowRootError(
+      `The shadow root with the reference ${nodeId} is not of type ShadowRoot`
+    );
+  }
+
+  // If null, which may be the case if the element has been unwrapped from a
+  // weak reference, it is always considered stale.
+  if (node === null || element.isDetached(node)) {
+    throw new lazy.error.DetachedShadowRootError(
+      `The shadow root with the reference ${nodeId} ` +
+        "is detached; either its node document is not the active document, " +
+        "or it is no longer connected to the DOM"
+    );
+  }
+
+  return node;
+};
+
+/**
+ * Determines if <var>obj<var> is an HTML or JS collection.
+ *
+ * @param {object} seq
+ *     Type to determine.
+ *
+ * @returns {boolean}
+ *     True if <var>seq</va> is a collection.
+ */
+element.isCollection = function (seq) {
+  switch (Object.prototype.toString.call(seq)) {
+    case "[object Arguments]":
+    case "[object Array]":
+    case "[object DOMTokenList]":
+    case "[object FileList]":
+    case "[object HTMLAllCollection]":
+    case "[object HTMLCollection]":
+    case "[object HTMLFormControlsCollection]":
+    case "[object HTMLOptionsCollection]":
+    case "[object NodeList]":
+      return true;
+
+    default:
+      return false;
+  }
+};
+
+/**
+ * Determines if <var>shadowRoot</var> is detached.
+ *
+ * A ShadowRoot is detached if its node document is not the active document
+ * or if the element node referred to as its host is stale.
+ *
+ * @param {ShadowRoot} shadowRoot
+ *     ShadowRoot to check for detached state.
+ *
+ * @returns {boolean}
+ *     True if <var>shadowRoot</var> is detached, false otherwise.
+ */
+element.isDetached = function (shadowRoot) {
+  return (
+    !shadowRoot.ownerDocument.isActive() || element.isStale(shadowRoot.host)
+  );
+};
+
+/**
+ * Determines if the node reference is known for the given browsing context.
+ *
+ * For WebDriver classic only nodes from the same browsing context are
+ * allowed to be accessed.
+ *
+ * @param {BrowsingContext} browsingContext
+ *     The browsing context the element has to be part of.
+ * @param {ElementIdentifier} nodeId
+ *     The WebElement reference identifier for a DOM element.
+ * @param {NodeCache} nodeCache
+ *     Node cache that holds already seen node references.
+ *
+ * @returns {boolean}
+ *     True if the element is known in the given browsing context.
+ */
+element.isNodeReferenceKnown = function (browsingContext, nodeId, nodeCache) {
+  const nodeDetails = nodeCache.getReferenceDetails(nodeId);
+  if (nodeDetails === null) {
+    return false;
+  }
+
+  if (nodeDetails.isTopBrowsingContext) {
+    // As long as Navigables are not available any cross-group navigation will
+    // cause a swap of the current top-level browsing context. The only unique
+    // identifier in such a case is the browser id the top-level browsing
+    // context actually lives in.
+    return nodeDetails.browserId === browsingContext.browserId;
+  }
+
+  return nodeDetails.browsingContextId === browsingContext.id;
+};
+
+/**
+ * Determines if <var>el</var> is stale.
+ *
+ * An element is stale if its node document is not the active document
+ * or if it is not connected.
+ *
+ * @param {Element} el
+ *     Element to check for staleness.
+ *
+ * @returns {boolean}
+ *     True if <var>el</var> is stale, false otherwise.
+ */
+element.isStale = function (el) {
+  if (!el.ownerGlobal) {
+    // Without a valid inner window the document is basically closed.
+    return true;
+  }
+
+  return !el.ownerDocument.isActive() || !el.isConnected;
+};
+
+/**
+ * Determine if <var>el</var> is selected or not.
+ *
+ * This operation only makes sense on
+ * <tt>&lt;input type=checkbox&gt;</tt>,
+ * <tt>&lt;input type=radio&gt;</tt>,
+ * and <tt>&gt;option&gt;</tt> elements.
+ *
+ * @param {Element} el
+ *     Element to test if selected.
+ *
+ * @returns {boolean}
+ *     True if element is selected, false otherwise.
+ */
+element.isSelected = function (el) {
+  if (!el) {
+    return false;
+  }
+
+  if (element.isXULElement(el)) {
+    if (XUL_CHECKED_ELS.has(el.tagName)) {
+      return el.checked;
+    } else if (XUL_SELECTED_ELS.has(el.tagName)) {
+      return el.selected;
+    }
+  } else if (element.isDOMElement(el)) {
+    if (el.localName == "input" && ["checkbox", "radio"].includes(el.type)) {
+      return el.checked;
+    } else if (el.localName == "option") {
+      return el.selected;
+    }
+  }
+
+  return false;
+};
+
+/**
+ * An element is considered read only if it is an
+ * <code>&lt;input&gt;</code> or <code>&lt;textarea&gt;</code>
+ * element whose <code>readOnly</code> content IDL attribute is set.
+ *
+ * @param {Element} el
+ *     Element to test is read only.
+ *
+ * @returns {boolean}
+ *     True if element is read only.
+ */
+element.isReadOnly = function (el) {
+  return (
+    element.isDOMElement(el) &&
+    ["input", "textarea"].includes(el.localName) &&
+    el.readOnly
+  );
+};
+
+/**
+ * An element is considered disabled if it is a an element
+ * that can be disabled, or it belongs to a container group which
+ * <code>disabled</code> content IDL attribute affects it.
+ *
+ * @param {Element} el
+ *     Element to test for disabledness.
+ *
+ * @returns {boolean}
+ *     True if element, or its container group, is disabled.
+ */
+element.isDisabled = function (el) {
+  if (!element.isDOMElement(el)) {
+    return false;
+  }
+
+  switch (el.localName) {
+    case "option":
+    case "optgroup":
+      if (el.disabled) {
+        return true;
+      }
+      let parent = element.findClosest(el, "optgroup,select");
+      return element.isDisabled(parent);
+
+    case "button":
+    case "input":
+    case "select":
+    case "textarea":
+      return el.disabled;
+
+    default:
+      return false;
+  }
+};
+
+/**
+ * Denotes elements that can be used for typing and clearing.
+ *
+ * Elements that are considered WebDriver-editable are non-readonly
+ * and non-disabled <code>&lt;input&gt;</code> elements in the Text,
+ * Search, URL, Telephone, Email, Password, Date, Month, Date and
+ * Time Local, Number, Range, Color, and File Upload states, and
+ * <code>&lt;textarea&gt;</code> elements.
+ *
+ * @param {Element} el
+ *     Element to test.
+ *
+ * @returns {boolean}
+ *     True if editable, false otherwise.
+ */
+element.isMutableFormControl = function (el) {
+  if (!element.isDOMElement(el)) {
+    return false;
+  }
+  if (element.isReadOnly(el) || element.isDisabled(el)) {
+    return false;
+  }
+
+  if (el.localName == "textarea") {
+    return true;
+  }
+
+  if (el.localName != "input") {
+    return false;
+  }
+
+  switch (el.type) {
+    case "color":
+    case "date":
+    case "datetime-local":
+    case "email":
+    case "file":
+    case "month":
+    case "number":
+    case "password":
+    case "range":
+    case "search":
+    case "tel":
+    case "text":
+    case "time":
+    case "url":
+    case "week":
+      return true;
+
+    default:
+      return false;
+  }
+};
+
+/**
+ * An editing host is a node that is either an HTML element with a
+ * <code>contenteditable</code> attribute, or the HTML element child
+ * of a document whose <code>designMode</code> is enabled.
+ *
+ * @param {Element} el
+ *     Element to determine if is an editing host.
+ *
+ * @returns {boolean}
+ *     True if editing host, false otherwise.
+ */
+element.isEditingHost = function (el) {
+  return (
+    element.isDOMElement(el) &&
+    (el.isContentEditable || el.ownerDocument.designMode == "on")
+  );
+};
+
+/**
+ * Determines if an element is editable according to WebDriver.
+ *
+ * An element is considered editable if it is not read-only or
+ * disabled, and one of the following conditions are met:
+ *
+ * <ul>
+ * <li>It is a <code>&lt;textarea&gt;</code> element.
+ *
+ * <li>It is an <code>&lt;input&gt;</code> element that is not of
+ * the <code>checkbox</code>, <code>radio</code>, <code>hidden</code>,
+ * <code>submit</code>, <code>button</code>, or <code>image</code> types.
+ *
+ * <li>It is content-editable.
+ *
+ * <li>It belongs to a document in design mode.
+ * </ul>
+ *
+ * @param {Element} el
+ *     Element to test if editable.
+ *
+ * @returns {boolean}
+ *     True if editable, false otherwise.
+ */
+element.isEditable = function (el) {
+  if (!element.isDOMElement(el)) {
+    return false;
+  }
+
+  if (element.isReadOnly(el) || element.isDisabled(el)) {
+    return false;
+  }
+
+  return element.isMutableFormControl(el) || element.isEditingHost(el);
+};
+
+/**
+ * This function generates a pair of coordinates relative to the viewport
+ * given a target element and coordinates relative to that element's
+ * top-left corner.
+ *
+ * @param {Node} node
+ *     Target node.
+ * @param {number=} xOffset
+ *     Horizontal offset relative to target's top-left corner.
+ *     Defaults to the centre of the target's bounding box.
+ * @param {number=} yOffset
+ *     Vertical offset relative to target's top-left corner.  Defaults to
+ *     the centre of the target's bounding box.
+ *
+ * @returns {Object<string, number>}
+ *     X- and Y coordinates.
+ *
+ * @throws TypeError
+ *     If <var>xOffset</var> or <var>yOffset</var> are not numbers.
+ */
+element.coordinates = function (
+  node,
+  xOffset = undefined,
+  yOffset = undefined
+) {
+  let box = node.getBoundingClientRect();
+
+  if (typeof xOffset == "undefined" || xOffset === null) {
+    xOffset = box.width / 2.0;
+  }
+  if (typeof yOffset == "undefined" || yOffset === null) {
+    yOffset = box.height / 2.0;
+  }
+
+  if (typeof yOffset != "number" || typeof xOffset != "number") {
+    throw new TypeError("Offset must be a number");
+  }
+
+  return {
+    x: box.left + xOffset,
+    y: box.top + yOffset,
+  };
+};
+
+/**
+ * This function returns true if the node is in the viewport.
+ *
+ * @param {Element} el
+ *     Target element.
+ * @param {number=} x
+ *     Horizontal offset relative to target.  Defaults to the centre of
+ *     the target's bounding box.
+ * @param {number=} y
+ *     Vertical offset relative to target.  Defaults to the centre of
+ *     the target's bounding box.
+ *
+ * @returns {boolean}
+ *     True if if <var>el</var> is in viewport, false otherwise.
+ */
+element.inViewport = function (el, x = undefined, y = undefined) {
+  let win = el.ownerGlobal;
+  let c = element.coordinates(el, x, y);
+  let vp = {
+    top: win.pageYOffset,
+    left: win.pageXOffset,
+    bottom: win.pageYOffset + win.innerHeight,
+    right: win.pageXOffset + win.innerWidth,
+  };
+
+  return (
+    vp.left <= c.x + win.pageXOffset &&
+    c.x + win.pageXOffset <= vp.right &&
+    vp.top <= c.y + win.pageYOffset &&
+    c.y + win.pageYOffset <= vp.bottom
+  );
+};
+
+/**
+ * Gets the element's container element.
+ *
+ * An element container is defined by the WebDriver
+ * specification to be an <tt>&lt;option&gt;</tt> element in a
+ * <a href="https://html.spec.whatwg.org/#concept-element-contexts">valid
+ * element context</a>, meaning that it has an ancestral element
+ * that is either <tt>&lt;datalist&gt;</tt> or <tt>&lt;select&gt;</tt>.
+ *
+ * If the element does not have a valid context, its container element
+ * is itself.
+ *
+ * @param {Element} el
+ *     Element to get the container of.
+ *
+ * @returns {Element}
+ *     Container element of <var>el</var>.
+ */
+element.getContainer = function (el) {
+  // Does <option> or <optgroup> have a valid context,
+  // meaning is it a child of <datalist> or <select>?
+  if (["option", "optgroup"].includes(el.localName)) {
+    return element.findClosest(el, "datalist,select") || el;
+  }
+
+  return el;
+};
+
+/**
+ * An element is in view if it is a member of its own pointer-interactable
+ * paint tree.
+ *
+ * This means an element is considered to be in view, but not necessarily
+ * pointer-interactable, if it is found somewhere in the
+ * <code>elementsFromPoint</code> list at <var>el</var>'s in-view
+ * centre coordinates.
+ *
+ * Before running the check, we change <var>el</var>'s pointerEvents
+ * style property to "auto", since elements without pointer events
+ * enabled do not turn up in the paint tree we get from
+ * document.elementsFromPoint.  This is a specialisation that is only
+ * relevant when checking if the element is in view.
+ *
+ * @param {Element} el
+ *     Element to check if is in view.
+ *
+ * @returns {boolean}
+ *     True if <var>el</var> is inside the viewport, or false otherwise.
+ */
+element.isInView = function (el) {
+  let originalPointerEvents = el.style.pointerEvents;
+
+  try {
+    el.style.pointerEvents = "auto";
+    const tree = element.getPointerInteractablePaintTree(el);
+
+    // Bug 1413493 - <tr> is not part of the returned paint tree yet. As
+    // workaround check the visibility based on the first contained cell.
+    if (el.localName === "tr" && el.cells && el.cells.length) {
+      return tree.includes(el.cells[0]);
+    }
+
+    return tree.includes(el);
+  } finally {
+    el.style.pointerEvents = originalPointerEvents;
+  }
+};
+
+/**
+ * This function throws the visibility of the element error if the element is
+ * not displayed or the given coordinates are not within the viewport.
+ *
+ * @param {Element} el
+ *     Element to check if visible.
+ * @param {number=} x
+ *     Horizontal offset relative to target.  Defaults to the centre of
+ *     the target's bounding box.
+ * @param {number=} y
+ *     Vertical offset relative to target.  Defaults to the centre of
+ *     the target's bounding box.
+ *
+ * @returns {boolean}
+ *     True if visible, false otherwise.
+ */
+element.isVisible = function (el, x = undefined, y = undefined) {
+  let win = el.ownerGlobal;
+
+  if (!lazy.atom.isElementDisplayed(el, win)) {
+    return false;
+  }
+
+  if (el.tagName.toLowerCase() == "body") {
+    return true;
+  }
+
+  if (!element.inViewport(el, x, y)) {
+    element.scrollIntoView(el);
+    if (!element.inViewport(el)) {
+      return false;
+    }
+  }
+  return true;
+};
+
+/**
+ * A pointer-interactable element is defined to be the first
+ * non-transparent element, defined by the paint order found at the centre
+ * point of its rectangle that is inside the viewport, excluding the size
+ * of any rendered scrollbars.
+ *
+ * An element is obscured if the pointer-interactable paint tree at its
+ * centre point is empty, or the first element in this tree is not an
+ * inclusive descendant of itself.
+ *
+ * @param {DOMElement} el
+ *     Element determine if is pointer-interactable.
+ *
+ * @returns {boolean}
+ *     True if element is obscured, false otherwise.
+ */
+element.isObscured = function (el) {
+  let tree = element.getPointerInteractablePaintTree(el);
+  return !el.contains(tree[0]);
+};
+
+// TODO(ato): Only used by deprecated action API
+// https://bugzil.la/1354578
+/**
+ * Calculates the in-view centre point of an element's client rect.
+ *
+ * The portion of an element that is said to be _in view_, is the
+ * intersection of two squares: the first square being the initial
+ * viewport, and the second a DOM element.  From this square we
+ * calculate the in-view _centre point_ and convert it into CSS pixels.
+ *
+ * Although Gecko's system internals allow click points to be
+ * given in floating point precision, the DOM operates in CSS pixels.
+ * When the in-view centre point is later used to retrieve a coordinate's
+ * paint tree, we need to ensure to operate in the same language.
+ *
+ * As a word of warning, there appears to be inconsistencies between
+ * how `DOMElement.elementsFromPoint` and `DOMWindowUtils.sendMouseEvent`
+ * internally rounds (ceils/floors) coordinates.
+ *
+ * @param {DOMRect} rect
+ *     Element off a DOMRect sequence produced by calling
+ *     `getClientRects` on an {@link Element}.
+ * @param {WindowProxy} win
+ *     Current window global.
+ *
+ * @returns {Map.<string, number>}
+ *     X and Y coordinates that denotes the in-view centre point of
+ *     `rect`.
+ */
+element.getInViewCentrePoint = function (rect, win) {
+  const { floor, max, min } = Math;
+
+  // calculate the intersection of the rect that is inside the viewport
+  let visible = {
+    left: max(0, min(rect.x, rect.x + rect.width)),
+    right: min(win.innerWidth, max(rect.x, rect.x + rect.width)),
+    top: max(0, min(rect.y, rect.y + rect.height)),
+    bottom: min(win.innerHeight, max(rect.y, rect.y + rect.height)),
+  };
+
+  // arrive at the centre point of the visible rectangle
+  let x = (visible.left + visible.right) / 2.0;
+  let y = (visible.top + visible.bottom) / 2.0;
+
+  // convert to CSS pixels, as centre point can be float
+  x = floor(x);
+  y = floor(y);
+
+  return { x, y };
+};
+
+/**
+ * Produces a pointer-interactable elements tree from a given element.
+ *
+ * The tree is defined by the paint order found at the centre point of
+ * the element's rectangle that is inside the viewport, excluding the size
+ * of any rendered scrollbars.
+ *
+ * @param {DOMElement} el
+ *     Element to determine if is pointer-interactable.
+ *
+ * @returns {Array.<DOMElement>}
+ *     Sequence of elements in paint order.
+ */
+element.getPointerInteractablePaintTree = function (el) {
+  const doc = el.ownerDocument;
+  const win = doc.defaultView;
+  const rootNode = el.getRootNode();
+
+  // pointer-interactable elements tree, step 1
+  if (!el.isConnected) {
+    return [];
+  }
+
+  // steps 2-3
+  let rects = el.getClientRects();
+  if (!rects.length) {
+    return [];
+  }
+
+  // step 4
+  let centre = element.getInViewCentrePoint(rects[0], win);
+
+  // step 5
+  return rootNode.elementsFromPoint(centre.x, centre.y);
+};
+
+// TODO(ato): Not implemented.
+// In fact, it's not defined in the spec.
+element.isKeyboardInteractable = () => true;
+
+/**
+ * Attempts to scroll into view |el|.
+ *
+ * @param {DOMElement} el
+ *     Element to scroll into view.
+ */
+element.scrollIntoView = function (el) {
+  if (el.scrollIntoView) {
+    el.scrollIntoView({ block: "end", inline: "nearest" });
+  }
+};
+
+/**
+ * Ascertains whether <var>obj</var> is a DOM-, SVG-, or XUL element.
+ *
+ * @param {object} obj
+ *     Object thought to be an <code>Element</code> or
+ *     <code>XULElement</code>.
+ *
+ * @returns {boolean}
+ *     True if <var>obj</var> is an element, false otherwise.
+ */
+element.isElement = function (obj) {
+  return element.isDOMElement(obj) || element.isXULElement(obj);
+};
+
+/**
+ * Returns the shadow root of an element.
+ *
+ * @param {Element} el
+ *     Element thought to have a <code>shadowRoot</code>
+ * @returns {ShadowRoot}
+ *     Shadow root of the element.
+ */
+element.getShadowRoot = function (el) {
+  const shadowRoot = el.openOrClosedShadowRoot;
+  if (!shadowRoot) {
+    throw new lazy.error.NoSuchShadowRootError();
+  }
+  return shadowRoot;
+};
+
+/**
+ * Ascertains whether <var>node</var> is a shadow root.
+ *
+ * @param {ShadowRoot} node
+ *   The node that will be checked to see if it has a shadow root
+ *
+ * @returns {boolean}
+ *     True if <var>node</var> is a shadow root, false otherwise.
+ */
+element.isShadowRoot = function (node) {
+  return (
+    node &&
+    node.nodeType === DOCUMENT_FRAGMENT_NODE &&
+    node.containingShadowRoot == node
+  );
+};
+
+/**
+ * Ascertains whether <var>obj</var> is a DOM element.
+ *
+ * @param {object} obj
+ *     Object to check.
+ *
+ * @returns {boolean}
+ *     True if <var>obj</var> is a DOM element, false otherwise.
+ */
+element.isDOMElement = function (obj) {
+  return obj && obj.nodeType == ELEMENT_NODE && !element.isXULElement(obj);
+};
+
+/**
+ * Ascertains whether <var>obj</var> is a XUL element.
+ *
+ * @param {object} obj
+ *     Object to check.
+ *
+ * @returns {boolean}
+ *     True if <var>obj</var> is a XULElement, false otherwise.
+ */
+element.isXULElement = function (obj) {
+  return obj && obj.nodeType === ELEMENT_NODE && obj.namespaceURI === XUL_NS;
+};
+
+/**
+ * Ascertains whether <var>node</var> is in a privileged document.
+ *
+ * @param {Node} node
+ *     Node to check.
+ *
+ * @returns {boolean}
+ *     True if <var>node</var> is in a privileged document,
+ *     false otherwise.
+ */
+element.isInPrivilegedDocument = function (node) {
+  return !!node?.nodePrincipal?.isSystemPrincipal;
+};
+
+/**
+ * Ascertains whether <var>obj</var> is a <code>WindowProxy</code>.
+ *
+ * @param {object} obj
+ *     Object to check.
+ *
+ * @returns {boolean}
+ *     True if <var>obj</var> is a DOM window.
+ */
+element.isDOMWindow = function (obj) {
+  // TODO(ato): This should use Object.prototype.toString.call(node)
+  // but it's not clear how to write a good xpcshell test for that,
+  // seeing as we stub out a WindowProxy.
+  return (
+    typeof obj == "object" &&
+    obj !== null &&
+    typeof obj.toString == "function" &&
+    obj.toString() == "[object Window]" &&
+    obj.self === obj
+  );
+};
+
+const boolEls = {
+  audio: ["autoplay", "controls", "loop", "muted"],
+  button: ["autofocus", "disabled", "formnovalidate"],
+  details: ["open"],
+  dialog: ["open"],
+  fieldset: ["disabled"],
+  form: ["novalidate"],
+  iframe: ["allowfullscreen"],
+  img: ["ismap"],
+  input: [
+    "autofocus",
+    "checked",
+    "disabled",
+    "formnovalidate",
+    "multiple",
+    "readonly",
+    "required",
+  ],
+  keygen: ["autofocus", "disabled"],
+  menuitem: ["checked", "default", "disabled"],
+  ol: ["reversed"],
+  optgroup: ["disabled"],
+  option: ["disabled", "selected"],
+  script: ["async", "defer"],
+  select: ["autofocus", "disabled", "multiple", "required"],
+  textarea: ["autofocus", "disabled", "readonly", "required"],
+  track: ["default"],
+  video: ["autoplay", "controls", "loop", "muted"],
+};
+
+/**
+ * Tests if the attribute is a boolean attribute on element.
+ *
+ * @param {Element} el
+ *     Element to test if <var>attr</var> is a boolean attribute on.
+ * @param {string} attr
+ *     Attribute to test is a boolean attribute.
+ *
+ * @returns {boolean}
+ *     True if the attribute is boolean, false otherwise.
+ */
+element.isBooleanAttribute = function (el, attr) {
+  if (!element.isDOMElement(el)) {
+    return false;
+  }
+
+  // global boolean attributes that apply to all HTML elements,
+  // except for custom elements
+  const customElement = !el.localName.includes("-");
+  if ((attr == "hidden" || attr == "itemscope") && customElement) {
+    return true;
+  }
+
+  if (!boolEls.hasOwnProperty(el.localName)) {
+    return false;
+  }
+  return boolEls[el.localName].includes(attr);
+};
+
+/**
+ * A web reference is an abstraction used to identify an element when
+ * it is transported via the protocol, between remote- and local ends.
+ *
+ * In Marionette this abstraction can represent DOM elements,
+ * WindowProxies, and XUL elements.
+ */
+export class WebReference {
+  /**
+   * @param {string} uuid
+   *     Identifier that must be unique across all browsing contexts
+   *     for the contract to be upheld.
+   */
+  constructor(uuid) {
+    this.uuid = lazy.assert.string(uuid);
+  }
+
+  /**
+   * Performs an equality check between this web element and
+   * <var>other</var>.
+   *
+   * @param {WebReference} other
+   *     Web element to compare with this.
+   *
+   * @returns {boolean}
+   *     True if this and <var>other</var> are the same.  False
+   *     otherwise.
+   */
+  is(other) {
+    return other instanceof WebReference && this.uuid === other.uuid;
+  }
+
+  toString() {
+    return `[object ${this.constructor.name} uuid=${this.uuid}]`;
+  }
+
+  /**
+   * Returns a new {@link WebReference} reference for a DOM or XUL element,
+   * <code>WindowProxy</code>, or <code>ShadowRoot</code>.
+   *
+   * @param {(Element|ShadowRoot|WindowProxy|MockXULElement)} node
+   *     Node to construct a web element reference for.
+   * @param {string=} uuid
+   *     Optional unique identifier of the WebReference if already known.
+   *     If not defined a new unique identifier will be created.
+   *
+   * @returns {WebReference}
+   *     Web reference for <var>node</var>.
+   *
+   * @throws {InvalidArgumentError}
+   *     If <var>node</var> is neither a <code>WindowProxy</code>,
+   *     DOM or XUL element, or <code>ShadowRoot</code>.
+   */
+  static from(node, uuid) {
+    if (uuid === undefined) {
+      uuid = lazy.generateUUID();
+    }
+
+    if (element.isShadowRoot(node) && !element.isInPrivilegedDocument(node)) {
+      // When we support Chrome Shadowroots we will need to
+      // do a check here of shadowroot.host being in a privileged document
+      // See Bug 1743541
+      return new ShadowRoot(uuid);
+    } else if (element.isElement(node)) {
+      return new WebElement(uuid);
+    } else if (element.isDOMWindow(node)) {
+      if (node.parent === node) {
+        return new WebWindow(uuid);
+      }
+      return new WebFrame(uuid);
+    }
+
+    throw new lazy.error.InvalidArgumentError(
+      "Expected DOM window/element " + lazy.pprint`or XUL element, got: ${node}`
+    );
+  }
+
+  /**
+   * Unmarshals a JSON Object to one of {@link ShadowRoot}, {@link WebElement},
+   * {@link WebFrame}, or {@link WebWindow}.
+   *
+   * @param {Object<string, string>} json
+   *     Web reference, which is supposed to be a JSON Object
+   *     where the key is one of the {@link WebReference} concrete
+   *     classes' UUID identifiers.
+   *
+   * @returns {WebReference}
+   *     Web reference for the JSON object.
+   *
+   * @throws {InvalidArgumentError}
+   *     If <var>json</var> is not a web reference.
+   */
+  static fromJSON(json) {
+    lazy.assert.object(json);
+    if (json instanceof WebReference) {
+      return json;
+    }
+    let keys = Object.keys(json);
+
+    for (let key of keys) {
+      switch (key) {
+        case ShadowRoot.Identifier:
+          return ShadowRoot.fromJSON(json);
+
+        case WebElement.Identifier:
+          return WebElement.fromJSON(json);
+
+        case WebFrame.Identifier:
+          return WebFrame.fromJSON(json);
+
+        case WebWindow.Identifier:
+          return WebWindow.fromJSON(json);
+      }
+    }
+
+    throw new lazy.error.InvalidArgumentError(
+      lazy.pprint`Expected web reference, got: ${json}`
+    );
+  }
+
+  /**
+   * Checks if <var>obj<var> is a {@link WebReference} reference.
+   *
+   * @param {Object<string, string>} obj
+   *     Object that represents a {@link WebReference}.
+   *
+   * @returns {boolean}
+   *     True if <var>obj</var> is a {@link WebReference}, false otherwise.
+   */
+  static isReference(obj) {
+    if (Object.prototype.toString.call(obj) != "[object Object]") {
+      return false;
+    }
+
+    if (
+      ShadowRoot.Identifier in obj ||
+      WebElement.Identifier in obj ||
+      WebFrame.Identifier in obj ||
+      WebWindow.Identifier in obj
+    ) {
+      return true;
+    }
+    return false;
+  }
+}
+
+/**
+ * DOM elements are represented as web elements when they are
+ * transported over the wire protocol.
+ */
+export class WebElement extends WebReference {
+  toJSON() {
+    return { [WebElement.Identifier]: this.uuid };
+  }
+
+  static fromJSON(json) {
+    const { Identifier } = WebElement;
+
+    if (!(Identifier in json)) {
+      throw new lazy.error.InvalidArgumentError(
+        lazy.pprint`Expected web element reference, got: ${json}`
+      );
+    }
+
+    let uuid = json[Identifier];
+    return new WebElement(uuid);
+  }
+
+  /**
+   * Constructs a {@link WebElement} from a string <var>uuid</var>.
+   *
+   * This whole function is a workaround for the fact that clients
+   * to Marionette occasionally pass <code>{id: <uuid>}</code> JSON
+   * Objects instead of web element representations.
+   *
+   * @param {string} uuid
+   *     UUID to be associated with the web reference.
+   *
+   * @returns {WebElement}
+   *     The web element reference.
+   *
+   * @throws {InvalidArgumentError}
+   *     If <var>uuid</var> is not a string.
+   */
+  static fromUUID(uuid) {
+    return new WebElement(uuid);
+  }
+}
+
+WebElement.Identifier = "element-6066-11e4-a52e-4f735466cecf";
+
+/**
+ * Shadow Root elements are represented as shadow root references when they are
+ * transported over the wire protocol
+ */
+export class ShadowRoot extends WebReference {
+  toJSON() {
+    return { [ShadowRoot.Identifier]: this.uuid };
+  }
+
+  static fromJSON(json) {
+    const { Identifier } = ShadowRoot;
+
+    if (!(Identifier in json)) {
+      throw new lazy.error.InvalidArgumentError(
+        lazy.pprint`Expected shadow root reference, got: ${json}`
+      );
+    }
+
+    let uuid = json[Identifier];
+    return new ShadowRoot(uuid);
+  }
+
+  /**
+   * Constructs a {@link ShadowRoot} from a string <var>uuid</var>.
+   *
+   * This whole function is a workaround for the fact that clients
+   * to Marionette occasionally pass <code>{id: <uuid>}</code> JSON
+   * Objects instead of shadow root representations.
+   *
+   * @param {string} uuid
+   *     UUID to be associated with the web reference.
+   *
+   * @returns {ShadowRoot}
+   *     The shadow root reference.
+   *
+   * @throws {InvalidArgumentError}
+   *     If <var>uuid</var> is not a string.
+   */
+  static fromUUID(uuid) {
+    lazy.assert.string(uuid);
+
+    return new ShadowRoot(uuid);
+  }
+}
+
+ShadowRoot.Identifier = "shadow-6066-11e4-a52e-4f735466cecf";
+
+/**
+ * Top-level browsing contexts, such as <code>WindowProxy</code>
+ * whose <code>opener</code> is null, are represented as web windows
+ * over the wire protocol.
+ */
+export class WebWindow extends WebReference {
+  toJSON() {
+    return { [WebWindow.Identifier]: this.uuid };
+  }
+
+  static fromJSON(json) {
+    if (!(WebWindow.Identifier in json)) {
+      throw new lazy.error.InvalidArgumentError(
+        lazy.pprint`Expected web window reference, got: ${json}`
+      );
+    }
+    let uuid = json[WebWindow.Identifier];
+    return new WebWindow(uuid);
+  }
+}
+
+WebWindow.Identifier = "window-fcc6-11e5-b4f8-330a88ab9d7f";
+
+/**
+ * Nested browsing contexts, such as the <code>WindowProxy</code>
+ * associated with <tt>&lt;frame&gt;</tt> and <tt>&lt;iframe&gt;</tt>,
+ * are represented as web frames over the wire protocol.
+ */
+export class WebFrame extends WebReference {
+  toJSON() {
+    return { [WebFrame.Identifier]: this.uuid };
+  }
+
+  static fromJSON(json) {
+    if (!(WebFrame.Identifier in json)) {
+      throw new lazy.error.InvalidArgumentError(
+        lazy.pprint`Expected web frame reference, got: ${json}`
+      );
+    }
+    let uuid = json[WebFrame.Identifier];
+    return new WebFrame(uuid);
+  }
+}
+
+WebFrame.Identifier = "frame-075b-4da1-b6ba-e579c2d3230a";
diff --git a/remote/marionette/evaluate.sys.mjs b/remote/marionette/evaluate.sys.mjs
new file mode 100644
index 0000000000..bdcce779a1
--- /dev/null
+++ b/remote/marionette/evaluate.sys.mjs
@@ -0,0 +1,356 @@
+/* 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 { clearTimeout, setTimeout } from "resource://gre/modules/Timer.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+});
+
+const ARGUMENTS = "__webDriverArguments";
+const CALLBACK = "__webDriverCallback";
+const COMPLETE = "__webDriverComplete";
+const DEFAULT_TIMEOUT = 10000; // ms
+const FINISH = "finish";
+
+/** @namespace */
+export const evaluate = {};
+
+/**
+ * Evaluate a script in given sandbox.
+ *
+ * The the provided `script` will be wrapped in an anonymous function
+ * with the `args` argument applied.
+ *
+ * The arguments provided by the `args<` argument are exposed
+ * through the `arguments` object available in the script context,
+ * and if the script is executed asynchronously with the `async`
+ * option, an additional last argument that is synonymous to the
+ * name `resolve` is appended, and can be accessed
+ * through `arguments[arguments.length - 1]`.
+ *
+ * The `timeout` option specifies the duration for how long the
+ * script should be allowed to run before it is interrupted and aborted.
+ * An interrupted script will cause a {@link ScriptTimeoutError} to occur.
+ *
+ * The `async` option indicates that the script will not return
+ * until the `resolve` callback is invoked,
+ * which is analogous to the last argument of the `arguments` object.
+ *
+ * The `file` option is used in error messages to provide information
+ * on the origin script file in the local end.
+ *
+ * The `line` option is used in error messages, along with `filename`,
+ * to provide the line number in the origin script file on the local end.
+ *
+ * @param {nsISandbox} sb
+ *     Sandbox the script will be evaluted in.
+ * @param {string} script
+ *     Script to evaluate.
+ * @param {Array.<?>=} args
+ *     A sequence of arguments to call the script with.
+ * @param {object=} options
+ * @param {boolean=} options.async
+ *     Indicates if the script should return immediately or wait for
+ *     the callback to be invoked before returning. Defaults to false.
+ * @param {string=} options.file
+ *     File location of the program in the client. Defaults to "dummy file".
+ * @param {number=} options.line
+ *     Line number of the program in the client. Defaults to 0.
+ * @param {number=} options.timeout
+ *     Duration in milliseconds before interrupting the script. Defaults to
+ *     DEFAULT_TIMEOUT.
+ *
+ * @returns {Promise}
+ *     A promise that when resolved will give you the return value from
+ *     the script.  Note that the return value requires serialisation before
+ *     it can be sent to the client.
+ *
+ * @throws {JavaScriptError}
+ *   If an {@link Error} was thrown whilst evaluating the script.
+ * @throws {ScriptTimeoutError}
+ *   If the script was interrupted due to script timeout.
+ */
+evaluate.sandbox = function (
+  sb,
+  script,
+  args = [],
+  {
+    async = false,
+    file = "dummy file",
+    line = 0,
+    timeout = DEFAULT_TIMEOUT,
+  } = {}
+) {
+  let unloadHandler;
+  let marionetteSandbox = sandbox.create(sb.window);
+
+  // timeout handler
+  let scriptTimeoutID, timeoutPromise;
+  if (timeout !== null) {
+    timeoutPromise = new Promise((resolve, reject) => {
+      scriptTimeoutID = setTimeout(() => {
+        reject(
+          new lazy.error.ScriptTimeoutError(`Timed out after ${timeout} ms`)
+        );
+      }, timeout);
+    });
+  }
+
+  let promise = new Promise((resolve, reject) => {
+    let src = "";
+    sb[COMPLETE] = resolve;
+    sb[ARGUMENTS] = sandbox.cloneInto(args, sb);
+
+    // callback function made private
+    // so that introspection is possible
+    // on the arguments object
+    if (async) {
+      sb[CALLBACK] = sb[COMPLETE];
+      src += `${ARGUMENTS}.push(rv => ${CALLBACK}(rv));`;
+    }
+
+    src += `(function() {
+      ${script}
+    }).apply(null, ${ARGUMENTS})`;
+
+    unloadHandler = sandbox.cloneInto(
+      () => reject(new lazy.error.JavaScriptError("Document was unloaded")),
+      marionetteSandbox
+    );
+    marionetteSandbox.window.addEventListener("unload", unloadHandler);
+
+    let promises = [
+      Cu.evalInSandbox(
+        src,
+        sb,
+        "1.8",
+        file,
+        line,
+        /* enforceFilenameRestrictions */ false
+      ),
+      timeoutPromise,
+    ];
+
+    // Wait for the immediate result of calling evalInSandbox, or a timeout.
+    // Only resolve the promise if the scriptPromise was resolved and is not
+    // async, because the latter has to call resolve() itself.
+    Promise.race(promises).then(
+      value => {
+        if (!async) {
+          resolve(value);
+        }
+      },
+      err => {
+        reject(err);
+      }
+    );
+  });
+
+  // This block is mainly for async scripts, which escape the inner promise
+  // when calling resolve() on their own. The timeout promise will be re-used
+  // to break out after the initially setup timeout.
+  return Promise.race([promise, timeoutPromise])
+    .catch(err => {
+      // Only raise valid errors for both the sync and async scripts.
+      if (err instanceof lazy.error.ScriptTimeoutError) {
+        throw err;
+      }
+      throw new lazy.error.JavaScriptError(err);
+    })
+    .finally(() => {
+      clearTimeout(scriptTimeoutID);
+      marionetteSandbox.window.removeEventListener("unload", unloadHandler);
+    });
+};
+
+/**
+ * `Cu.isDeadWrapper` does not return true for a dead sandbox that
+ * was assosciated with and extension popup.  This provides a way to
+ * still test for a dead object.
+ *
+ * @param {object} obj
+ *     A potentially dead object.
+ * @param {string} prop
+ *     Name of a property on the object.
+ *
+ * @returns {boolean}
+ *     True if <var>obj</var> is dead, false otherwise.
+ */
+evaluate.isDead = function (obj, prop) {
+  try {
+    obj[prop];
+  } catch (e) {
+    if (e.message.includes("dead object")) {
+      return true;
+    }
+    throw e;
+  }
+  return false;
+};
+
+export const sandbox = {};
+
+/**
+ * Provides a safe way to take an object defined in a privileged scope and
+ * create a structured clone of it in a less-privileged scope.  It returns
+ * a reference to the clone.
+ *
+ * Unlike for {@link Components.utils.cloneInto}, `obj` may contain
+ * functions and DOM elements.
+ */
+sandbox.cloneInto = function (obj, sb) {
+  return Cu.cloneInto(obj, sb, { cloneFunctions: true, wrapReflectors: true });
+};
+
+/**
+ * Augment given sandbox by an adapter that has an `exports` map
+ * property, or a normal map, of function names and function references.
+ *
+ * @param {Sandbox} sb
+ *     The sandbox to augment.
+ * @param {object} adapter
+ *     Object that holds an `exports` property, or a map, of function
+ *     names and function references.
+ *
+ * @returns {Sandbox}
+ *     The augmented sandbox.
+ */
+sandbox.augment = function (sb, adapter) {
+  function* entries(obj) {
+    for (let key of Object.keys(obj)) {
+      yield [key, obj[key]];
+    }
+  }
+
+  let funcs = adapter.exports || entries(adapter);
+  for (let [name, func] of funcs) {
+    sb[name] = func;
+  }
+
+  return sb;
+};
+
+/**
+ * Creates a sandbox.
+ *
+ * @param {Window} win
+ *     The DOM Window object.
+ * @param {nsIPrincipal=} principal
+ *     An optional, custom principal to prefer over the Window.  Useful if
+ *     you need elevated security permissions.
+ *
+ * @returns {Sandbox}
+ *     The created sandbox.
+ */
+sandbox.create = function (win, principal = null, opts = {}) {
+  let p = principal || win;
+  opts = Object.assign(
+    {
+      sameZoneAs: win,
+      sandboxPrototype: win,
+      wantComponents: true,
+      wantXrays: true,
+      wantGlobalProperties: ["ChromeUtils"],
+    },
+    opts
+  );
+  return new Cu.Sandbox(p, opts);
+};
+
+/**
+ * Creates a mutable sandbox, where changes to the global scope
+ * will have lasting side-effects.
+ *
+ * @param {Window} win
+ *     The DOM Window object.
+ *
+ * @returns {Sandbox}
+ *     The created sandbox.
+ */
+sandbox.createMutable = function (win) {
+  let opts = {
+    wantComponents: false,
+    wantXrays: false,
+  };
+  // Note: We waive Xrays here to match potentially-accidental old behavior.
+  return Cu.waiveXrays(sandbox.create(win, null, opts));
+};
+
+sandbox.createSystemPrincipal = function (win) {
+  let principal = Cc["@mozilla.org/systemprincipal;1"].createInstance(
+    Ci.nsIPrincipal
+  );
+  return sandbox.create(win, principal);
+};
+
+sandbox.createSimpleTest = function (win, harness) {
+  let sb = sandbox.create(win);
+  sb = sandbox.augment(sb, harness);
+  sb[FINISH] = () => sb[COMPLETE](harness.generate_results());
+  return sb;
+};
+
+/**
+ * Sandbox storage.  When the user requests a sandbox by a specific name,
+ * if one exists in the storage this will be used as long as its window
+ * reference is still valid.
+ *
+ * @memberof evaluate
+ */
+export class Sandboxes {
+  /**
+   * @param {function(): Window} windowFn
+   *     A function that returns the references to the current Window
+   *     object.
+   */
+  constructor(windowFn) {
+    this.windowFn_ = windowFn;
+    this.boxes_ = new Map();
+  }
+
+  get window_() {
+    return this.windowFn_();
+  }
+
+  /**
+   * Factory function for getting a sandbox by name, or failing that,
+   * creating a new one.
+   *
+   * If the sandbox' window does not match the provided window, a new one
+   * will be created.
+   *
+   * @param {string} name
+   *     The name of the sandbox to get or create.
+   * @param {boolean=} [fresh=false] fresh
+   *     Remove old sandbox by name first, if it exists.
+   *
+   * @returns {Sandbox}
+   *     A used or fresh sandbox.
+   */
+  get(name = "default", fresh = false) {
+    let sb = this.boxes_.get(name);
+    if (sb) {
+      if (fresh || evaluate.isDead(sb, "window") || sb.window != this.window_) {
+        this.boxes_.delete(name);
+        return this.get(name, false);
+      }
+    } else {
+      if (name == "system") {
+        sb = sandbox.createSystemPrincipal(this.window_);
+      } else {
+        sb = sandbox.create(this.window_);
+      }
+      this.boxes_.set(name, sb);
+    }
+    return sb;
+  }
+
+  /** Clears cache of sandboxes. */
+  clear() {
+    this.boxes_.clear();
+  }
+}
diff --git a/remote/marionette/event.sys.mjs b/remote/marionette/event.sys.mjs
new file mode 100644
index 0000000000..cf5ff717a2
--- /dev/null
+++ b/remote/marionette/event.sys.mjs
@@ -0,0 +1,317 @@
+/* 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/. */
+
+/* eslint-disable no-restricted-globals */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  keyData: "chrome://remote/content/shared/webdriver/KeyData.sys.mjs",
+});
+
+/** Provides functionality for creating and sending DOM events. */
+export const event = {};
+
+XPCOMUtils.defineLazyGetter(lazy, "dblclickTimer", () => {
+  return Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+});
+
+const _eventUtils = new WeakMap();
+
+function _getEventUtils(win) {
+  if (!_eventUtils.has(win)) {
+    const eventUtilsObject = {
+      window: win,
+      parent: win,
+      _EU_Ci: Ci,
+      _EU_Cc: Cc,
+    };
+    Services.scriptloader.loadSubScript(
+      "chrome://remote/content/external/EventUtils.js",
+      eventUtilsObject
+    );
+    _eventUtils.set(win, eventUtilsObject);
+  }
+  return _eventUtils.get(win);
+}
+
+//  Max interval between two clicks that should result in a dblclick (in ms)
+const DBLCLICK_INTERVAL = 640;
+
+event.MouseEvents = {
+  click: 0,
+  dblclick: 1,
+  mousedown: 2,
+  mouseup: 3,
+  mouseover: 4,
+  mouseout: 5,
+};
+
+event.Modifiers = {
+  shiftKey: 0,
+  ctrlKey: 1,
+  altKey: 2,
+  metaKey: 3,
+};
+
+event.MouseButton = {
+  isPrimary(button) {
+    return button === 0;
+  },
+  isAuxiliary(button) {
+    return button === 1;
+  },
+  isSecondary(button) {
+    return button === 2;
+  },
+};
+
+event.DoubleClickTracker = {
+  firstClick: false,
+  isClicked() {
+    return event.DoubleClickTracker.firstClick;
+  },
+  setClick() {
+    if (!event.DoubleClickTracker.firstClick) {
+      event.DoubleClickTracker.firstClick = true;
+      event.DoubleClickTracker.startTimer();
+    }
+  },
+  resetClick() {
+    event.DoubleClickTracker.firstClick = false;
+    event.DoubleClickTracker.cancelTimer();
+  },
+  startTimer() {
+    lazy.dblclickTimer.initWithCallback(
+      event.DoubleClickTracker.resetClick,
+      DBLCLICK_INTERVAL,
+      Ci.nsITimer.TYPE_ONE_SHOT
+    );
+  },
+  cancelTimer() {
+    lazy.dblclickTimer.cancel();
+  },
+};
+
+// Only used by legacyactions.js
+event.parseModifiers_ = function (modifiers, win) {
+  return _getEventUtils(win)._parseModifiers(modifiers);
+};
+
+/**
+ * Synthesise a mouse event at a point.
+ *
+ * If the type is specified in opts, an mouse event of that type is
+ * fired. Otherwise, a mousedown followed by a mouseup is performed.
+ *
+ * @param {number} left
+ *     Offset from viewport left, in CSS pixels
+ * @param {number} top
+ *     Offset from viewport top, in CSS pixels
+ * @param {object} opts
+ *     Object which may contain the properties "shiftKey", "ctrlKey",
+ *     "altKey", "metaKey", "accessKey", "clickCount", "button", and
+ *     "type".
+ * @param {Window} win
+ *     Window object.
+ *
+ * @returns {boolean} defaultPrevented
+ */
+event.synthesizeMouseAtPoint = function (left, top, opts, win) {
+  return _getEventUtils(win).synthesizeMouseAtPoint(left, top, opts, win);
+};
+
+/**
+ * Synthesise a touch event at a point.
+ *
+ * If the type is specified in opts, a touch event of that type is
+ * fired. Otherwise, a touchstart followed by a touchend is performed.
+ *
+ * @param {number} left
+ *     Offset from viewport left, in CSS pixels
+ * @param {number} top
+ *     Offset from viewport top, in CSS pixels
+ * @param {object} opts
+ *     Object which may contain the properties "id", "rx", "ry", "angle",
+ *     "force", "shiftKey", "ctrlKey", "altKey", "metaKey", "accessKey",
+ *     "type".
+ * @param {Window} win
+ *     Window object.
+ *
+ * @returns {boolean} defaultPrevented
+ */
+event.synthesizeTouchAtPoint = function (left, top, opts, win) {
+  return _getEventUtils(win).synthesizeTouchAtPoint(left, top, opts, win);
+};
+
+/**
+ * Synthesise a wheel scroll event at a point.
+ *
+ * @param {number} left
+ *     Offset from viewport left, in CSS pixels
+ * @param {number} top
+ *     Offset from viewport top, in CSS pixels
+ * @param {object} opts
+ *     Object which may contain the properties "shiftKey", "ctrlKey",
+ *     "altKey", "metaKey", "accessKey", "deltaX", "deltaY", "deltaZ",
+ *     "deltaMode", "lineOrPageDeltaX", "lineOrPageDeltaY", "isMomentum",
+ *     "isNoLineOrPageDelta", "isCustomizedByPrefs", "expectedOverflowDeltaX",
+ *     "expectedOverflowDeltaY"
+ * @param {Window} win
+ *     Window object.
+ */
+event.synthesizeWheelAtPoint = function (left, top, opts, win) {
+  return _getEventUtils(win).synthesizeWheelAtPoint(left, top, opts, win);
+};
+
+event.synthesizeMultiTouch = function (opts, win) {
+  const modifiers = _getEventUtils(win)._parseModifiers(opts);
+  win.windowUtils.sendTouchEvent(
+    opts.type,
+    opts.id,
+    opts.x,
+    opts.y,
+    opts.rx,
+    opts.ry,
+    opts.angle,
+    opts.force,
+    opts.tiltx,
+    opts.tilty,
+    opts.twist,
+    modifiers
+  );
+};
+
+/**
+ * Synthesize a keydown event for a single key.
+ *
+ * @param {object} key
+ *     Key data as returned by keyData.getData
+ * @param {Window} win
+ *     Window object.
+ */
+event.sendKeyDown = function (key, win) {
+  event.sendSingleKey(key, win, "keydown");
+};
+
+/**
+ * Synthesize a keyup event for a single key.
+ *
+ * @param {object} key
+ *     Key data as returned by keyData.getData
+ * @param {Window} win
+ *     Window object.
+ */
+event.sendKeyUp = function (key, win) {
+  event.sendSingleKey(key, win, "keyup");
+};
+
+/**
+ * Synthesize a key event for a single key.
+ *
+ * @param {object} key
+ *     Key data as returned by keyData.getData
+ * @param {Window} win
+ *     Window object.
+ * @param {string=} type
+ *     Event to emit. By default the full keydown/keypressed/keyup event
+ *     sequence is emitted.
+ */
+event.sendSingleKey = function (key, win, type = null) {
+  let keyValue = key.key;
+  if (!key.printable) {
+    keyValue = `KEY_${keyValue}`;
+  }
+  const event = {
+    code: key.code,
+    location: key.location,
+    altKey: key.altKey ?? false,
+    shiftKey: key.shiftKey ?? false,
+    ctrlKey: key.ctrlKey ?? false,
+    metaKey: key.metaKey ?? false,
+    repeat: key.repeat ?? false,
+  };
+  if (type) {
+    event.type = type;
+  }
+  _getEventUtils(win).synthesizeKey(keyValue, event, win);
+};
+
+/**
+ * Send a string as a series of keypresses.
+ *
+ * @param {string} keyString
+ *     Sequence of characters to send as key presses
+ * @param {Window} win
+ *     Window object
+ */
+event.sendKeys = function (keyString, win) {
+  const modifiers = {};
+  for (let modifier in event.Modifiers) {
+    modifiers[modifier] = false;
+  }
+
+  for (let i = 0; i < keyString.length; i++) {
+    let keyValue = keyString.charAt(i);
+    if (modifiers.shiftKey) {
+      keyValue = lazy.keyData.getShiftedKey(keyValue);
+    }
+    const data = lazy.keyData.getData(keyValue);
+    const key = { ...data, ...modifiers };
+    if (data.modifier) {
+      // Negating the state of the modifier here is not spec compliant but
+      // makes us compatible to Chrome's behavior for now. That's fine unless
+      // we know the correct behavior.
+      //
+      // @see: https://github.com/w3c/webdriver/issues/1734
+      modifiers[data.modifier] = !modifiers[data.modifier];
+    }
+    event.sendSingleKey(key, win);
+  }
+};
+
+event.sendEvent = function (eventType, el, modifiers = {}, opts = {}) {
+  opts.canBubble = opts.canBubble || true;
+
+  let doc = el.ownerDocument || el.document;
+  let ev = doc.createEvent("Event");
+
+  ev.shiftKey = modifiers.shift;
+  ev.metaKey = modifiers.meta;
+  ev.altKey = modifiers.alt;
+  ev.ctrlKey = modifiers.ctrl;
+
+  ev.initEvent(eventType, opts.canBubble, true);
+  el.dispatchEvent(ev);
+};
+
+event.mouseover = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("mouseover", el, modifiers, opts);
+};
+
+event.mousemove = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("mousemove", el, modifiers, opts);
+};
+
+event.mousedown = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("mousedown", el, modifiers, opts);
+};
+
+event.mouseup = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("mouseup", el, modifiers, opts);
+};
+
+event.click = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("click", el, modifiers, opts);
+};
+
+event.change = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("change", el, modifiers, opts);
+};
+
+event.input = function (el, modifiers = {}, opts = {}) {
+  return event.sendEvent("input", el, modifiers, opts);
+};
diff --git a/remote/marionette/interaction.sys.mjs b/remote/marionette/interaction.sys.mjs
new file mode 100644
index 0000000000..4adbd9716b
--- /dev/null
+++ b/remote/marionette/interaction.sys.mjs
@@ -0,0 +1,782 @@
+/* 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/. */
+
+/* eslint-disable no-restricted-globals */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+
+  accessibility: "chrome://remote/content/marionette/accessibility.sys.mjs",
+  atom: "chrome://remote/content/marionette/atom.sys.mjs",
+  element: "chrome://remote/content/marionette/element.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  event: "chrome://remote/content/marionette/event.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+  TimedPromise: "chrome://remote/content/marionette/sync.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+/** XUL elements that support disabled attribute. */
+const DISABLED_ATTRIBUTE_SUPPORTED_XUL = new Set([
+  "ARROWSCROLLBOX",
+  "BUTTON",
+  "CHECKBOX",
+  "COMMAND",
+  "DESCRIPTION",
+  "KEY",
+  "KEYSET",
+  "LABEL",
+  "MENU",
+  "MENUITEM",
+  "MENULIST",
+  "MENUSEPARATOR",
+  "RADIO",
+  "RADIOGROUP",
+  "RICHLISTBOX",
+  "RICHLISTITEM",
+  "TAB",
+  "TABS",
+  "TOOLBARBUTTON",
+  "TREE",
+]);
+
+/**
+ * Common form controls that user can change the value property
+ * interactively.
+ */
+const COMMON_FORM_CONTROLS = new Set(["input", "textarea", "select"]);
+
+/**
+ * Input elements that do not fire <tt>input</tt> and <tt>change</tt>
+ * events when value property changes.
+ */
+const INPUT_TYPES_NO_EVENT = new Set([
+  "checkbox",
+  "radio",
+  "file",
+  "hidden",
+  "image",
+  "reset",
+  "button",
+  "submit",
+]);
+
+/** @namespace */
+export const interaction = {};
+
+/**
+ * Interact with an element by clicking it.
+ *
+ * The element is scrolled into view before visibility- or interactability
+ * checks are performed.
+ *
+ * Selenium-style visibility checks will be performed
+ * if <var>specCompat</var> is false (default).  Otherwise
+ * pointer-interactability checks will be performed.  If either of these
+ * fail an {@link ElementNotInteractableError} is thrown.
+ *
+ * If <var>strict</var> is enabled (defaults to disabled), further
+ * accessibility checks will be performed, and these may result in an
+ * {@link ElementNotAccessibleError} being returned.
+ *
+ * When <var>el</var> is not enabled, an {@link InvalidElementStateError}
+ * is returned.
+ *
+ * @param {(DOMElement|XULElement)} el
+ *     Element to click.
+ * @param {boolean=} [strict=false] strict
+ *     Enforce strict accessibility tests.
+ * @param {boolean=} [specCompat=false] specCompat
+ *     Use WebDriver specification compatible interactability definition.
+ *
+ * @throws {ElementNotInteractableError}
+ *     If either Selenium-style visibility check or
+ *     pointer-interactability check fails.
+ * @throws {ElementClickInterceptedError}
+ *     If <var>el</var> is obscured by another element and a click would
+ *     not hit, in <var>specCompat</var> mode.
+ * @throws {ElementNotAccessibleError}
+ *     If <var>strict</var> is true and element is not accessible.
+ * @throws {InvalidElementStateError}
+ *     If <var>el</var> is not enabled.
+ */
+interaction.clickElement = async function (
+  el,
+  strict = false,
+  specCompat = false
+) {
+  const a11y = lazy.accessibility.get(strict);
+  if (lazy.element.isXULElement(el)) {
+    await chromeClick(el, a11y);
+  } else if (specCompat) {
+    await webdriverClickElement(el, a11y);
+  } else {
+    lazy.logger.trace(`Using non spec-compatible element click`);
+    await seleniumClickElement(el, a11y);
+  }
+};
+
+async function webdriverClickElement(el, a11y) {
+  const win = getWindow(el);
+
+  // step 3
+  if (el.localName == "input" && el.type == "file") {
+    throw new lazy.error.InvalidArgumentError(
+      "Cannot click <input type=file> elements"
+    );
+  }
+
+  let containerEl = lazy.element.getContainer(el);
+
+  // step 4
+  if (!lazy.element.isInView(containerEl)) {
+    lazy.element.scrollIntoView(containerEl);
+  }
+
+  // step 5
+  // TODO(ato): wait for containerEl to be in view
+
+  // step 6
+  // if we cannot bring the container element into the viewport
+  // there is no point in checking if it is pointer-interactable
+  if (!lazy.element.isInView(containerEl)) {
+    throw new lazy.error.ElementNotInteractableError(
+      lazy.pprint`Element ${el} could not be scrolled into view`
+    );
+  }
+
+  // step 7
+  let rects = containerEl.getClientRects();
+  let clickPoint = lazy.element.getInViewCentrePoint(rects[0], win);
+
+  if (lazy.element.isObscured(containerEl)) {
+    throw new lazy.error.ElementClickInterceptedError(
+      null,
+      {},
+      containerEl,
+      clickPoint
+    );
+  }
+
+  let acc = await a11y.assertAccessible(el, true);
+  a11y.assertVisible(acc, el, true);
+  a11y.assertEnabled(acc, el, true);
+  a11y.assertActionable(acc, el);
+
+  // step 8
+  if (el.localName == "option") {
+    interaction.selectOption(el);
+  } else {
+    // step 9
+    let clicked = interaction.flushEventLoop(containerEl);
+
+    // Synthesize a pointerMove action.
+    lazy.event.synthesizeMouseAtPoint(
+      clickPoint.x,
+      clickPoint.y,
+      {
+        type: "mousemove",
+      },
+      win
+    );
+
+    // Synthesize a pointerDown + pointerUp action.
+    lazy.event.synthesizeMouseAtPoint(clickPoint.x, clickPoint.y, {}, win);
+
+    await clicked;
+  }
+
+  // step 10
+  // if the click causes navigation, the post-navigation checks are
+  // handled by navigate.js
+}
+
+async function chromeClick(el, a11y) {
+  if (!lazy.atom.isElementEnabled(el)) {
+    throw new lazy.error.InvalidElementStateError("Element is not enabled");
+  }
+
+  let acc = await a11y.assertAccessible(el, true);
+  a11y.assertVisible(acc, el, true);
+  a11y.assertEnabled(acc, el, true);
+  a11y.assertActionable(acc, el);
+
+  if (el.localName == "option") {
+    interaction.selectOption(el);
+  } else {
+    el.click();
+  }
+}
+
+async function seleniumClickElement(el, a11y) {
+  let win = getWindow(el);
+
+  let visibilityCheckEl = el;
+  if (el.localName == "option") {
+    visibilityCheckEl = lazy.element.getContainer(el);
+  }
+
+  if (!lazy.element.isVisible(visibilityCheckEl)) {
+    throw new lazy.error.ElementNotInteractableError();
+  }
+
+  if (!lazy.atom.isElementEnabled(el)) {
+    throw new lazy.error.InvalidElementStateError("Element is not enabled");
+  }
+
+  let acc = await a11y.assertAccessible(el, true);
+  a11y.assertVisible(acc, el, true);
+  a11y.assertEnabled(acc, el, true);
+  a11y.assertActionable(acc, el);
+
+  if (el.localName == "option") {
+    interaction.selectOption(el);
+  } else {
+    let rects = el.getClientRects();
+    let centre = lazy.element.getInViewCentrePoint(rects[0], win);
+    let opts = {};
+    lazy.event.synthesizeMouseAtPoint(centre.x, centre.y, opts, win);
+  }
+}
+
+/**
+ * Select <tt>&lt;option&gt;</tt> element in a <tt>&lt;select&gt;</tt>
+ * list.
+ *
+ * Because the dropdown list of select elements are implemented using
+ * native widget technology, our trusted synthesised events are not able
+ * to reach them.  Dropdowns are instead handled mimicking DOM events,
+ * which for obvious reasons is not ideal, but at the current point in
+ * time considered to be good enough.
+ *
+ * @param {HTMLOptionElement} el
+ *     Option element to select.
+ *
+ * @throws {TypeError}
+ *     If <var>el</var> is a XUL element or not an <tt>&lt;option&gt;</tt>
+ *     element.
+ * @throws {Error}
+ *     If unable to find <var>el</var>'s parent <tt>&lt;select&gt;</tt>
+ *     element.
+ */
+interaction.selectOption = function (el) {
+  if (lazy.element.isXULElement(el)) {
+    throw new TypeError("XUL dropdowns not supported");
+  }
+  if (el.localName != "option") {
+    throw new TypeError(lazy.pprint`Expected <option> element, got ${el}`);
+  }
+
+  let containerEl = lazy.element.getContainer(el);
+
+  lazy.event.mouseover(containerEl);
+  lazy.event.mousemove(containerEl);
+  lazy.event.mousedown(containerEl);
+  containerEl.focus();
+
+  if (!el.disabled) {
+    // Clicking <option> in <select> should not be deselected if selected.
+    // However, clicking one in a <select multiple> should toggle
+    // selectedness the way holding down Control works.
+    if (containerEl.multiple) {
+      el.selected = !el.selected;
+    } else if (!el.selected) {
+      el.selected = true;
+    }
+    lazy.event.input(containerEl);
+    lazy.event.change(containerEl);
+  }
+
+  lazy.event.mouseup(containerEl);
+  lazy.event.click(containerEl);
+  containerEl.blur();
+};
+
+/**
+ * Clears the form control or the editable element, if required.
+ *
+ * Before clearing the element, it will attempt to scroll it into
+ * view if it is not already in the viewport.  An error is raised
+ * if the element cannot be brought into view.
+ *
+ * If the element is a submittable form control and it is empty
+ * (it has no value or it has no files associated with it, in the
+ * case it is a <code>&lt;input type=file&gt;</code> element) or
+ * it is an editing host and its <code>innerHTML</code> content IDL
+ * attribute is empty, this function acts as a no-op.
+ *
+ * @param {Element} el
+ *     Element to clear.
+ *
+ * @throws {InvalidElementStateError}
+ *     If element is disabled, read-only, non-editable, not a submittable
+ *     element or not an editing host, or cannot be scrolled into view.
+ */
+interaction.clearElement = function (el) {
+  if (lazy.element.isDisabled(el)) {
+    throw new lazy.error.InvalidElementStateError(
+      lazy.pprint`Element is disabled: ${el}`
+    );
+  }
+  if (lazy.element.isReadOnly(el)) {
+    throw new lazy.error.InvalidElementStateError(
+      lazy.pprint`Element is read-only: ${el}`
+    );
+  }
+  if (!lazy.element.isEditable(el)) {
+    throw new lazy.error.InvalidElementStateError(
+      lazy.pprint`Unable to clear element that cannot be edited: ${el}`
+    );
+  }
+
+  if (!lazy.element.isInView(el)) {
+    lazy.element.scrollIntoView(el);
+  }
+  if (!lazy.element.isInView(el)) {
+    throw new lazy.error.ElementNotInteractableError(
+      lazy.pprint`Element ${el} could not be scrolled into view`
+    );
+  }
+
+  if (lazy.element.isEditingHost(el)) {
+    clearContentEditableElement(el);
+  } else {
+    clearResettableElement(el);
+  }
+};
+
+function clearContentEditableElement(el) {
+  if (el.innerHTML === "") {
+    return;
+  }
+  el.focus();
+  el.innerHTML = "";
+  el.blur();
+}
+
+function clearResettableElement(el) {
+  if (!lazy.element.isMutableFormControl(el)) {
+    throw new lazy.error.InvalidElementStateError(
+      lazy.pprint`Not an editable form control: ${el}`
+    );
+  }
+
+  let isEmpty;
+  switch (el.type) {
+    case "file":
+      isEmpty = !el.files.length;
+      break;
+
+    default:
+      isEmpty = el.value === "";
+      break;
+  }
+
+  if (el.validity.valid && isEmpty) {
+    return;
+  }
+
+  el.focus();
+  el.value = "";
+  lazy.event.change(el);
+  el.blur();
+}
+
+/**
+ * Waits until the event loop has spun enough times to process the
+ * DOM events generated by clicking an element, or until the document
+ * is unloaded.
+ *
+ * @param {Element} el
+ *     Element that is expected to receive the click.
+ *
+ * @returns {Promise}
+ *     Promise is resolved once <var>el</var> has been clicked
+ *     (its <code>click</code> event fires), the document is unloaded,
+ *     or a 500 ms timeout is reached.
+ */
+interaction.flushEventLoop = async function (el) {
+  const win = el.ownerGlobal;
+  let unloadEv, clickEv;
+
+  let spinEventLoop = resolve => {
+    unloadEv = resolve;
+    clickEv = event => {
+      lazy.logger.trace(`Received DOM event click for ${event.target}`);
+      if (win.closed) {
+        resolve();
+      } else {
+        lazy.setTimeout(resolve, 0);
+      }
+    };
+
+    win.addEventListener("unload", unloadEv, { mozSystemGroup: true });
+    el.addEventListener("click", clickEv, { mozSystemGroup: true });
+  };
+  let removeListeners = () => {
+    // only one event fires
+    win.removeEventListener("unload", unloadEv);
+    el.removeEventListener("click", clickEv);
+  };
+
+  return new lazy.TimedPromise(spinEventLoop, {
+    timeout: 500,
+    throws: null,
+  }).then(removeListeners);
+};
+
+/**
+ * If <var>el<var> is a textual form control, or is contenteditable,
+ * and no previous selection state exists, move the caret to the end
+ * of the form control.
+ *
+ * The element has to be a <code>&lt;input type=text&gt;</code> or
+ * <code>&lt;textarea&gt;</code> element, or have the contenteditable
+ * attribute set, for the cursor to be moved.
+ *
+ * @param {Element} el
+ *     Element to potential move the caret in.
+ */
+interaction.moveCaretToEnd = function (el) {
+  if (!lazy.element.isDOMElement(el)) {
+    return;
+  }
+
+  let isTextarea = el.localName == "textarea";
+  let isInputText = el.localName == "input" && el.type == "text";
+
+  if (isTextarea || isInputText) {
+    if (el.selectionEnd == 0) {
+      let len = el.value.length;
+      el.setSelectionRange(len, len);
+    }
+  } else if (el.isContentEditable) {
+    let selection = getWindow(el).getSelection();
+    selection.setPosition(el, el.childNodes.length);
+  }
+};
+
+/**
+ * Performs checks if <var>el</var> is keyboard-interactable.
+ *
+ * To decide if an element is keyboard-interactable various properties,
+ * and computed CSS styles have to be evaluated. Whereby it has to be taken
+ * into account that the element can be part of a container (eg. option),
+ * and as such the container has to be checked instead.
+ *
+ * @param {Element} el
+ *     Element to check.
+ *
+ * @returns {boolean}
+ *     True if element is keyboard-interactable, false otherwise.
+ */
+interaction.isKeyboardInteractable = function (el) {
+  const win = getWindow(el);
+
+  // body and document element are always keyboard-interactable
+  if (el.localName === "body" || el === win.document.documentElement) {
+    return true;
+  }
+
+  // context menu popups do not take the focus from the document.
+  const menuPopup = el.closest("menupopup");
+  if (menuPopup) {
+    if (menuPopup.state !== "open") {
+      // closed menupopups are not keyboard interactable.
+      return false;
+    }
+
+    const menuItem = el.closest("menuitem");
+    if (menuItem) {
+      // hidden or disabled menu items are not keyboard interactable.
+      return !menuItem.disabled && !menuItem.hidden;
+    }
+
+    return true;
+  }
+
+  return Services.focus.elementIsFocusable(el, 0);
+};
+
+/**
+ * Updates an `<input type=file>`'s file list with given `paths`.
+ *
+ * Hereby will the file list be appended with `paths` if the
+ * element allows multiple files. Otherwise the list will be
+ * replaced.
+ *
+ * @param {HTMLInputElement} el
+ *     An `input type=file` element.
+ * @param {Array.<string>} paths
+ *     List of full paths to any of the files to be uploaded.
+ *
+ * @throws {InvalidArgumentError}
+ *     If `path` doesn't exist.
+ */
+interaction.uploadFiles = async function (el, paths) {
+  let files = [];
+
+  if (el.hasAttribute("multiple")) {
+    // for multiple file uploads new files will be appended
+    files = Array.prototype.slice.call(el.files);
+  } else if (paths.length > 1) {
+    throw new lazy.error.InvalidArgumentError(
+      lazy.pprint`Element ${el} doesn't accept multiple files`
+    );
+  }
+
+  for (let path of paths) {
+    let file;
+
+    try {
+      file = await File.createFromFileName(path);
+    } catch (e) {
+      throw new lazy.error.InvalidArgumentError("File not found: " + path);
+    }
+
+    files.push(file);
+  }
+
+  el.mozSetFileArray(files);
+};
+
+/**
+ * Sets a form element's value.
+ *
+ * @param {DOMElement} el
+ *     An form element, e.g. input, textarea, etc.
+ * @param {string} value
+ *     The value to be set.
+ *
+ * @throws {TypeError}
+ *     If <var>el</var> is not an supported form element.
+ */
+interaction.setFormControlValue = function (el, value) {
+  if (!COMMON_FORM_CONTROLS.has(el.localName)) {
+    throw new TypeError("This function is for form elements only");
+  }
+
+  el.value = value;
+
+  if (INPUT_TYPES_NO_EVENT.has(el.type)) {
+    return;
+  }
+
+  lazy.event.input(el);
+  lazy.event.change(el);
+};
+
+/**
+ * Send keys to element.
+ *
+ * @param {DOMElement|XULElement} el
+ *     Element to send key events to.
+ * @param {Array.<string>} value
+ *     Sequence of keystrokes to send to the element.
+ * @param {object=} options
+ * @param {boolean=} options.strictFileInteractability
+ *     Run interactability checks on `<input type=file>` elements.
+ * @param {boolean=} options.accessibilityChecks
+ *     Enforce strict accessibility tests.
+ * @param {boolean=} options.webdriverClick
+ *     Use WebDriver specification compatible interactability definition.
+ */
+interaction.sendKeysToElement = async function (
+  el,
+  value,
+  {
+    strictFileInteractability = false,
+    accessibilityChecks = false,
+    webdriverClick = false,
+  } = {}
+) {
+  const a11y = lazy.accessibility.get(accessibilityChecks);
+
+  if (webdriverClick) {
+    await webdriverSendKeysToElement(
+      el,
+      value,
+      a11y,
+      strictFileInteractability
+    );
+  } else {
+    await legacySendKeysToElement(el, value, a11y);
+  }
+};
+
+async function webdriverSendKeysToElement(
+  el,
+  value,
+  a11y,
+  strictFileInteractability
+) {
+  const win = getWindow(el);
+
+  if (el.type !== "file" || strictFileInteractability) {
+    let containerEl = lazy.element.getContainer(el);
+
+    lazy.element.scrollIntoView(containerEl);
+
+    // TODO: Wait for element to be keyboard-interactible
+    if (!interaction.isKeyboardInteractable(containerEl)) {
+      throw new lazy.error.ElementNotInteractableError(
+        lazy.pprint`Element ${el} is not reachable by keyboard`
+      );
+    }
+
+    if (win.document.activeElement !== containerEl) {
+      containerEl.focus();
+      // This validates the correct element types internally
+      interaction.moveCaretToEnd(containerEl);
+    }
+  }
+
+  let acc = await a11y.assertAccessible(el, true);
+  a11y.assertActionable(acc, el);
+
+  if (el.type == "file") {
+    let paths = value.split("\n");
+    await interaction.uploadFiles(el, paths);
+
+    lazy.event.input(el);
+    lazy.event.change(el);
+  } else if (el.type == "date" || el.type == "time") {
+    interaction.setFormControlValue(el, value);
+  } else {
+    lazy.event.sendKeys(value, win);
+  }
+}
+
+async function legacySendKeysToElement(el, value, a11y) {
+  const win = getWindow(el);
+
+  if (el.type == "file") {
+    el.focus();
+    await interaction.uploadFiles(el, [value]);
+
+    lazy.event.input(el);
+    lazy.event.change(el);
+  } else if (el.type == "date" || el.type == "time") {
+    interaction.setFormControlValue(el, value);
+  } else {
+    let visibilityCheckEl = el;
+    if (el.localName == "option") {
+      visibilityCheckEl = lazy.element.getContainer(el);
+    }
+
+    if (!lazy.element.isVisible(visibilityCheckEl)) {
+      throw new lazy.error.ElementNotInteractableError(
+        "Element is not visible"
+      );
+    }
+
+    let acc = await a11y.assertAccessible(el, true);
+    a11y.assertActionable(acc, el);
+
+    interaction.moveCaretToEnd(el);
+    el.focus();
+    lazy.event.sendKeys(value, win);
+  }
+}
+
+/**
+ * Determine the element displayedness of an element.
+ *
+ * @param {DOMElement|XULElement} el
+ *     Element to determine displayedness of.
+ * @param {boolean=} [strict=false] strict
+ *     Enforce strict accessibility tests.
+ *
+ * @returns {boolean}
+ *     True if element is displayed, false otherwise.
+ */
+interaction.isElementDisplayed = function (el, strict = false) {
+  let win = getWindow(el);
+  let displayed = lazy.atom.isElementDisplayed(el, win);
+
+  let a11y = lazy.accessibility.get(strict);
+  return a11y.assertAccessible(el).then(acc => {
+    a11y.assertVisible(acc, el, displayed);
+    return displayed;
+  });
+};
+
+/**
+ * Check if element is enabled.
+ *
+ * @param {DOMElement|XULElement} el
+ *     Element to test if is enabled.
+ *
+ * @returns {boolean}
+ *     True if enabled, false otherwise.
+ */
+interaction.isElementEnabled = function (el, strict = false) {
+  let enabled = true;
+  let win = getWindow(el);
+
+  if (lazy.element.isXULElement(el)) {
+    // check if XUL element supports disabled attribute
+    if (DISABLED_ATTRIBUTE_SUPPORTED_XUL.has(el.tagName.toUpperCase())) {
+      if (
+        el.hasAttribute("disabled") &&
+        el.getAttribute("disabled") === "true"
+      ) {
+        enabled = false;
+      }
+    }
+  } else if (
+    ["application/xml", "text/xml"].includes(win.document.contentType)
+  ) {
+    enabled = false;
+  } else {
+    enabled = lazy.atom.isElementEnabled(el, { frame: win });
+  }
+
+  let a11y = lazy.accessibility.get(strict);
+  return a11y.assertAccessible(el).then(acc => {
+    a11y.assertEnabled(acc, el, enabled);
+    return enabled;
+  });
+};
+
+/**
+ * Determines if the referenced element is selected or not, with
+ * an additional accessibility check if <var>strict</var> is true.
+ *
+ * This operation only makes sense on input elements of the checkbox-
+ * and radio button states, and option elements.
+ *
+ * @param {(DOMElement|XULElement)} el
+ *     Element to test if is selected.
+ * @param {boolean=} [strict=false] strict
+ *     Enforce strict accessibility tests.
+ *
+ * @returns {boolean}
+ *     True if element is selected, false otherwise.
+ *
+ * @throws {ElementNotAccessibleError}
+ *     If <var>el</var> is not accessible when <var>strict</var> is true.
+ */
+interaction.isElementSelected = function (el, strict = false) {
+  let selected = lazy.element.isSelected(el);
+
+  let a11y = lazy.accessibility.get(strict);
+  return a11y.assertAccessible(el).then(acc => {
+    a11y.assertSelected(acc, el, selected);
+    return selected;
+  });
+};
+
+function getWindow(el) {
+  // eslint-disable-next-line mozilla/use-ownerGlobal
+  return el.ownerDocument.defaultView;
+}
diff --git a/remote/marionette/jar.mn b/remote/marionette/jar.mn
new file mode 100644
index 0000000000..0d1478c1cc
--- /dev/null
+++ b/remote/marionette/jar.mn
@@ -0,0 +1,52 @@
+# 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/.
+
+remote.jar:
+% content remote %content/
+  content/marionette/accessibility.sys.mjs (accessibility.sys.mjs)
+  content/marionette/actors/MarionetteCommandsChild.sys.mjs (actors/MarionetteCommandsChild.sys.mjs)
+  content/marionette/actors/MarionetteCommandsParent.sys.mjs (actors/MarionetteCommandsParent.sys.mjs)
+  content/marionette/actors/MarionetteEventsChild.sys.mjs (actors/MarionetteEventsChild.sys.mjs)
+  content/marionette/actors/MarionetteEventsParent.sys.mjs (actors/MarionetteEventsParent.sys.mjs)
+  content/marionette/actors/MarionetteReftestChild.sys.mjs (actors/MarionetteReftestChild.sys.mjs)
+  content/marionette/actors/MarionetteReftestParent.sys.mjs (actors/MarionetteReftestParent.sys.mjs)
+  content/marionette/addon.sys.mjs (addon.sys.mjs)
+  content/marionette/atom.sys.mjs (atom.sys.mjs)
+  content/marionette/browser.sys.mjs (browser.sys.mjs)
+  content/marionette/cert.sys.mjs (cert.sys.mjs)
+  content/marionette/cookie.sys.mjs (cookie.sys.mjs)
+  content/marionette/driver.sys.mjs (driver.sys.mjs)
+  content/marionette/element.sys.mjs (element.sys.mjs)
+  content/marionette/evaluate.sys.mjs (evaluate.sys.mjs)
+  content/marionette/event.sys.mjs  (event.sys.mjs)
+  content/marionette/interaction.sys.mjs (interaction.sys.mjs)
+  content/marionette/json.sys.mjs (json.sys.mjs)
+  content/marionette/l10n.sys.mjs (l10n.sys.mjs)
+  content/marionette/legacyaction.sys.mjs (legacyaction.sys.mjs)
+  content/marionette/message.sys.mjs (message.sys.mjs)
+  content/marionette/modal.sys.mjs (modal.sys.mjs)
+  content/marionette/navigate.sys.mjs (navigate.sys.mjs)
+  content/marionette/packets.sys.mjs (packets.sys.mjs)
+  content/marionette/permissions.sys.mjs (permissions.sys.mjs)
+  content/marionette/prefs.sys.mjs (prefs.sys.mjs)
+  content/marionette/reftest.sys.mjs (reftest.sys.mjs)
+  content/marionette/reftest.xhtml (chrome/reftest.xhtml)
+  content/marionette/reftest-content.js (reftest-content.js)
+  content/marionette/server.sys.mjs (server.sys.mjs)
+  content/marionette/stream-utils.sys.mjs (stream-utils.sys.mjs)
+  content/marionette/sync.sys.mjs (sync.sys.mjs)
+  content/marionette/transport.sys.mjs (transport.sys.mjs)
+#ifdef ENABLE_TESTS
+  content/marionette/test_dialog.dtd (chrome/test_dialog.dtd)
+  content/marionette/test_dialog.properties (chrome/test_dialog.properties)
+  content/marionette/test_dialog.xhtml (chrome/test_dialog.xhtml)
+  content/marionette/test_menupopup.xhtml (chrome/test_menupopup.xhtml)
+  content/marionette/test_nested_iframe.xhtml (chrome/test_nested_iframe.xhtml)
+  content/marionette/test_no_xul.xhtml (chrome/test_no_xul.xhtml)
+  content/marionette/test.xhtml (chrome/test.xhtml)
+  content/marionette/test2.xhtml (chrome/test2.xhtml)
+#ifdef MOZ_CODE_COVERAGE
+  content/marionette/PerTestCoverageUtils.sys.mjs (../../tools/code-coverage/PerTestCoverageUtils.sys.mjs)
+#endif
+#endif
diff --git a/remote/marionette/json.sys.mjs b/remote/marionette/json.sys.mjs
new file mode 100644
index 0000000000..8cae4f3283
--- /dev/null
+++ b/remote/marionette/json.sys.mjs
@@ -0,0 +1,249 @@
+/* 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, {
+  element: "chrome://remote/content/marionette/element.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+  ShadowRoot: "chrome://remote/content/marionette/element.sys.mjs",
+  WebElement: "chrome://remote/content/marionette/element.sys.mjs",
+  WebReference: "chrome://remote/content/marionette/element.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+/** @namespace */
+export const json = {};
+
+/**
+ * Clone an object including collections.
+ *
+ * @param {object} value
+ *     Object to be cloned.
+ * @param {Set} seen
+ *     List of objects already processed.
+ * @param {Function} cloneAlgorithm
+ *     The clone algorithm to invoke for individual list entries or object
+ *     properties.
+ *
+ * @returns {object}
+ *     The cloned object.
+ */
+function cloneObject(value, seen, cloneAlgorithm) {
+  // Only proceed with cloning an object if it hasn't been seen yet.
+  if (seen.has(value)) {
+    throw new lazy.error.JavaScriptError("Cyclic object value");
+  }
+  seen.add(value);
+
+  let result;
+
+  if (lazy.element.isCollection(value)) {
+    result = [...value].map(entry => cloneAlgorithm(entry, seen));
+  } else {
+    // arbitrary objects
+    result = {};
+    for (let prop in value) {
+      try {
+        result[prop] = cloneAlgorithm(value[prop], seen);
+      } catch (e) {
+        if (e.result == Cr.NS_ERROR_NOT_IMPLEMENTED) {
+          lazy.logger.debug(`Skipping ${prop}: ${e.message}`);
+        } else {
+          throw e;
+        }
+      }
+    }
+  }
+
+  seen.delete(value);
+
+  return result;
+}
+
+/**
+ * Clone arbitrary objects to JSON-safe primitives that can be
+ * transported across processes and over the Marionette protocol.
+ *
+ * The marshaling rules are as follows:
+ *
+ * - Primitives are returned as is.
+ *
+ * - Collections, such as `Array`, `NodeList`, `HTMLCollection`
+ *   et al. are transformed to arrays and then recursed.
+ *
+ * - Elements and ShadowRoots that are not known WebReference's are added to
+ *   the `NodeCache`. For both the associated unique web reference identifier
+ *   is returned.
+ *
+ * - Objects with custom JSON representations, i.e. if they have
+ *   a callable `toJSON` function, are returned verbatim.  This means
+ *   their internal integrity _are not_ checked.  Be careful.
+ *
+ * - If a cyclic references is detected a JavaScriptError is thrown.
+ *
+ * @param {object} value
+ *     Object to be cloned.
+ * @param {NodeCache} nodeCache
+ *     Node cache that holds already seen WebElement and ShadowRoot references.
+ *
+ * @returns {object}
+ *     Same object as provided by `value` with the WebDriver specific
+ *     elements replaced by WebReference's.
+ *
+ * @throws {JavaScriptError}
+ *     If an object contains cyclic references.
+ * @throws {StaleElementReferenceError}
+ *     If the element has gone stale, indicating it is no longer
+ *     attached to the DOM.
+ */
+json.clone = function (value, nodeCache) {
+  function cloneJSON(value, seen) {
+    if (seen === undefined) {
+      seen = new Set();
+    }
+
+    if ([undefined, null].includes(value)) {
+      return null;
+    }
+
+    const type = typeof value;
+
+    if (["boolean", "number", "string"].includes(type)) {
+      // Primitive values
+      return value;
+    }
+
+    // Evaluation of code might take place in mutable sandboxes, which are
+    // created to waive XRays by default. As such DOM nodes would have to be
+    // unwaived before accessing properties like "ownerGlobal" is possible.
+    //
+    // Until bug 1743788 is fixed there might be the possibility that more
+    // objects might need to be unwaived as well.
+    const isNode = Node.isInstance(value);
+    if (isNode) {
+      value = Cu.unwaiveXrays(value);
+    }
+
+    if (isNode && lazy.element.isElement(value)) {
+      // Convert DOM elements to WebReference instances.
+
+      if (lazy.element.isStale(value)) {
+        // Don't create a reference for stale elements.
+        throw new lazy.error.StaleElementReferenceError(
+          lazy.pprint`The element ${value} is no longer attached to the DOM`
+        );
+      }
+
+      const nodeRef = nodeCache.getOrCreateNodeReference(value);
+      return lazy.WebReference.from(value, nodeRef).toJSON();
+    }
+
+    if (isNode && lazy.element.isShadowRoot(value)) {
+      // Convert ShadowRoot instances to WebReference references.
+
+      if (lazy.element.isDetached(value)) {
+        // Don't create a reference for detached shadow roots.
+        throw new lazy.error.DetachedShadowRootError(
+          lazy.pprint`The ShadowRoot ${value} is no longer attached to the DOM`
+        );
+      }
+
+      const nodeRef = nodeCache.getOrCreateNodeReference(value);
+      return lazy.WebReference.from(value, nodeRef).toJSON();
+    }
+
+    if (typeof value.toJSON == "function") {
+      // custom JSON representation
+      let unsafeJSON;
+      try {
+        unsafeJSON = value.toJSON();
+      } catch (e) {
+        throw new lazy.error.JavaScriptError(`toJSON() failed with: ${e}`);
+      }
+
+      return cloneJSON(unsafeJSON, seen);
+    }
+
+    // Collections and arbitrary objects
+    return cloneObject(value, seen, cloneJSON);
+  }
+
+  return cloneJSON(value, new Set());
+};
+
+/**
+ * Deserialize an arbitrary object.
+ *
+ * @param {object} value
+ *     Arbitrary object.
+ * @param {NodeCache} nodeCache
+ *     Node cache that holds already seen WebElement and ShadowRoot references.
+ * @param {WindowProxy} win
+ *     Current window.
+ *
+ * @returns {object}
+ *     Same object as provided by `value` with the WebDriver specific
+ *     references replaced with real JavaScript objects.
+ *
+ * @throws {NoSuchElementError}
+ *     If the WebElement reference has not been seen before.
+ * @throws {StaleElementReferenceError}
+ *     If the element is stale, indicating it is no longer attached to the DOM.
+ */
+json.deserialize = function (value, nodeCache, win) {
+  function deserializeJSON(value, seen) {
+    if (seen === undefined) {
+      seen = new Set();
+    }
+
+    if (value === undefined || value === null) {
+      return value;
+    }
+
+    switch (typeof value) {
+      case "boolean":
+      case "number":
+      case "string":
+      default:
+        return value;
+
+      case "object":
+        if (lazy.WebReference.isReference(value)) {
+          // Create a WebReference based on the WebElement identifier.
+          const webRef = lazy.WebReference.fromJSON(value);
+
+          if (webRef instanceof lazy.ShadowRoot) {
+            return lazy.element.getKnownShadowRoot(
+              win.browsingContext,
+              webRef.uuid,
+              nodeCache
+            );
+          }
+
+          if (webRef instanceof lazy.WebElement) {
+            return lazy.element.getKnownElement(
+              win.browsingContext,
+              webRef.uuid,
+              nodeCache
+            );
+          }
+
+          // WebFrame and WebWindow not supported yet
+          throw new lazy.error.UnsupportedOperationError();
+        }
+
+        return cloneObject(value, seen, deserializeJSON);
+    }
+  }
+
+  return deserializeJSON(value, new Set());
+};
diff --git a/remote/marionette/l10n.sys.mjs b/remote/marionette/l10n.sys.mjs
new file mode 100644
index 0000000000..80ad95b67b
--- /dev/null
+++ b/remote/marionette/l10n.sys.mjs
@@ -0,0 +1,103 @@
+/* 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/. */
+
+/**
+ * An API which allows Marionette to handle localized content.
+ *
+ * The localization (https://mzl.la/2eUMjyF) of UI elements in Gecko
+ * based applications is done via entities and properties. For static
+ * values entities are used, which are located in .dtd files. Whereby for
+ * dynamically updated content the values come from .property files. Both
+ * types of elements can be identifed via a unique id, and the translated
+ * content retrieved.
+ */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "domParser", () => {
+  const parser = new DOMParser();
+  parser.forceEnableDTD();
+  return parser;
+});
+
+/** @namespace */
+export const l10n = {};
+
+/**
+ * Retrieve the localized string for the specified entity id.
+ *
+ * Example:
+ *     localizeEntity(["chrome://branding/locale/brand.dtd"], "brandShortName")
+ *
+ * @param {Array.<string>} urls
+ *     Array of .dtd URLs.
+ * @param {string} id
+ *     The ID of the entity to retrieve the localized string for.
+ *
+ * @returns {string}
+ *     The localized string for the requested entity.
+ */
+l10n.localizeEntity = function (urls, id) {
+  // Build a string which contains all possible entity locations
+  let locations = [];
+  urls.forEach((url, index) => {
+    locations.push(`<!ENTITY % dtd_${index} SYSTEM "${url}">%dtd_${index};`);
+  });
+
+  // Use the DOM parser to resolve the entity and extract its real value
+  let header = `<?xml version="1.0"?><!DOCTYPE elem [${locations.join("")}]>`;
+  let elem = `<elem id="elementID">&${id};</elem>`;
+  let doc = lazy.domParser.parseFromString(header + elem, "text/xml");
+  let element = doc.querySelector("elem[id='elementID']");
+
+  if (element === null) {
+    throw new lazy.error.NoSuchElementError(
+      `Entity with id='${id}' hasn't been found`
+    );
+  }
+
+  return element.textContent;
+};
+
+/**
+ * Retrieve the localized string for the specified property id.
+ *
+ * Example:
+ *
+ *     localizeProperty(
+ *         ["chrome://global/locale/findbar.properties"], "FastFind");
+ *
+ * @param {Array.<string>} urls
+ *     Array of .properties URLs.
+ * @param {string} id
+ *     The ID of the property to retrieve the localized string for.
+ *
+ * @returns {string}
+ *     The localized string for the requested property.
+ */
+l10n.localizeProperty = function (urls, id) {
+  let property = null;
+
+  for (let url of urls) {
+    let bundle = Services.strings.createBundle(url);
+    try {
+      property = bundle.GetStringFromName(id);
+      break;
+    } catch (e) {}
+  }
+
+  if (property === null) {
+    throw new lazy.error.NoSuchElementError(
+      `Property with ID '${id}' hasn't been found`
+    );
+  }
+
+  return property;
+};
diff --git a/remote/marionette/legacyaction.sys.mjs b/remote/marionette/legacyaction.sys.mjs
new file mode 100644
index 0000000000..6790ba55c7
--- /dev/null
+++ b/remote/marionette/legacyaction.sys.mjs
@@ -0,0 +1,640 @@
+/* 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/. */
+
+/* eslint-disable no-restricted-globals */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Preferences: "resource://gre/modules/Preferences.sys.mjs",
+
+  accessibility: "chrome://remote/content/marionette/accessibility.sys.mjs",
+  element: "chrome://remote/content/marionette/element.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  json: "chrome://remote/content/marionette/json.sys.mjs",
+  event: "chrome://remote/content/marionette/event.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  WebElement: "chrome://remote/content/marionette/element.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+const CONTEXT_MENU_DELAY_PREF = "ui.click_hold_context_menus.delay";
+const DEFAULT_CONTEXT_MENU_DELAY = 750; // ms
+
+/** @namespace */
+export const legacyaction = {};
+
+const action = legacyaction;
+
+/**
+ * Functionality for (single finger) action chains.
+ */
+action.Chain = function () {
+  // for assigning unique ids to all touches
+  this.nextTouchId = 1000;
+  // keep track of active Touches
+  this.touchIds = {};
+  // last touch for each fingerId
+  this.lastCoordinates = null;
+  this.isTap = false;
+  this.scrolling = false;
+  // whether to send mouse event
+  this.mouseEventsOnly = false;
+  this.checkTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+
+  // determines if we create touch events
+  this.inputSource = null;
+};
+
+/**
+ * Create a touch based event.
+ *
+ * @param {Element} elem
+ *        The Element on which the touch event should be created.
+ * @param {number} x
+ *        x coordinate relative to the viewport.
+ * @param {number} y
+ *        y coordinate relative to the viewport.
+ * @param {number} touchId
+ *        Touch event id used by legacyactions.
+ */
+action.Chain.prototype.createATouch = function (elem, x, y, touchId) {
+  const doc = elem.ownerDocument;
+  const win = doc.defaultView;
+  const [clientX, clientY, pageX, pageY, screenX, screenY] =
+    this.getCoordinateInfo(elem, x, y);
+  const atouch = doc.createTouch(
+    win,
+    elem,
+    touchId,
+    pageX,
+    pageY,
+    screenX,
+    screenY,
+    clientX,
+    clientY
+  );
+  return atouch;
+};
+
+action.Chain.prototype.dispatchActions = function (
+  args,
+  touchId,
+  container,
+  seenEls
+) {
+  this.seenEls = seenEls;
+  this.container = container;
+  let commandArray = lazy.json.deserialize(args, seenEls, container.frame);
+
+  if (touchId == null) {
+    touchId = this.nextTSouchId++;
+  }
+
+  if (!container.frame.document.createTouch) {
+    this.mouseEventsOnly = true;
+  }
+
+  let keyModifiers = {
+    shiftKey: false,
+    ctrlKey: false,
+    altKey: false,
+    metaKey: false,
+  };
+
+  return new Promise(resolve => {
+    this.actions(commandArray, touchId, 0, keyModifiers, resolve);
+  }).catch(this.resetValues.bind(this));
+};
+
+/**
+ * This function emit mouse event.
+ *
+ * @param {Document} doc
+ *     Current document.
+ * @param {string} type
+ *     Type of event to dispatch.
+ * @param {number} elClientX
+ *     X coordinate of the mouse relative to the viewport.
+ * @param {number} elClientY
+ *     Y coordinate of the mouse relative to the viewport.
+ * @param {number} button
+ *     The button number.
+ * @param {number} clickCount
+ *     Number of clicks, button notes the mouse button.
+ * @param {object} modifiers
+ *     An object of modifier keys present.
+ */
+action.Chain.prototype.emitMouseEvent = function (
+  doc,
+  type,
+  elClientX,
+  elClientY,
+  button,
+  clickCount,
+  modifiers
+) {
+  lazy.logger.debug(
+    `Emitting ${type} mouse event ` +
+      `at coordinates (${elClientX}, ${elClientY}) ` +
+      `relative to the viewport, ` +
+      `button: ${button}, ` +
+      `clickCount: ${clickCount}`
+  );
+
+  let win = doc.defaultView;
+  let domUtils = win.windowUtils;
+
+  let mods;
+  if (typeof modifiers != "undefined") {
+    mods = lazy.event.parseModifiers_(modifiers, win);
+  } else {
+    mods = 0;
+  }
+
+  domUtils.sendMouseEvent(
+    type,
+    elClientX,
+    elClientY,
+    button || 0,
+    clickCount || 1,
+    mods,
+    false,
+    0,
+    this.inputSource
+  );
+};
+
+action.Chain.prototype.emitTouchEvent = function (doc, type, touch) {
+  lazy.logger.info(
+    `Emitting Touch event of type ${type} ` +
+      `to element with id: ${touch.target.id} ` +
+      `and tag name: ${touch.target.tagName} ` +
+      `at coordinates (${touch.clientX}), ` +
+      `${touch.clientY}) relative to the viewport`
+  );
+
+  const win = doc.defaultView;
+  if (win.docShell.asyncPanZoomEnabled && this.scrolling) {
+    lazy.logger.debug(
+      `Cannot emit touch event with asyncPanZoomEnabled and legacyactions.scrolling`
+    );
+    return;
+  }
+
+  // we get here if we're not in asyncPacZoomEnabled land, or if we're
+  // the main process
+  win.windowUtils.sendTouchEvent(
+    type,
+    [touch.identifier],
+    [touch.clientX],
+    [touch.clientY],
+    [touch.radiusX],
+    [touch.radiusY],
+    [touch.rotationAngle],
+    [touch.force],
+    [0],
+    [0],
+    [0],
+    0
+  );
+};
+
+/**
+ * Reset any persisted values after a command completes.
+ */
+action.Chain.prototype.resetValues = function () {
+  this.container = null;
+  this.seenEls = null;
+  this.mouseEventsOnly = false;
+};
+
+/**
+ * Function that performs a single tap.
+ */
+action.Chain.prototype.singleTap = async function (
+  el,
+  corx,
+  cory,
+  capabilities
+) {
+  const doc = el.ownerDocument;
+  // after this block, the element will be scrolled into view
+  let visible = lazy.element.isVisible(el, corx, cory);
+  if (!visible) {
+    throw new lazy.error.ElementNotInteractableError(
+      "Element is not currently visible and may not be manipulated"
+    );
+  }
+
+  let a11y = lazy.accessibility.get(capabilities["moz:accessibilityChecks"]);
+  let acc = await a11y.assertAccessible(el, true);
+  a11y.assertVisible(acc, el, visible);
+  a11y.assertActionable(acc, el);
+  if (!doc.createTouch) {
+    this.mouseEventsOnly = true;
+  }
+  let c = lazy.element.coordinates(el, corx, cory);
+  if (!this.mouseEventsOnly) {
+    let touchId = this.nextTouchId++;
+    let touch = this.createATouch(el, c.x, c.y, touchId);
+    this.emitTouchEvent(doc, "touchstart", touch);
+    this.emitTouchEvent(doc, "touchend", touch);
+  }
+  this.mouseTap(doc, c.x, c.y);
+};
+
+/**
+ * Emit events for each action in the provided chain.
+ *
+ * To emit touch events for each finger, one might send a [["press", id],
+ * ["wait", 5], ["release"]] chain.
+ *
+ * @param {Array.<Array<?>>} chain
+ *     A multi-dimensional array of actions.
+ * @param {Object<string, number>} touchId
+ *     Represents the finger ID.
+ * @param {number} i
+ *     Keeps track of the current action of the chain.
+ * @param {Object<string, boolean>} keyModifiers
+ *     Keeps track of keyDown/keyUp pairs through an action chain.
+ * @param {function(?)} cb
+ *     Called on success.
+ *
+ * @returns {Object<string, number>}
+ *     Last finger ID, or an empty object.
+ */
+action.Chain.prototype.actions = function (
+  chain,
+  touchId,
+  i,
+  keyModifiers,
+  cb
+) {
+  if (i == chain.length) {
+    cb(touchId || null);
+    this.resetValues();
+    return;
+  }
+
+  let pack = chain[i];
+  let command = pack[0];
+  let webEl;
+  let el;
+  let c;
+  i++;
+
+  if (!["press", "wait", "keyDown", "keyUp", "click"].includes(command)) {
+    // if mouseEventsOnly, then touchIds isn't used
+    if (!(touchId in this.touchIds) && !this.mouseEventsOnly) {
+      this.resetValues();
+      throw new lazy.error.WebDriverError("Element has not been pressed");
+    }
+  }
+
+  switch (command) {
+    case "keyDown":
+      lazy.event.sendKeyDown(pack[1], keyModifiers, this.container.frame);
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "keyUp":
+      lazy.event.sendKeyUp(pack[1], keyModifiers, this.container.frame);
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "click":
+      webEl = lazy.WebElement.fromUUID(pack[1]);
+      el = this.seenEls.get(webEl);
+      let button = pack[2];
+      let clickCount = pack[3];
+      c = lazy.element.coordinates(el);
+      this.mouseTap(
+        el.ownerDocument,
+        c.x,
+        c.y,
+        button,
+        clickCount,
+        keyModifiers
+      );
+      if (button == 2) {
+        this.emitMouseEvent(
+          el.ownerDocument,
+          "contextmenu",
+          c.x,
+          c.y,
+          button,
+          clickCount,
+          keyModifiers
+        );
+      }
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "press":
+      if (this.lastCoordinates) {
+        this.generateEvents(
+          "cancel",
+          this.lastCoordinates[0],
+          this.lastCoordinates[1],
+          touchId,
+          null,
+          keyModifiers
+        );
+        this.resetValues();
+        throw new lazy.error.WebDriverError(
+          "Invalid Command: press cannot follow an active touch event"
+        );
+      }
+
+      // look ahead to check if we're scrolling,
+      // needed for APZ touch dispatching
+      if (i != chain.length && chain[i][0].includes("move")) {
+        this.scrolling = true;
+      }
+      webEl = lazy.WebElement.fromUUID(pack[1]);
+      el = this.seenEls.get(webEl);
+      c = lazy.element.coordinates(el, pack[2], pack[3]);
+      touchId = this.generateEvents("press", c.x, c.y, null, el, keyModifiers);
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "release":
+      this.generateEvents(
+        "release",
+        this.lastCoordinates[0],
+        this.lastCoordinates[1],
+        touchId,
+        null,
+        keyModifiers
+      );
+      this.actions(chain, null, i, keyModifiers, cb);
+      this.scrolling = false;
+      break;
+
+    case "move":
+      webEl = lazy.WebElement.fromUUID(pack[1]);
+      el = this.seenEls.get(webEl);
+      c = lazy.element.coordinates(el);
+      this.generateEvents("move", c.x, c.y, touchId, null, keyModifiers);
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "moveByOffset":
+      this.generateEvents(
+        "move",
+        this.lastCoordinates[0] + pack[1],
+        this.lastCoordinates[1] + pack[2],
+        touchId,
+        null,
+        keyModifiers
+      );
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+
+    case "wait":
+      if (pack[1] != null) {
+        let time = pack[1] * 1000;
+
+        // standard waiting time to fire contextmenu
+        let standard = lazy.Preferences.get(
+          CONTEXT_MENU_DELAY_PREF,
+          DEFAULT_CONTEXT_MENU_DELAY
+        );
+
+        if (time >= standard && this.isTap) {
+          chain.splice(i, 0, ["longPress"], ["wait", (time - standard) / 1000]);
+          time = standard;
+        }
+        this.checkTimer.initWithCallback(
+          () => this.actions(chain, touchId, i, keyModifiers, cb),
+          time,
+          Ci.nsITimer.TYPE_ONE_SHOT
+        );
+      } else {
+        this.actions(chain, touchId, i, keyModifiers, cb);
+      }
+      break;
+
+    case "cancel":
+      this.generateEvents(
+        "cancel",
+        this.lastCoordinates[0],
+        this.lastCoordinates[1],
+        touchId,
+        null,
+        keyModifiers
+      );
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      this.scrolling = false;
+      break;
+
+    case "longPress":
+      this.generateEvents(
+        "contextmenu",
+        this.lastCoordinates[0],
+        this.lastCoordinates[1],
+        touchId,
+        null,
+        keyModifiers
+      );
+      this.actions(chain, touchId, i, keyModifiers, cb);
+      break;
+  }
+};
+
+/**
+ * Given an element and a pair of coordinates, returns an array of the
+ * form [clientX, clientY, pageX, pageY, screenX, screenY].
+ */
+action.Chain.prototype.getCoordinateInfo = function (el, corx, cory) {
+  let win = el.ownerGlobal;
+  return [
+    corx, // clientX
+    cory, // clientY
+    corx + win.pageXOffset, // pageX
+    cory + win.pageYOffset, // pageY
+    corx + win.mozInnerScreenX, // screenX
+    cory + win.mozInnerScreenY, // screenY
+  ];
+};
+
+/**
+ * @param {string} type
+ *     The event type (eg "tap", "press", ...).
+ * @param {number} x
+ *     X coordinate of the location to generate the event that is relative
+ *     to the viewport.
+ * @param {number} y
+ *     Y coordinate of the location to generate the event that is relative
+ *     to the viewport.
+ * @param {number} touchId
+ *     The current touch id.
+ * @param {Element} target
+ *     The Element on which the events should be created.
+ */
+action.Chain.prototype.generateEvents = function (
+  type,
+  x,
+  y,
+  touchId,
+  target,
+  keyModifiers
+) {
+  this.lastCoordinates = [x, y];
+  let doc = this.container.frame.document;
+
+  switch (type) {
+    case "tap":
+      if (this.mouseEventsOnly) {
+        let touch = this.createATouch(target, x, y, touchId);
+        this.mouseTap(
+          touch.target.ownerDocument,
+          touch.clientX,
+          touch.clientY,
+          null,
+          null,
+          keyModifiers
+        );
+      } else {
+        touchId = this.nextTouchId++;
+        let touch = this.createATouch(target, x, y, touchId);
+        this.emitTouchEvent(doc, "touchstart", touch);
+        this.emitTouchEvent(doc, "touchend", touch);
+        this.mouseTap(
+          touch.target.ownerDocument,
+          touch.clientX,
+          touch.clientY,
+          null,
+          null,
+          keyModifiers
+        );
+      }
+      this.lastCoordinates = null;
+      break;
+
+    case "press":
+      this.isTap = true;
+      if (this.mouseEventsOnly) {
+        this.emitMouseEvent(doc, "mousemove", x, y, null, null, keyModifiers);
+        this.emitMouseEvent(doc, "mousedown", x, y, null, null, keyModifiers);
+      } else {
+        touchId = this.nextTouchId++;
+        let touch = this.createATouch(target, x, y, touchId);
+        this.emitTouchEvent(doc, "touchstart", touch);
+        this.touchIds[touchId] = touch;
+        return touchId;
+      }
+      break;
+
+    case "release":
+      if (this.mouseEventsOnly) {
+        let [x, y] = this.lastCoordinates;
+        this.emitMouseEvent(doc, "mouseup", x, y, null, null, keyModifiers);
+      } else {
+        let touch = this.touchIds[touchId];
+        let [x, y] = this.lastCoordinates;
+
+        touch = this.createATouch(touch.target, x, y, touchId);
+        this.emitTouchEvent(doc, "touchend", touch);
+
+        if (this.isTap) {
+          this.mouseTap(
+            touch.target.ownerDocument,
+            touch.clientX,
+            touch.clientY,
+            null,
+            null,
+            keyModifiers
+          );
+        }
+        delete this.touchIds[touchId];
+      }
+
+      this.isTap = false;
+      this.lastCoordinates = null;
+      break;
+
+    case "cancel":
+      this.isTap = false;
+      if (this.mouseEventsOnly) {
+        let [x, y] = this.lastCoordinates;
+        this.emitMouseEvent(doc, "mouseup", x, y, null, null, keyModifiers);
+      } else {
+        this.emitTouchEvent(doc, "touchcancel", this.touchIds[touchId]);
+        delete this.touchIds[touchId];
+      }
+      this.lastCoordinates = null;
+      break;
+
+    case "move":
+      this.isTap = false;
+      if (this.mouseEventsOnly) {
+        this.emitMouseEvent(doc, "mousemove", x, y, null, null, keyModifiers);
+      } else {
+        let touch = this.createATouch(
+          this.touchIds[touchId].target,
+          x,
+          y,
+          touchId
+        );
+        this.touchIds[touchId] = touch;
+        this.emitTouchEvent(doc, "touchmove", touch);
+      }
+      break;
+
+    case "contextmenu":
+      this.isTap = false;
+      let event = this.container.frame.document.createEvent("MouseEvents");
+      if (this.mouseEventsOnly) {
+        target = doc.elementFromPoint(
+          this.lastCoordinates[0],
+          this.lastCoordinates[1]
+        );
+      } else {
+        target = this.touchIds[touchId].target;
+      }
+
+      let [clientX, clientY, , , screenX, screenY] = this.getCoordinateInfo(
+        target,
+        x,
+        y
+      );
+
+      event.initMouseEvent(
+        "contextmenu",
+        true,
+        true,
+        target.ownerGlobal,
+        1,
+        screenX,
+        screenY,
+        clientX,
+        clientY,
+        false,
+        false,
+        false,
+        false,
+        0,
+        null
+      );
+      target.dispatchEvent(event);
+      break;
+
+    default:
+      throw new lazy.error.WebDriverError("Unknown event type: " + type);
+  }
+  return null;
+};
+
+action.Chain.prototype.mouseTap = function (doc, x, y, button, count, mod) {
+  this.emitMouseEvent(doc, "mousemove", x, y, button, count, mod);
+  this.emitMouseEvent(doc, "mousedown", x, y, button, count, mod);
+  this.emitMouseEvent(doc, "mouseup", x, y, button, count, mod);
+};
diff --git a/remote/marionette/message.sys.mjs b/remote/marionette/message.sys.mjs
new file mode 100644
index 0000000000..d8b5dd60f9
--- /dev/null
+++ b/remote/marionette/message.sys.mjs
@@ -0,0 +1,329 @@
+/* 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, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
+/** Representation of the packets transproted over the wire. */
+export class Message {
+  /**
+   * @param {number} messageID
+   *     Message ID unique identifying this message.
+   */
+  constructor(messageID) {
+    this.id = lazy.assert.integer(messageID);
+  }
+
+  toString() {
+    function replacer(key, value) {
+      if (typeof value === "string") {
+        return lazy.truncate`${value}`;
+      }
+      return value;
+    }
+
+    return JSON.stringify(this.toPacket(), replacer);
+  }
+
+  /**
+   * Converts a data packet into a {@link Command} or {@link Response}.
+   *
+   * @param {Array.<number, number, ?, ?>} data
+   *     A four element array where the elements, in sequence, signifies
+   *     message type, message ID, method name or error, and parameters
+   *     or result.
+   *
+   * @returns {Message}
+   *     Based on the message type, a {@link Command} or {@link Response}
+   *     instance.
+   *
+   * @throws {TypeError}
+   *     If the message type is not recognised.
+   */
+  static fromPacket(data) {
+    const [type] = data;
+
+    switch (type) {
+      case Command.Type:
+        return Command.fromPacket(data);
+
+      case Response.Type:
+        return Response.fromPacket(data);
+
+      default:
+        throw new TypeError(
+          "Unrecognised message type in packet: " + JSON.stringify(data)
+        );
+    }
+  }
+}
+
+/**
+ * Messages may originate from either the server or the client.
+ * Because the remote protocol is full duplex, both endpoints may be
+ * the origin of both commands and responses.
+ *
+ * @enum
+ * @see {@link Message}
+ */
+Message.Origin = {
+  /** Indicates that the message originates from the client. */
+  Client: 0,
+  /** Indicates that the message originates from the server. */
+  Server: 1,
+};
+
+/**
+ * A command is a request from the client to run a series of remote end
+ * steps and return a fitting response.
+ *
+ * The command can be synthesised from the message passed over the
+ * Marionette socket using the {@link fromPacket} function.  The format of
+ * a message is:
+ *
+ * <pre>
+ *     [<var>type</var>, <var>id</var>, <var>name</var>, <var>params</var>]
+ * </pre>
+ *
+ * where
+ *
+ * <dl>
+ *   <dt><var>type</var> (integer)
+ *   <dd>
+ *     Must be zero (integer).  Zero means that this message is
+ *     a command.
+ *
+ *   <dt><var>id</var> (integer)
+ *   <dd>
+ *     Integer used as a sequence number.  The server replies with
+ *     the same ID for the response.
+ *
+ *   <dt><var>name</var> (string)
+ *   <dd>
+ *     String representing the command name with an associated set
+ *     of remote end steps.
+ *
+ *   <dt><var>params</var> (JSON Object or null)
+ *   <dd>
+ *     Object of command function arguments.  The keys of this object
+ *     must be strings, but the values can be arbitrary values.
+ * </dl>
+ *
+ * A command has an associated message <var>id</var> that prevents
+ * the dispatcher from sending responses in the wrong order.
+ *
+ * The command may also have optional error- and result handlers that
+ * are called when the client returns with a response.  These are
+ * <code>function onerror({Object})</code>,
+ * <code>function onresult({Object})</code>, and
+ * <code>function onresult({Response})</code>:
+ *
+ * @param {number} messageID
+ *     Message ID unique identifying this message.
+ * @param {string} name
+ *     Command name.
+ * @param {Object<string, ?>} params
+ *     Command parameters.
+ */
+export class Command extends Message {
+  constructor(messageID, name, params = {}) {
+    super(messageID);
+
+    this.name = lazy.assert.string(name);
+    this.parameters = lazy.assert.object(params);
+
+    this.onerror = null;
+    this.onresult = null;
+
+    this.origin = Message.Origin.Client;
+    this.sent = false;
+  }
+
+  /**
+   * Calls the error- or result handler associated with this command.
+   * This function can be replaced with a custom response handler.
+   *
+   * @param {Response} resp
+   *     The response to pass on to the result or error to the
+   *     <code>onerror</code> or <code>onresult</code> handlers to.
+   */
+  onresponse(resp) {
+    if (this.onerror && resp.error) {
+      this.onerror(resp.error);
+    } else if (this.onresult && resp.body) {
+      this.onresult(resp.body);
+    }
+  }
+
+  /**
+   * Encodes the command to a packet.
+   *
+   * @returns {Array}
+   *     Packet.
+   */
+  toPacket() {
+    return [Command.Type, this.id, this.name, this.parameters];
+  }
+
+  /**
+   * Converts a data packet into {@link Command}.
+   *
+   * @param {Array.<number, number, *, *>} payload
+   *     A four element array where the elements, in sequence, signifies
+   *     message type, message ID, command name, and parameters.
+   *
+   * @returns {Command}
+   *     Representation of packet.
+   *
+   * @throws {TypeError}
+   *     If the message type is not recognised.
+   */
+  static fromPacket(payload) {
+    let [type, msgID, name, params] = payload;
+    lazy.assert.that(n => n === Command.Type)(type);
+
+    // if parameters are given but null, treat them as undefined
+    if (params === null) {
+      params = undefined;
+    }
+
+    return new Command(msgID, name, params);
+  }
+}
+
+Command.Type = 0;
+
+/**
+ * @callback ResponseCallback
+ *
+ * @param {Response} resp
+ *     Response to handle.
+ */
+
+/**
+ * Represents the response returned from the remote end after execution
+ * of its corresponding command.
+ *
+ * The response is a mutable object passed to each command for
+ * modification through the available setters.  To send data in a response,
+ * you modify the body property on the response.  The body property can
+ * also be replaced completely.
+ *
+ * The response is sent implicitly by
+ * {@link server.TCPConnection#execute when a command has finished
+ * executing, and any modifications made subsequent to that will have
+ * no effect.
+ *
+ * @param {number} messageID
+ *     Message ID tied to the corresponding command request this is
+ *     a response for.
+ * @param {ResponseHandler} respHandler
+ *     Function callback called on sending the response.
+ */
+export class Response extends Message {
+  constructor(messageID, respHandler = () => {}) {
+    super(messageID);
+
+    this.respHandler_ = lazy.assert.callable(respHandler);
+
+    this.error = null;
+    this.body = { value: null };
+
+    this.origin = Message.Origin.Server;
+    this.sent = false;
+  }
+
+  /**
+   * Sends response conditionally, given a predicate.
+   *
+   * @param {function(Response): boolean} predicate
+   *     A predicate taking a Response object and returning a boolean.
+   */
+  sendConditionally(predicate) {
+    if (predicate(this)) {
+      this.send();
+    }
+  }
+
+  /**
+   * Sends response using the response handler provided on
+   * construction.
+   *
+   * @throws {RangeError}
+   *     If the response has already been sent.
+   */
+  send() {
+    if (this.sent) {
+      throw new RangeError("Response has already been sent: " + this);
+    }
+    this.respHandler_(this);
+    this.sent = true;
+  }
+
+  /**
+   * Send error to client.
+   *
+   * Turns the response into an error response, clears any previously
+   * set body data, and sends it using the response handler provided
+   * on construction.
+   *
+   * @param {Error} err
+   *     The Error instance to send.
+   *
+   * @throws {Error}
+   *     If <var>err</var> is not a {@link WebDriverError}, the error
+   *     is propagated, i.e. rethrown.
+   */
+  sendError(err) {
+    this.error = lazy.error.wrap(err).toJSON();
+    this.body = null;
+    this.send();
+
+    // propagate errors which are implementation problems
+    if (!lazy.error.isWebDriverError(err)) {
+      throw err;
+    }
+  }
+
+  /**
+   * Encodes the response to a packet.
+   *
+   * @returns {Array}
+   *     Packet.
+   */
+  toPacket() {
+    return [Response.Type, this.id, this.error, this.body];
+  }
+
+  /**
+   * Converts a data packet into {@link Response}.
+   *
+   * @param {Array.<number, number, ?, ?>} payload
+   *     A four element array where the elements, in sequence, signifies
+   *     message type, message ID, error, and result.
+   *
+   * @returns {Response}
+   *     Representation of packet.
+   *
+   * @throws {TypeError}
+   *     If the message type is not recognised.
+   */
+  static fromPacket(payload) {
+    let [type, msgID, err, body] = payload;
+    lazy.assert.that(n => n === Response.Type)(type);
+
+    let resp = new Response(msgID);
+    resp.error = lazy.assert.string(err);
+
+    resp.body = body;
+    return resp;
+  }
+}
+
+Response.Type = 1;
diff --git a/remote/marionette/modal.sys.mjs b/remote/marionette/modal.sys.mjs
new file mode 100644
index 0000000000..eb3a96170b
--- /dev/null
+++ b/remote/marionette/modal.sys.mjs
@@ -0,0 +1,377 @@
+/* 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, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+const COMMON_DIALOG = "chrome://global/content/commonDialog.xhtml";
+
+/** @namespace */
+export const modal = {
+  ACTION_CLOSED: "closed",
+  ACTION_OPENED: "opened",
+};
+
+/**
+ * Check for already existing modal or tab modal dialogs
+ *
+ * @param {browser.Context} context
+ *     Reference to the browser context to check for existent dialogs.
+ *
+ * @returns {modal.Dialog}
+ *     Returns instance of the Dialog class, or `null` if no modal dialog
+ *     is present.
+ */
+modal.findModalDialogs = function (context) {
+  // First check if there is a modal dialog already present for the
+  // current browser window.
+  for (let win of Services.wm.getEnumerator(null)) {
+    // TODO: Use BrowserWindowTracker.getTopWindow for modal dialogs without
+    // an opener.
+    if (
+      win.document.documentURI === COMMON_DIALOG &&
+      win.opener &&
+      win.opener === context.window
+    ) {
+      lazy.logger.trace("Found open window modal prompt");
+      return new modal.Dialog(() => context, win);
+    }
+  }
+
+  if (lazy.AppInfo.isAndroid) {
+    const geckoViewPrompts = context.window.prompts();
+    if (geckoViewPrompts.length) {
+      lazy.logger.trace("Found open GeckoView prompt");
+      const prompt = geckoViewPrompts[0];
+      return new modal.Dialog(() => context, prompt);
+    }
+  }
+
+  const contentBrowser = context.contentBrowser;
+
+  // If no modal dialog has been found yet, also check for tab and content modal
+  // dialogs for the current tab.
+  //
+  // TODO: Find an adequate implementation for Firefox on Android (bug 1708105)
+  if (contentBrowser?.tabDialogBox) {
+    let dialogs = contentBrowser.tabDialogBox.getTabDialogManager().dialogs;
+    if (dialogs.length) {
+      lazy.logger.trace("Found open tab modal prompt");
+      return new modal.Dialog(() => context, dialogs[0].frameContentWindow);
+    }
+
+    dialogs = contentBrowser.tabDialogBox.getContentDialogManager().dialogs;
+
+    // Even with the dialog manager handing back a dialog, the `Dialog` property
+    // gets lazily added. If it's not set yet, ignore the dialog for now.
+    if (dialogs.length && dialogs[0].frameContentWindow.Dialog) {
+      lazy.logger.trace("Found open content prompt");
+      return new modal.Dialog(() => context, dialogs[0].frameContentWindow);
+    }
+  }
+
+  // If no modal dialog has been found yet, check for old non SubDialog based
+  // content modal dialogs. Even with those deprecated in Firefox 89 we should
+  // keep supporting applications that don't have them implemented yet.
+  if (contentBrowser?.tabModalPromptBox) {
+    const prompts = contentBrowser.tabModalPromptBox.listPrompts();
+    if (prompts.length) {
+      lazy.logger.trace("Found open old-style content prompt");
+      return new modal.Dialog(() => context, null);
+    }
+  }
+
+  return null;
+};
+
+/**
+ * Observer for modal and tab modal dialogs.
+ *
+ * @param {function(): browser.Context} curBrowserFn
+ *     Function that returns the current |browser.Context|.
+ *
+ * @returns {modal.DialogObserver}
+ *     Returns instance of the DialogObserver class.
+ */
+modal.DialogObserver = class {
+  constructor(curBrowserFn) {
+    this._curBrowserFn = curBrowserFn;
+
+    this.callbacks = new Set();
+    this.register();
+  }
+
+  register() {
+    Services.obs.addObserver(this, "common-dialog-loaded");
+    Services.obs.addObserver(this, "domwindowopened");
+    Services.obs.addObserver(this, "geckoview-prompt-show");
+    Services.obs.addObserver(this, "tabmodal-dialog-loaded");
+
+    // Register event listener for all already open windows
+    for (let win of Services.wm.getEnumerator(null)) {
+      win.addEventListener("DOMModalDialogClosed", this);
+    }
+  }
+
+  unregister() {
+    Services.obs.removeObserver(this, "common-dialog-loaded");
+    Services.obs.removeObserver(this, "domwindowopened");
+    Services.obs.removeObserver(this, "geckoview-prompt-show");
+    Services.obs.removeObserver(this, "tabmodal-dialog-loaded");
+
+    // Unregister event listener for all open windows
+    for (let win of Services.wm.getEnumerator(null)) {
+      win.removeEventListener("DOMModalDialogClosed", this);
+    }
+  }
+
+  cleanup() {
+    this.callbacks.clear();
+    this.unregister();
+  }
+
+  handleEvent(event) {
+    lazy.logger.trace(`Received event ${event.type}`);
+
+    const chromeWin = event.target.opener
+      ? event.target.opener.ownerGlobal
+      : event.target.ownerGlobal;
+
+    if (chromeWin != this._curBrowserFn().window) {
+      return;
+    }
+
+    this.callbacks.forEach(callback => {
+      callback(modal.ACTION_CLOSED, event.target);
+    });
+  }
+
+  observe(subject, topic) {
+    lazy.logger.trace(`Received observer notification ${topic}`);
+
+    const curBrowser = this._curBrowserFn();
+
+    switch (topic) {
+      // This topic is only used by the old-style content modal dialogs like
+      // alert, confirm, and prompt. It can be removed when only the new
+      // subdialog based content modals remain. Those will be made default in
+      // Firefox 89, and this case is deprecated.
+      case "tabmodal-dialog-loaded":
+        const container = curBrowser.contentBrowser.closest(
+          ".browserSidebarContainer"
+        );
+        if (!container.contains(subject)) {
+          return;
+        }
+        this.callbacks.forEach(callback =>
+          callback(modal.ACTION_OPENED, subject)
+        );
+        break;
+
+      case "common-dialog-loaded":
+        const modalType = subject.Dialog.args.modalType;
+
+        if (
+          modalType === Services.prompt.MODAL_TYPE_TAB ||
+          modalType === Services.prompt.MODAL_TYPE_CONTENT
+        ) {
+          // Find the container of the dialog in the parent document, and ensure
+          // it is a descendant of the same container as the current browser.
+          const container = curBrowser.contentBrowser.closest(
+            ".browserSidebarContainer"
+          );
+          if (!container.contains(subject.docShell.chromeEventHandler)) {
+            return;
+          }
+        } else if (
+          subject.ownerGlobal != curBrowser.window &&
+          subject.opener?.ownerGlobal != curBrowser.window
+        ) {
+          return;
+        }
+
+        this.callbacks.forEach(callback =>
+          callback(modal.ACTION_OPENED, subject)
+        );
+        break;
+
+      case "domwindowopened":
+        subject.addEventListener("DOMModalDialogClosed", this);
+        break;
+
+      case "geckoview-prompt-show":
+        for (let win of Services.wm.getEnumerator(null)) {
+          const prompt = win.prompts().find(item => item.id == subject.id);
+          if (prompt) {
+            this.callbacks.forEach(callback =>
+              callback(modal.ACTION_OPENED, prompt)
+            );
+            return;
+          }
+        }
+        break;
+    }
+  }
+
+  /**
+   * Add dialog handler by function reference.
+   *
+   * @param {Function} callback
+   *     The handler to be added.
+   */
+  add(callback) {
+    if (this.callbacks.has(callback)) {
+      return;
+    }
+    this.callbacks.add(callback);
+  }
+
+  /**
+   * Remove dialog handler by function reference.
+   *
+   * @param {Function} callback
+   *     The handler to be removed.
+   */
+  remove(callback) {
+    if (!this.callbacks.has(callback)) {
+      return;
+    }
+    this.callbacks.delete(callback);
+  }
+
+  /**
+   * Returns a promise that waits for the dialog to be closed.
+   */
+  async dialogClosed() {
+    return new Promise(resolve => {
+      const dialogClosed = (action, dialog) => {
+        if (action == modal.ACTION_CLOSED) {
+          this.remove(dialogClosed);
+          resolve();
+        }
+      };
+
+      this.add(dialogClosed);
+    });
+  }
+};
+
+/**
+ * Represents a modal dialog.
+ *
+ * @param {function(): browser.Context} curBrowserFn
+ *     Function that returns the current |browser.Context|.
+ * @param {DOMWindow} dialog
+ *     DOMWindow of the dialog.
+ */
+modal.Dialog = class {
+  constructor(curBrowserFn, dialog) {
+    this.curBrowserFn_ = curBrowserFn;
+    this.win_ = Cu.getWeakReference(dialog);
+  }
+
+  get args() {
+    if (lazy.AppInfo.isAndroid) {
+      return this.window.args;
+    }
+    let tm = this.tabModal;
+    return tm ? tm.args : null;
+  }
+
+  get curBrowser_() {
+    return this.curBrowserFn_();
+  }
+
+  get isOpen() {
+    if (lazy.AppInfo.isAndroid) {
+      return this.window !== null;
+    }
+    if (!this.ui) {
+      return false;
+    }
+    return true;
+  }
+
+  get isWindowModal() {
+    return [
+      Services.prompt.MODAL_TYPE_WINDOW,
+      Services.prompt.MODAL_TYPE_INTERNAL_WINDOW,
+    ].includes(this.args.modalType);
+  }
+
+  get tabModal() {
+    let win = this.window;
+    if (win) {
+      return win.Dialog;
+    }
+    return this.curBrowser_.getTabModal();
+  }
+
+  get text() {
+    if (lazy.AppInfo.isAndroid) {
+      return this.window.getPromptText();
+    }
+    return this.ui.infoBody.textContent;
+  }
+
+  get ui() {
+    let tm = this.tabModal;
+    return tm ? tm.ui : null;
+  }
+
+  /**
+   * For Android, this returns a GeckoViewPrompter, which can be used to control prompts.
+   * Otherwise, this returns the ChromeWindow associated with an open dialog window if
+   * it is currently attached to the DOM.
+   */
+  get window() {
+    if (this.win_) {
+      let win = this.win_.get();
+      if (win && (lazy.AppInfo.isAndroid || win.parent)) {
+        return win;
+      }
+    }
+    return null;
+  }
+
+  set text(inputText) {
+    if (lazy.AppInfo.isAndroid) {
+      this.window.setInputText(inputText);
+    } else {
+      // see toolkit/components/prompts/content/commonDialog.js
+      let { loginTextbox } = this.ui;
+      loginTextbox.value = inputText;
+    }
+  }
+
+  accept() {
+    if (lazy.AppInfo.isAndroid) {
+      // GeckoView does not have a UI, so the methods are called directly
+      this.window.acceptPrompt();
+    } else {
+      const { button0 } = this.ui;
+      button0.click();
+    }
+  }
+
+  dismiss() {
+    if (lazy.AppInfo.isAndroid) {
+      // GeckoView does not have a UI, so the methods are called directly
+      this.window.dismissPrompt();
+    } else {
+      const { button0, button1 } = this.ui;
+      (button1 ? button1 : button0).click();
+    }
+  }
+};
diff --git a/remote/marionette/moz.build b/remote/marionette/moz.build
new file mode 100644
index 0000000000..bb061a2ee7
--- /dev/null
+++ b/remote/marionette/moz.build
@@ -0,0 +1,10 @@
+# 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/.
+
+JAR_MANIFESTS += ["jar.mn"]
+
+XPCSHELL_TESTS_MANIFESTS += ["test/xpcshell/xpcshell.ini"]
+
+with Files("**"):
+    BUG_COMPONENT = ("Remote Protocol", "Marionette")
diff --git a/remote/marionette/navigate.sys.mjs b/remote/marionette/navigate.sys.mjs
new file mode 100644
index 0000000000..a7c000f9eb
--- /dev/null
+++ b/remote/marionette/navigate.sys.mjs
@@ -0,0 +1,428 @@
+/* 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",
+  EventDispatcher:
+    "chrome://remote/content/marionette/actors/MarionetteEventsParent.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  modal: "chrome://remote/content/marionette/modal.sys.mjs",
+  PageLoadStrategy:
+    "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  ProgressListener: "chrome://remote/content/shared/Navigate.sys.mjs",
+  TimedPromise: "chrome://remote/content/marionette/sync.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+// Timeouts used to check if a new navigation has been initiated.
+const TIMEOUT_BEFOREUNLOAD_EVENT = 200;
+const TIMEOUT_UNLOAD_EVENT = 5000;
+
+/** @namespace */
+export const navigate = {};
+
+/**
+ * Checks the value of readyState for the current page
+ * load activity, and resolves the command if the load
+ * has been finished. It also takes care of the selected
+ * page load strategy.
+ *
+ * @param {PageLoadStrategy} pageLoadStrategy
+ *     Strategy when navigation is considered as finished.
+ * @param {object} eventData
+ * @param {string} eventData.documentURI
+ *     Current document URI of the document.
+ * @param {string} eventData.readyState
+ *     Current ready state of the document.
+ *
+ * @returns {boolean}
+ *     True if the page load has been finished.
+ */
+function checkReadyState(pageLoadStrategy, eventData = {}) {
+  const { documentURI, readyState } = eventData;
+
+  const result = { error: null, finished: false };
+
+  switch (readyState) {
+    case "interactive":
+      if (documentURI.startsWith("about:certerror")) {
+        result.error = new lazy.error.InsecureCertificateError();
+        result.finished = true;
+      } else if (/about:.*(error)\?/.exec(documentURI)) {
+        result.error = new lazy.error.UnknownError(
+          `Reached error page: ${documentURI}`
+        );
+        result.finished = true;
+
+        // Return early with a page load strategy of eager, and also
+        // special-case about:blocked pages which should be treated as
+        // non-error pages but do not raise a pageshow event. about:blank
+        // is also treaded specifically here, because it gets temporary
+        // loaded for new content processes, and we only want to rely on
+        // complete loads for it.
+      } else if (
+        (pageLoadStrategy === lazy.PageLoadStrategy.Eager &&
+          documentURI != "about:blank") ||
+        /about:blocked\?/.exec(documentURI)
+      ) {
+        result.finished = true;
+      }
+      break;
+
+    case "complete":
+      result.finished = true;
+      break;
+  }
+
+  return result;
+}
+
+/**
+ * Determines if we expect to get a DOM load event (DOMContentLoaded)
+ * on navigating to the <code>future</code> URL.
+ *
+ * @param {URL} current
+ *     URL the browser is currently visiting.
+ * @param {object} options
+ * @param {BrowsingContext=} options.browsingContext
+ *     The current browsing context. Needed for targets of _parent and _top.
+ * @param {URL=} options.future
+ *     Destination URL, if known.
+ * @param {target=} options.target
+ *     Link target, if known.
+ *
+ * @returns {boolean}
+ *     Full page load would be expected if future is followed.
+ *
+ * @throws TypeError
+ *     If <code>current</code> is not defined, or any of
+ *     <code>current</code> or <code>future</code>  are invalid URLs.
+ */
+navigate.isLoadEventExpected = function (current, options = {}) {
+  const { browsingContext, future, target } = options;
+
+  if (typeof current == "undefined") {
+    throw new TypeError("Expected at least one URL");
+  }
+
+  if (["_parent", "_top"].includes(target) && !browsingContext) {
+    throw new TypeError(
+      "Expected browsingContext when target is _parent or _top"
+    );
+  }
+
+  // Don't wait if the navigation happens in a different browsing context
+  if (
+    target === "_blank" ||
+    (target === "_parent" && browsingContext.parent) ||
+    (target === "_top" && browsingContext.top != browsingContext)
+  ) {
+    return false;
+  }
+
+  // Assume we will go somewhere exciting
+  if (typeof future == "undefined") {
+    return true;
+  }
+
+  // Assume javascript:<whatever> will modify the current document
+  // but this is not an entirely safe assumption to make,
+  // considering it could be used to set window.location
+  if (future.protocol == "javascript:") {
+    return false;
+  }
+
+  // If hashes are present and identical
+  if (
+    current.href.includes("#") &&
+    future.href.includes("#") &&
+    current.hash === future.hash
+  ) {
+    return false;
+  }
+
+  return true;
+};
+
+/**
+ * Load the given URL in the specified browsing context.
+ *
+ * @param {CanonicalBrowsingContext} browsingContext
+ *     Browsing context to load the URL into.
+ * @param {string} url
+ *     URL to navigate to.
+ */
+navigate.navigateTo = async function (browsingContext, url) {
+  const opts = {
+    loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_IS_LINK,
+    triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+    // Fake user activation.
+    hasValidUserGestureActivation: true,
+  };
+  browsingContext.fixupAndLoadURIString(url, opts);
+};
+
+/**
+ * Reload the page.
+ *
+ * @param {CanonicalBrowsingContext} browsingContext
+ *     Browsing context to refresh.
+ */
+navigate.refresh = async function (browsingContext) {
+  const flags = Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CACHE;
+  browsingContext.reload(flags);
+};
+
+/**
+ * Execute a callback and wait for a possible navigation to complete
+ *
+ * @param {GeckoDriver} driver
+ *     Reference to driver instance.
+ * @param {Function} callback
+ *     Callback to execute that might trigger a navigation.
+ * @param {object} options
+ * @param {BrowsingContext=} options.browsingContext
+ *     Browsing context to observe. Defaults to the current browsing context.
+ * @param {boolean=} options.loadEventExpected
+ *     If false, return immediately and don't wait for
+ *     the navigation to be completed. Defaults to true.
+ * @param {boolean=} options.requireBeforeUnload
+ *     If false and no beforeunload event is fired, abort waiting
+ *     for the navigation. Defaults to true.
+ */
+navigate.waitForNavigationCompleted = async function waitForNavigationCompleted(
+  driver,
+  callback,
+  options = {}
+) {
+  const {
+    browsingContextFn = driver.getBrowsingContext.bind(driver),
+    loadEventExpected = true,
+    requireBeforeUnload = true,
+  } = options;
+
+  const browsingContext = browsingContextFn();
+  const chromeWindow = browsingContext.topChromeWindow;
+  const pageLoadStrategy = driver.currentSession.pageLoadStrategy;
+
+  // Return immediately if no load event is expected
+  if (!loadEventExpected) {
+    await callback();
+    return Promise.resolve();
+  }
+
+  // When not waiting for page load events, do not return until the navigation has actually started.
+  if (pageLoadStrategy === lazy.PageLoadStrategy.None) {
+    const listener = new lazy.ProgressListener(browsingContext.webProgress, {
+      resolveWhenStarted: true,
+      waitForExplicitStart: true,
+    });
+    const navigated = listener.start();
+    navigated.finally(() => {
+      if (listener.isStarted) {
+        listener.stop();
+      }
+    });
+
+    await callback();
+    await navigated;
+
+    return Promise.resolve();
+  }
+
+  let rejectNavigation;
+  let resolveNavigation;
+
+  let browsingContextChanged = false;
+  let seenBeforeUnload = false;
+  let seenUnload = false;
+
+  let unloadTimer;
+
+  const checkDone = ({ finished, error }) => {
+    if (finished) {
+      if (error) {
+        rejectNavigation(error);
+      } else {
+        resolveNavigation();
+      }
+    }
+  };
+
+  const onDialogOpened = action => {
+    if (action === lazy.modal.ACTION_OPENED) {
+      lazy.logger.trace("Canceled page load listener because a dialog opened");
+      checkDone({ finished: true });
+    }
+  };
+
+  const onTimer = timer => {
+    // In the case when a document has a beforeunload handler
+    // registered, the currently active command will return immediately
+    // due to the modal dialog observer.
+    //
+    // Otherwise the timeout waiting for the document to start
+    // navigating is increased by 5000 ms to ensure a possible load
+    // event is not missed. In the common case such an event should
+    // occur pretty soon after beforeunload, and we optimise for this.
+    if (seenBeforeUnload) {
+      seenBeforeUnload = false;
+      unloadTimer.initWithCallback(
+        onTimer,
+        TIMEOUT_UNLOAD_EVENT,
+        Ci.nsITimer.TYPE_ONE_SHOT
+      );
+
+      // If no page unload has been detected, ensure to properly stop
+      // the load listener, and return from the currently active command.
+    } else if (!seenUnload) {
+      lazy.logger.trace(
+        "Canceled page load listener because no navigation " +
+          "has been detected"
+      );
+      checkDone({ finished: true });
+    }
+  };
+
+  const onNavigation = (eventName, data) => {
+    const browsingContext = browsingContextFn();
+
+    // Ignore events from other browsing contexts than the selected one.
+    if (data.browsingContext != browsingContext) {
+      return;
+    }
+
+    lazy.logger.trace(
+      lazy.truncate`[${data.browsingContext.id}] Received event ${data.type} for ${data.documentURI}`
+    );
+
+    switch (data.type) {
+      case "beforeunload":
+        seenBeforeUnload = true;
+        break;
+
+      case "pagehide":
+        seenUnload = true;
+        break;
+
+      case "hashchange":
+      case "popstate":
+        checkDone({ finished: true });
+        break;
+
+      case "DOMContentLoaded":
+      case "pageshow":
+        // Don't require an unload event when a top-level browsing context
+        // change occurred.
+        if (!seenUnload && !browsingContextChanged) {
+          return;
+        }
+        const result = checkReadyState(pageLoadStrategy, data);
+        checkDone(result);
+        break;
+    }
+  };
+
+  // In the case when the currently selected frame is closed,
+  // there will be no further load events. Stop listening immediately.
+  const onBrowsingContextDiscarded = (subject, topic, why) => {
+    // If the BrowsingContext is being discarded to be replaced by another
+    // context, we don't want to stop waiting for the pageload to complete, as
+    // we will continue listening to the newly created context.
+    if (subject == browsingContextFn() && why != "replace") {
+      lazy.logger.trace(
+        "Canceled page load listener " +
+          `because browsing context with id ${subject.id} has been removed`
+      );
+      checkDone({ finished: true });
+    }
+  };
+
+  // Detect changes to the top-level browsing context to not
+  // necessarily require an unload event.
+  const onBrowsingContextChanged = event => {
+    if (event.target === driver.curBrowser.contentBrowser) {
+      browsingContextChanged = true;
+    }
+  };
+
+  const onUnload = event => {
+    lazy.logger.trace(
+      "Canceled page load listener " +
+        "because the top-browsing context has been closed"
+    );
+    checkDone({ finished: true });
+  };
+
+  chromeWindow.addEventListener("TabClose", onUnload);
+  chromeWindow.addEventListener("unload", onUnload);
+  driver.curBrowser.tabBrowser?.addEventListener(
+    "XULFrameLoaderCreated",
+    onBrowsingContextChanged
+  );
+  driver.dialogObserver.add(onDialogOpened);
+  Services.obs.addObserver(
+    onBrowsingContextDiscarded,
+    "browsing-context-discarded"
+  );
+
+  lazy.EventDispatcher.on("page-load", onNavigation);
+
+  return new lazy.TimedPromise(
+    async (resolve, reject) => {
+      rejectNavigation = reject;
+      resolveNavigation = resolve;
+
+      try {
+        await callback();
+
+        // Certain commands like clickElement can cause a navigation. Setup a timer
+        // to check if a "beforeunload" event has been emitted within the given
+        // time frame. If not resolve the Promise.
+        if (!requireBeforeUnload) {
+          unloadTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+          unloadTimer.initWithCallback(
+            onTimer,
+            TIMEOUT_BEFOREUNLOAD_EVENT,
+            Ci.nsITimer.TYPE_ONE_SHOT
+          );
+        }
+      } catch (e) {
+        // Executing the callback above could destroy the actor pair before the
+        // command returns. Such an error has to be ignored.
+        if (e.name !== "AbortError") {
+          checkDone({ finished: true, error: e });
+        }
+      }
+    },
+    {
+      errorMessage: "Navigation timed out",
+      timeout: driver.currentSession.timeouts.pageLoad,
+    }
+  ).finally(() => {
+    // Clean-up all registered listeners and timers
+    Services.obs.removeObserver(
+      onBrowsingContextDiscarded,
+      "browsing-context-discarded"
+    );
+    chromeWindow.removeEventListener("TabClose", onUnload);
+    chromeWindow.removeEventListener("unload", onUnload);
+    driver.curBrowser.tabBrowser?.removeEventListener(
+      "XULFrameLoaderCreated",
+      onBrowsingContextChanged
+    );
+    driver.dialogObserver?.remove(onDialogOpened);
+    unloadTimer?.cancel();
+
+    lazy.EventDispatcher.off("page-load", onNavigation);
+  });
+};
diff --git a/remote/marionette/packets.sys.mjs b/remote/marionette/packets.sys.mjs
new file mode 100644
index 0000000000..2c69fca70b
--- /dev/null
+++ b/remote/marionette/packets.sys.mjs
@@ -0,0 +1,426 @@
+/* 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, {
+  StreamUtils: "chrome://remote/content/marionette/stream-utils.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "unicodeConverter", () => {
+  const unicodeConverter = Cc[
+    "@mozilla.org/intl/scriptableunicodeconverter"
+  ].createInstance(Ci.nsIScriptableUnicodeConverter);
+  unicodeConverter.charset = "UTF-8";
+
+  return unicodeConverter;
+});
+
+/**
+ * Packets contain read / write functionality for the different packet types
+ * supported by the debugging protocol, so that a transport can focus on
+ * delivery and queue management without worrying too much about the specific
+ * packet types.
+ *
+ * They are intended to be "one use only", so a new packet should be
+ * instantiated for each incoming or outgoing packet.
+ *
+ * A complete Packet type should expose at least the following:
+ *   read(stream, scriptableStream)
+ *     Called when the input stream has data to read
+ *   write(stream)
+ *     Called when the output stream is ready to write
+ *   get done()
+ *     Returns true once the packet is done being read / written
+ *   destroy()
+ *     Called to clean up at the end of use
+ */
+
+const defer = function () {
+  let deferred = {
+    promise: new Promise((resolve, reject) => {
+      deferred.resolve = resolve;
+      deferred.reject = reject;
+    }),
+  };
+  return deferred;
+};
+
+// The transport's previous check ensured the header length did not
+// exceed 20 characters.  Here, we opt for the somewhat smaller, but still
+// large limit of 1 TiB.
+const PACKET_LENGTH_MAX = Math.pow(2, 40);
+
+/**
+ * A generic Packet processing object (extended by two subtypes below).
+ *
+ * @class
+ */
+export function Packet(transport) {
+  this._transport = transport;
+  this._length = 0;
+}
+
+/**
+ * Attempt to initialize a new Packet based on the incoming packet header
+ * we've received so far.  We try each of the types in succession, trying
+ * JSON packets first since they are much more common.
+ *
+ * @param {string} header
+ *     Packet header string to attempt parsing.
+ * @param {DebuggerTransport} transport
+ *     Transport instance that will own the packet.
+ *
+ * @returns {Packet}
+ *     Parsed packet of the matching type, or null if no types matched.
+ */
+Packet.fromHeader = function (header, transport) {
+  return (
+    JSONPacket.fromHeader(header, transport) ||
+    BulkPacket.fromHeader(header, transport)
+  );
+};
+
+Packet.prototype = {
+  get length() {
+    return this._length;
+  },
+
+  set length(length) {
+    if (length > PACKET_LENGTH_MAX) {
+      throw new Error(
+        "Packet length " +
+          length +
+          " exceeds the max length of " +
+          PACKET_LENGTH_MAX
+      );
+    }
+    this._length = length;
+  },
+
+  destroy() {
+    this._transport = null;
+  },
+};
+
+/**
+ * With a JSON packet (the typical packet type sent via the transport),
+ * data is transferred as a JSON packet serialized into a string,
+ * with the string length prepended to the packet, followed by a colon
+ * ([length]:[packet]). The contents of the JSON packet are specified in
+ * the Remote Debugging Protocol specification.
+ *
+ * @param {DebuggerTransport} transport
+ *     Transport instance that will own the packet.
+ */
+export function JSONPacket(transport) {
+  Packet.call(this, transport);
+  this._data = "";
+  this._done = false;
+}
+
+/**
+ * Attempt to initialize a new JSONPacket based on the incoming packet
+ * header we've received so far.
+ *
+ * @param {string} header
+ *     Packet header string to attempt parsing.
+ * @param {DebuggerTransport} transport
+ *     Transport instance that will own the packet.
+ *
+ * @returns {JSONPacket}
+ *     Parsed packet, or null if it's not a match.
+ */
+JSONPacket.fromHeader = function (header, transport) {
+  let match = this.HEADER_PATTERN.exec(header);
+
+  if (!match) {
+    return null;
+  }
+
+  let packet = new JSONPacket(transport);
+  packet.length = +match[1];
+  return packet;
+};
+
+JSONPacket.HEADER_PATTERN = /^(\d+):$/;
+
+JSONPacket.prototype = Object.create(Packet.prototype);
+
+Object.defineProperty(JSONPacket.prototype, "object", {
+  /**
+   * Gets the object (not the serialized string) being read or written.
+   */
+  get() {
+    return this._object;
+  },
+
+  /**
+   * Sets the object to be sent when write() is called.
+   */
+  set(object) {
+    this._object = object;
+    let data = JSON.stringify(object);
+    this._data = lazy.unicodeConverter.ConvertFromUnicode(data);
+    this.length = this._data.length;
+  },
+});
+
+JSONPacket.prototype.read = function (stream, scriptableStream) {
+  // Read in more packet data.
+  this._readData(stream, scriptableStream);
+
+  if (!this.done) {
+    // Don't have a complete packet yet.
+    return;
+  }
+
+  let json = this._data;
+  try {
+    json = lazy.unicodeConverter.ConvertToUnicode(json);
+    this._object = JSON.parse(json);
+  } catch (e) {
+    let msg =
+      "Error parsing incoming packet: " +
+      json +
+      " (" +
+      e +
+      " - " +
+      e.stack +
+      ")";
+    console.error(msg);
+    dump(msg + "\n");
+    return;
+  }
+
+  this._transport._onJSONObjectReady(this._object);
+};
+
+JSONPacket.prototype._readData = function (stream, scriptableStream) {
+  let bytesToRead = Math.min(
+    this.length - this._data.length,
+    stream.available()
+  );
+  this._data += scriptableStream.readBytes(bytesToRead);
+  this._done = this._data.length === this.length;
+};
+
+JSONPacket.prototype.write = function (stream) {
+  if (this._outgoing === undefined) {
+    // Format the serialized packet to a buffer
+    this._outgoing = this.length + ":" + this._data;
+  }
+
+  let written = stream.write(this._outgoing, this._outgoing.length);
+  this._outgoing = this._outgoing.slice(written);
+  this._done = !this._outgoing.length;
+};
+
+Object.defineProperty(JSONPacket.prototype, "done", {
+  get() {
+    return this._done;
+  },
+});
+
+JSONPacket.prototype.toString = function () {
+  return JSON.stringify(this._object, null, 2);
+};
+
+/**
+ * With a bulk packet, data is transferred by temporarily handing over
+ * the transport's input or output stream to the application layer for
+ * writing data directly.  This can be much faster for large data sets,
+ * and avoids various stages of copies and data duplication inherent in
+ * the JSON packet type.  The bulk packet looks like:
+ *
+ *     bulk [actor] [type] [length]:[data]
+ *
+ * The interpretation of the data portion depends on the kind of actor and
+ * the packet's type.  See the Remote Debugging Protocol Stream Transport
+ * spec for more details.
+ *
+ * @param {DebuggerTransport} transport
+ *     Transport instance that will own the packet.
+ */
+export function BulkPacket(transport) {
+  Packet.call(this, transport);
+  this._done = false;
+  this._readyForWriting = defer();
+}
+
+/**
+ * Attempt to initialize a new BulkPacket based on the incoming packet
+ * header we've received so far.
+ *
+ * @param {string} header
+ *     Packet header string to attempt parsing.
+ * @param {DebuggerTransport} transport
+ *     Transport instance that will own the packet.
+ *
+ * @returns {BulkPacket}
+ *     Parsed packet, or null if it's not a match.
+ */
+BulkPacket.fromHeader = function (header, transport) {
+  let match = this.HEADER_PATTERN.exec(header);
+
+  if (!match) {
+    return null;
+  }
+
+  let packet = new BulkPacket(transport);
+  packet.header = {
+    actor: match[1],
+    type: match[2],
+    length: +match[3],
+  };
+  return packet;
+};
+
+BulkPacket.HEADER_PATTERN = /^bulk ([^: ]+) ([^: ]+) (\d+):$/;
+
+BulkPacket.prototype = Object.create(Packet.prototype);
+
+BulkPacket.prototype.read = function (stream) {
+  // Temporarily pause monitoring of the input stream
+  this._transport.pauseIncoming();
+
+  let deferred = defer();
+
+  this._transport._onBulkReadReady({
+    actor: this.actor,
+    type: this.type,
+    length: this.length,
+    copyTo: output => {
+      let copying = lazy.StreamUtils.copyStream(stream, output, this.length);
+      deferred.resolve(copying);
+      return copying;
+    },
+    stream,
+    done: deferred,
+  });
+
+  // Await the result of reading from the stream
+  deferred.promise.then(() => {
+    this._done = true;
+    this._transport.resumeIncoming();
+  }, this._transport.close);
+
+  // Ensure this is only done once
+  this.read = () => {
+    throw new Error("Tried to read() a BulkPacket's stream multiple times.");
+  };
+};
+
+BulkPacket.prototype.write = function (stream) {
+  if (this._outgoingHeader === undefined) {
+    // Format the serialized packet header to a buffer
+    this._outgoingHeader =
+      "bulk " + this.actor + " " + this.type + " " + this.length + ":";
+  }
+
+  // Write the header, or whatever's left of it to write.
+  if (this._outgoingHeader.length) {
+    let written = stream.write(
+      this._outgoingHeader,
+      this._outgoingHeader.length
+    );
+    this._outgoingHeader = this._outgoingHeader.slice(written);
+    return;
+  }
+
+  // Temporarily pause the monitoring of the output stream
+  this._transport.pauseOutgoing();
+
+  let deferred = defer();
+
+  this._readyForWriting.resolve({
+    copyFrom: input => {
+      let copying = lazy.StreamUtils.copyStream(input, stream, this.length);
+      deferred.resolve(copying);
+      return copying;
+    },
+    stream,
+    done: deferred,
+  });
+
+  // Await the result of writing to the stream
+  deferred.promise.then(() => {
+    this._done = true;
+    this._transport.resumeOutgoing();
+  }, this._transport.close);
+
+  // Ensure this is only done once
+  this.write = () => {
+    throw new Error("Tried to write() a BulkPacket's stream multiple times.");
+  };
+};
+
+Object.defineProperty(BulkPacket.prototype, "streamReadyForWriting", {
+  get() {
+    return this._readyForWriting.promise;
+  },
+});
+
+Object.defineProperty(BulkPacket.prototype, "header", {
+  get() {
+    return {
+      actor: this.actor,
+      type: this.type,
+      length: this.length,
+    };
+  },
+
+  set(header) {
+    this.actor = header.actor;
+    this.type = header.type;
+    this.length = header.length;
+  },
+});
+
+Object.defineProperty(BulkPacket.prototype, "done", {
+  get() {
+    return this._done;
+  },
+});
+
+BulkPacket.prototype.toString = function () {
+  return "Bulk: " + JSON.stringify(this.header, null, 2);
+};
+
+/**
+ * RawPacket is used to test the transport's error handling of malformed
+ * packets, by writing data directly onto the stream.
+ *
+ * @param {DebuggerTransport} transport
+ *     The transport instance that will own the packet.
+ * @param {string} data
+ *     The raw string to send out onto the stream.
+ */
+export function RawPacket(transport, data) {
+  Packet.call(this, transport);
+  this._data = data;
+  this.length = data.length;
+  this._done = false;
+}
+
+RawPacket.prototype = Object.create(Packet.prototype);
+
+RawPacket.prototype.read = function () {
+  // this has not yet been needed for testing
+  throw new Error("Not implemented");
+};
+
+RawPacket.prototype.write = function (stream) {
+  let written = stream.write(this._data, this._data.length);
+  this._data = this._data.slice(written);
+  this._done = !this._data.length;
+};
+
+Object.defineProperty(RawPacket.prototype, "done", {
+  get() {
+    return this._done;
+  },
+});
diff --git a/remote/marionette/permissions.sys.mjs b/remote/marionette/permissions.sys.mjs
new file mode 100644
index 0000000000..d67e2c46b7
--- /dev/null
+++ b/remote/marionette/permissions.sys.mjs
@@ -0,0 +1,69 @@
+/* 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, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  MarionettePrefs: "chrome://remote/content/marionette/prefs.sys.mjs",
+});
+
+/** @namespace */
+export const permissions = {};
+
+/**
+ * Set a permission's state.
+ * Note: Currently just a shim to support testdriver's set_permission.
+ *
+ * @param {object} descriptor
+ *     Descriptor with the `name` property.
+ * @param {string} state
+ *     State of the permission. It can be `granted`, `denied` or `prompt`.
+ * @param {boolean} oneRealm
+ *     Currently ignored
+ *
+ * @throws {UnsupportedOperationError}
+ *     If `marionette.setpermission.enabled` is not set or
+ *     an unsupported permission is used.
+ */
+permissions.set = function (descriptor, state, oneRealm) {
+  if (!lazy.MarionettePrefs.setPermissionEnabled) {
+    throw new lazy.error.UnsupportedOperationError(
+      "'Set Permission' is not available"
+    );
+  }
+
+  if (state === "prompt") {
+    throw new lazy.error.UnsupportedOperationError(
+      "'Set Permission' doesn't support prompt"
+    );
+  }
+
+  // This is not a real implementation of the permissions API.
+  // Instead the purpose of this implementation is to have web-platform-tests
+  // that use `set_permission()` not fail.
+  // Each test needs the corresponding testing pref to make it actually work.
+  const { name } = descriptor;
+  if (["clipboard-write", "clipboard-read"].includes(name)) {
+    if (
+      Services.prefs.getBoolPref("dom.events.testing.asyncClipboard", false)
+    ) {
+      return;
+    }
+    throw new lazy.error.UnsupportedOperationError(
+      "'Set Permission' expected dom.events.testing.asyncClipboard to be set"
+    );
+  } else if (name === "notifications") {
+    if (Services.prefs.getBoolPref("notification.prompt.testing", false)) {
+      return;
+    }
+    throw new lazy.error.UnsupportedOperationError(
+      "'Set Permission' expected notification.prompt.testing to be set"
+    );
+  }
+
+  throw new lazy.error.UnsupportedOperationError(
+    `'Set Permission' doesn't support '${name}'`
+  );
+};
diff --git a/remote/marionette/prefs.sys.mjs b/remote/marionette/prefs.sys.mjs
new file mode 100644
index 0000000000..17df13d0fd
--- /dev/null
+++ b/remote/marionette/prefs.sys.mjs
@@ -0,0 +1,175 @@
+/* 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 { PREF_BOOL, PREF_INT, PREF_INVALID, PREF_STRING } = Ci.nsIPrefBranch;
+
+export class Branch {
+  /**
+   * @param {string=} branch
+   *     Preference subtree.  Uses root tree given `null`.
+   */
+  constructor(branch) {
+    this._branch = Services.prefs.getBranch(branch);
+  }
+
+  /**
+   * Gets value of `pref` in its known type.
+   *
+   * @param {string} pref
+   *     Preference name.
+   * @param {*=} fallback
+   *     Fallback value to return if `pref` does not exist.
+   *
+   * @returns {(string|boolean|number)}
+   *     Value of `pref`, or the `fallback` value if `pref` does
+   *     not exist.
+   *
+   * @throws {TypeError}
+   *     If `pref` is not a recognised preference and no `fallback`
+   *     value has been provided.
+   */
+  get(pref, fallback = null) {
+    switch (this._branch.getPrefType(pref)) {
+      case PREF_STRING:
+        return this._branch.getStringPref(pref);
+
+      case PREF_BOOL:
+        return this._branch.getBoolPref(pref);
+
+      case PREF_INT:
+        return this._branch.getIntPref(pref);
+
+      case PREF_INVALID:
+      default:
+        if (fallback != null) {
+          return fallback;
+        }
+        throw new TypeError(`Unrecognised preference: ${pref}`);
+    }
+  }
+
+  /**
+   * Sets the value of `pref`.
+   *
+   * @param {string} pref
+   *     Preference name.
+   * @param {(string|boolean|number)} value
+   *     `pref`'s new value.
+   *
+   * @throws {TypeError}
+   *     If `value` is not the correct type for `pref`.
+   */
+  set(pref, value) {
+    let typ;
+    if (typeof value != "undefined" && value != null) {
+      typ = value.constructor.name;
+    }
+
+    switch (typ) {
+      case "String":
+        // Unicode compliant
+        return this._branch.setStringPref(pref, value);
+
+      case "Boolean":
+        return this._branch.setBoolPref(pref, value);
+
+      case "Number":
+        return this._branch.setIntPref(pref, value);
+
+      default:
+        throw new TypeError(`Illegal preference type value: ${typ}`);
+    }
+  }
+}
+
+/**
+ * Provides shortcuts for lazily getting and setting typed Marionette
+ * preferences.
+ *
+ * Some of Marionette's preferences are stored using primitive values
+ * that internally are represented by complex types.
+ *
+ * Because we cannot trust the input of many of these preferences,
+ * this class provides abstraction that lets us safely deal with
+ * potentially malformed input.
+ *
+ * A further complication is that we cannot rely on `Preferences.sys.mjs`
+ * in Marionette.  See https://bugzilla.mozilla.org/show_bug.cgi?id=1357517
+ * for further details.
+ */
+class MarionetteBranch extends Branch {
+  constructor(branch = "marionette.") {
+    super(branch);
+  }
+
+  /**
+   * The `marionette.debugging.clicktostart` preference delays
+   * server startup until a modal dialogue has been clicked to allow
+   * time for user to set breakpoints in the Browser Toolbox.
+   *
+   * @returns {boolean}
+   */
+  get clickToStart() {
+    return this.get("debugging.clicktostart", false);
+  }
+
+  /**
+   * The `marionette.port` preference, detailing which port
+   * the TCP server should listen on.
+   *
+   * @returns {number}
+   */
+  get port() {
+    return this.get("port", 2828);
+  }
+
+  set port(newPort) {
+    this.set("port", newPort);
+  }
+
+  /**
+   * Gets the `marionette.setpermission.enabled` preference, should
+   * only be used for testdriver's set_permission API.
+   *
+   * @returns {boolean}
+   */
+  get setPermissionEnabled() {
+    return this.get("setpermission.enabled", false);
+  }
+}
+
+/** Reads a JSON serialised blob stored in the environment. */
+export class EnvironmentPrefs {
+  /**
+   * Reads the environment variable `key` and tries to parse it as
+   * JSON Object, then provides an iterator over its keys and values.
+   *
+   * If the environment variable is not set, this function returns empty.
+   *
+   * @param {string} key
+   *     Environment variable.
+   *
+   * @returns {Iterable.<string, (string|boolean|number)>}
+   */
+  static *from(key) {
+    if (!Services.env.exists(key)) {
+      return;
+    }
+
+    let prefs;
+    try {
+      prefs = JSON.parse(Services.env.get(key));
+    } catch (e) {
+      throw new TypeError(`Unable to parse prefs from ${key}`, e);
+    }
+
+    for (let prefName of Object.keys(prefs)) {
+      yield [prefName, prefs[prefName]];
+    }
+  }
+}
+
+// There is a future potential of exposing this as Marionette.prefs.port
+// if we introduce a Marionette.jsm module.
+export const MarionettePrefs = new MarionetteBranch();
diff --git a/remote/marionette/reftest-content.js b/remote/marionette/reftest-content.js
new file mode 100644
index 0000000000..3c0712f232
--- /dev/null
+++ b/remote/marionette/reftest-content.js
@@ -0,0 +1,65 @@
+/* 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/. */
+
+/* eslint-env mozilla/frame-script */
+
+"use strict";
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+
+XPCOMUtils.defineLazyScriptGetter(
+  this,
+  "PrintUtils",
+  "chrome://global/content/printUtils.js"
+);
+
+// This is an implementation of nsIBrowserDOMWindow that handles only opening
+// print browsers, because the "open a new window fallback" is just too slow
+// in some cases and causes timeouts.
+function BrowserDOMWindow() {}
+BrowserDOMWindow.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIBrowserDOMWindow"]),
+
+  _maybeOpen(aOpenWindowInfo, aWhere) {
+    if (aWhere == Ci.nsIBrowserDOMWindow.OPEN_PRINT_BROWSER) {
+      return PrintUtils.handleStaticCloneCreatedForPrint(aOpenWindowInfo);
+    }
+    return null;
+  },
+
+  createContentWindow(
+    aURI,
+    aOpenWindowInfo,
+    aWhere,
+    aFlags,
+    aTriggeringPrincipal,
+    aCsp
+  ) {
+    return this._maybeOpen(aOpenWindowInfo, aWhere)?.browsingContext;
+  },
+
+  openURI(aURI, aOpenWindowInfo, aWhere, aFlags, aTriggeringPrincipal, aCsp) {
+    return this._maybeOpen(aOpenWindowInfo, aWhere)?.browsingContext;
+  },
+
+  createContentWindowInFrame(aURI, aParams, aWhere, aFlags, aName) {
+    return this._maybeOpen(aParams.openWindowInfo, aWhere);
+  },
+
+  openURIInFrame(aURI, aParams, aWhere, aFlags, aName) {
+    return this._maybeOpen(aParams.openWindowInfo, aWhere);
+  },
+
+  canClose() {
+    return true;
+  },
+
+  get tabCount() {
+    return 1;
+  },
+};
+
+window.browserDOMWindow = new BrowserDOMWindow();
diff --git a/remote/marionette/reftest.sys.mjs b/remote/marionette/reftest.sys.mjs
new file mode 100644
index 0000000000..635f2196ef
--- /dev/null
+++ b/remote/marionette/reftest.sys.mjs
@@ -0,0 +1,907 @@
+/* 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, {
+  clearTimeout: "resource://gre/modules/Timer.sys.mjs",
+  E10SUtils: "resource://gre/modules/E10SUtils.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  capture: "chrome://remote/content/shared/Capture.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  navigate: "chrome://remote/content/marionette/navigate.sys.mjs",
+  print: "chrome://remote/content/shared/PDF.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+const XHTML_NS = "http://www.w3.org/1999/xhtml";
+const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+const SCREENSHOT_MODE = {
+  unexpected: 0,
+  fail: 1,
+  always: 2,
+};
+
+const STATUS = {
+  PASS: "PASS",
+  FAIL: "FAIL",
+  ERROR: "ERROR",
+  TIMEOUT: "TIMEOUT",
+};
+
+const DEFAULT_REFTEST_WIDTH = 600;
+const DEFAULT_REFTEST_HEIGHT = 600;
+
+// reftest-print page dimensions in cm
+const CM_PER_INCH = 2.54;
+const DEFAULT_PAGE_WIDTH = 5 * CM_PER_INCH;
+const DEFAULT_PAGE_HEIGHT = 3 * CM_PER_INCH;
+const DEFAULT_PAGE_MARGIN = 0.5 * CM_PER_INCH;
+
+// CSS 96 pixels per inch, compared to pdf.js default 72 pixels per inch
+const DEFAULT_PDF_RESOLUTION = 96 / 72;
+
+/**
+ * Implements an fast runner for web-platform-tests format reftests
+ * c.f. http://web-platform-tests.org/writing-tests/reftests.html.
+ *
+ * @namespace
+ */
+export const reftest = {};
+
+/**
+ * @memberof reftest
+ * @class Runner
+ */
+reftest.Runner = class {
+  constructor(driver) {
+    this.driver = driver;
+    this.canvasCache = new DefaultMap(undefined, () => new Map([[null, []]]));
+    this.isPrint = null;
+    this.windowUtils = null;
+    this.lastURL = null;
+    this.useRemoteTabs = lazy.AppInfo.browserTabsRemoteAutostart;
+    this.useRemoteSubframes = lazy.AppInfo.fissionAutostart;
+  }
+
+  /**
+   * Setup the required environment for running reftests.
+   *
+   * This will open a non-browser window in which the tests will
+   * be loaded, and set up various caches for the reftest run.
+   *
+   * @param {Object<number>} urlCount
+   *     Object holding a map of URL: number of times the URL
+   *     will be opened during the reftest run, where that's
+   *     greater than 1.
+   * @param {string} screenshotMode
+   *     String enum representing when screenshots should be taken
+   */
+  setup(urlCount, screenshotMode, isPrint = false) {
+    this.isPrint = isPrint;
+
+    lazy.assert.open(this.driver.getBrowsingContext({ top: true }));
+    this.parentWindow = this.driver.getCurrentWindow();
+
+    this.screenshotMode =
+      SCREENSHOT_MODE[screenshotMode] || SCREENSHOT_MODE.unexpected;
+
+    this.urlCount = Object.keys(urlCount || {}).reduce(
+      (map, key) => map.set(key, urlCount[key]),
+      new Map()
+    );
+
+    if (isPrint) {
+      this.loadPdfJs();
+    }
+
+    ChromeUtils.registerWindowActor("MarionetteReftest", {
+      kind: "JSWindowActor",
+      parent: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteReftestParent.sys.mjs",
+      },
+      child: {
+        esModuleURI:
+          "chrome://remote/content/marionette/actors/MarionetteReftestChild.sys.mjs",
+        events: {
+          load: { mozSystemGroup: true, capture: true },
+        },
+      },
+      allFrames: true,
+    });
+  }
+
+  /**
+   * Cleanup the environment once the reftest is finished.
+   */
+  teardown() {
+    // Abort the current test if any.
+    this.abort();
+
+    // Unregister the JSWindowActors.
+    ChromeUtils.unregisterWindowActor("MarionetteReftest");
+  }
+
+  async ensureWindow(timeout, width, height) {
+    lazy.logger.debug(`ensuring we have a window ${width}x${height}`);
+
+    if (this.reftestWin && !this.reftestWin.closed) {
+      let browserRect = this.reftestWin.gBrowser.getBoundingClientRect();
+      if (browserRect.width === width && browserRect.height === height) {
+        return this.reftestWin;
+      }
+      lazy.logger.debug(`current: ${browserRect.width}x${browserRect.height}`);
+    }
+
+    let reftestWin;
+    if (lazy.AppInfo.isAndroid) {
+      lazy.logger.debug("Using current window");
+      reftestWin = this.parentWindow;
+      await lazy.navigate.waitForNavigationCompleted(this.driver, () => {
+        const browsingContext = this.driver.getBrowsingContext();
+        lazy.navigate.navigateTo(browsingContext, "about:blank");
+      });
+    } else {
+      lazy.logger.debug("Using separate window");
+      if (this.reftestWin && !this.reftestWin.closed) {
+        this.reftestWin.close();
+      }
+      reftestWin = await this.openWindow(width, height);
+    }
+
+    this.setupWindow(reftestWin, width, height);
+    this.windowUtils = reftestWin.windowUtils;
+    this.reftestWin = reftestWin;
+
+    let windowHandle = lazy.windowManager.getWindowProperties(reftestWin);
+    await this.driver.setWindowHandle(windowHandle, true);
+
+    const url = await this.driver._getCurrentURL();
+    this.lastURL = url.href;
+    lazy.logger.debug(`loaded initial URL: ${this.lastURL}`);
+
+    let browserRect = reftestWin.gBrowser.getBoundingClientRect();
+    lazy.logger.debug(`new: ${browserRect.width}x${browserRect.height}`);
+
+    return reftestWin;
+  }
+
+  async openWindow(width, height) {
+    lazy.assert.positiveInteger(width);
+    lazy.assert.positiveInteger(height);
+
+    let reftestWin = this.parentWindow.open(
+      "chrome://remote/content/marionette/reftest.xhtml",
+      "reftest",
+      `chrome,height=${height},width=${width}`
+    );
+
+    await new Promise(resolve => {
+      reftestWin.addEventListener("load", resolve, { once: true });
+    });
+    return reftestWin;
+  }
+
+  setupWindow(reftestWin, width, height) {
+    let browser;
+    if (lazy.AppInfo.isAndroid) {
+      browser = reftestWin.document.getElementsByTagName("browser")[0];
+      browser.setAttribute("remote", "false");
+    } else {
+      browser = reftestWin.document.createElementNS(XUL_NS, "xul:browser");
+      browser.permanentKey = {};
+      browser.setAttribute("id", "browser");
+      browser.setAttribute("type", "content");
+      browser.setAttribute("primary", "true");
+      browser.setAttribute("remote", this.useRemoteTabs ? "true" : "false");
+    }
+    // Make sure the browser element is exactly the right size, no matter
+    // what size our window is
+    const windowStyle = `
+      padding: 0px;
+      margin: 0px;
+      border:none;
+      min-width: ${width}px; min-height: ${height}px;
+      max-width: ${width}px; max-height: ${height}px;
+      color-scheme: env(-moz-content-preferred-color-scheme);
+    `;
+    browser.setAttribute("style", windowStyle);
+
+    if (!lazy.AppInfo.isAndroid) {
+      let doc = reftestWin.document.documentElement;
+      while (doc.firstChild) {
+        doc.firstChild.remove();
+      }
+      doc.appendChild(browser);
+    }
+    if (reftestWin.BrowserApp) {
+      reftestWin.BrowserApp = browser;
+    }
+    reftestWin.gBrowser = browser;
+    return reftestWin;
+  }
+
+  async abort() {
+    if (this.reftestWin && this.reftestWin != this.parentWindow) {
+      await this.driver.closeChromeWindow();
+      let parentHandle = lazy.windowManager.getWindowProperties(
+        this.parentWindow
+      );
+      await this.driver.setWindowHandle(parentHandle);
+    }
+    this.reftestWin = null;
+  }
+
+  /**
+   * Run a specific reftest.
+   *
+   * The assumed semantics are those of web-platform-tests where
+   * references form a tree and each test must meet all the conditions
+   * to reach one leaf node of the tree in order for the overall test
+   * to pass.
+   *
+   * @param {string} testUrl
+   *     URL of the test itself.
+   * @param {Array.<Array>} references
+   *     Array representing a tree of references to try.
+   *
+   *     Each item in the array represents a single reference node and
+   *     has the form <code>[referenceUrl, references, relation]</code>,
+   *     where <var>referenceUrl</var> is a string to the URL, relation
+   *     is either <code>==</code> or <code>!=</code> depending on the
+   *     type of reftest, and references is another array containing
+   *     items of the same form, representing further comparisons treated
+   *     as AND with the current item. Sibling entries are treated as OR.
+   *
+   *     For example with testUrl of T:
+   *
+   *     <pre><code>
+   *       references = [[A, [[B, [], ==]], ==]]
+   *       Must have T == A AND A == B to pass
+   *
+   *       references = [[A, [], ==], [B, [], !=]
+   *       Must have T == A OR T != B
+   *
+   *       references = [[A, [[B, [], ==], [C, [], ==]], ==], [D, [], ]]
+   *       Must have (T == A AND A == B) OR (T == A AND A == C) OR (T == D)
+   *     </code></pre>
+   *
+   * @param {string} expected
+   *     Expected test outcome (e.g. <tt>PASS</tt>, <tt>FAIL</tt>).
+   * @param {number} timeout
+   *     Test timeout in milliseconds.
+   *
+   * @returns {object}
+   *     Result object with fields status, message and extra.
+   */
+  async run(
+    testUrl,
+    references,
+    expected,
+    timeout,
+    pageRanges = {},
+    width = DEFAULT_REFTEST_WIDTH,
+    height = DEFAULT_REFTEST_HEIGHT
+  ) {
+    let timerId;
+
+    let timeoutPromise = new Promise(resolve => {
+      timerId = lazy.setTimeout(() => {
+        resolve({ status: STATUS.TIMEOUT, message: null, extra: {} });
+      }, timeout);
+    });
+
+    let testRunner = (async () => {
+      let result;
+      try {
+        result = await this.runTest(
+          testUrl,
+          references,
+          expected,
+          timeout,
+          pageRanges,
+          width,
+          height
+        );
+      } catch (e) {
+        result = {
+          status: STATUS.ERROR,
+          message: String(e),
+          stack: e.stack,
+          extra: {},
+        };
+      }
+      return result;
+    })();
+
+    let result = await Promise.race([testRunner, timeoutPromise]);
+    lazy.clearTimeout(timerId);
+    if (result.status === STATUS.TIMEOUT) {
+      await this.abort();
+    }
+
+    return result;
+  }
+
+  async runTest(
+    testUrl,
+    references,
+    expected,
+    timeout,
+    pageRanges,
+    width,
+    height
+  ) {
+    let win = await this.ensureWindow(timeout, width, height);
+
+    function toBase64(screenshot) {
+      let dataURL = screenshot.canvas.toDataURL();
+      return dataURL.split(",")[1];
+    }
+
+    let result = {
+      status: STATUS.FAIL,
+      message: "",
+      stack: null,
+      extra: {},
+    };
+
+    let screenshotData = [];
+
+    let stack = [];
+    for (let i = references.length - 1; i >= 0; i--) {
+      let item = references[i];
+      stack.push([testUrl, ...item]);
+    }
+
+    let done = false;
+
+    while (stack.length && !done) {
+      let [lhsUrl, rhsUrl, references, relation, extras = {}] = stack.pop();
+      result.message += `Testing ${lhsUrl} ${relation} ${rhsUrl}\n`;
+
+      let comparison;
+      try {
+        comparison = await this.compareUrls(
+          win,
+          lhsUrl,
+          rhsUrl,
+          relation,
+          timeout,
+          pageRanges,
+          extras
+        );
+      } catch (e) {
+        comparison = {
+          lhs: null,
+          rhs: null,
+          passed: false,
+          error: e,
+          msg: null,
+        };
+      }
+      if (comparison.msg) {
+        result.message += `${comparison.msg}\n`;
+      }
+      if (comparison.error !== null) {
+        result.status = STATUS.ERROR;
+        result.message += String(comparison.error);
+        result.stack = comparison.error.stack;
+      }
+
+      function recordScreenshot() {
+        let encodedLHS = comparison.lhs ? toBase64(comparison.lhs) : "";
+        let encodedRHS = comparison.rhs ? toBase64(comparison.rhs) : "";
+        screenshotData.push([
+          { url: lhsUrl, screenshot: encodedLHS },
+          relation,
+          { url: rhsUrl, screenshot: encodedRHS },
+        ]);
+      }
+
+      if (this.screenshotMode === SCREENSHOT_MODE.always) {
+        recordScreenshot();
+      }
+
+      if (comparison.passed) {
+        if (references.length) {
+          for (let i = references.length - 1; i >= 0; i--) {
+            let item = references[i];
+            stack.push([rhsUrl, ...item]);
+          }
+        } else {
+          // Reached a leaf node so all of one reference chain passed
+          result.status = STATUS.PASS;
+          if (
+            this.screenshotMode <= SCREENSHOT_MODE.fail &&
+            expected != result.status
+          ) {
+            recordScreenshot();
+          }
+          done = true;
+        }
+      } else if (!stack.length || result.status == STATUS.ERROR) {
+        // If we don't have any alternatives to try then this will be
+        // the last iteration, so save the failing screenshots if required.
+        let isFail = this.screenshotMode === SCREENSHOT_MODE.fail;
+        let isUnexpected = this.screenshotMode === SCREENSHOT_MODE.unexpected;
+        if (isFail || (isUnexpected && expected != result.status)) {
+          recordScreenshot();
+        }
+      }
+
+      // Return any reusable canvases to the pool
+      let cacheKey = width + "x" + height;
+      let canvasPool = this.canvasCache.get(cacheKey).get(null);
+      [comparison.lhs, comparison.rhs].map(screenshot => {
+        if (screenshot !== null && screenshot.reuseCanvas) {
+          canvasPool.push(screenshot.canvas);
+        }
+      });
+      lazy.logger.debug(
+        `Canvas pool (${cacheKey}) is of length ${canvasPool.length}`
+      );
+    }
+
+    if (screenshotData.length) {
+      // For now the tbpl formatter only accepts one screenshot, so just
+      // return the last one we took.
+      let lastScreenshot = screenshotData[screenshotData.length - 1];
+      // eslint-disable-next-line camelcase
+      result.extra.reftest_screenshots = lastScreenshot;
+    }
+
+    return result;
+  }
+
+  async compareUrls(
+    win,
+    lhsUrl,
+    rhsUrl,
+    relation,
+    timeout,
+    pageRanges,
+    extras
+  ) {
+    lazy.logger.info(`Testing ${lhsUrl} ${relation} ${rhsUrl}`);
+
+    if (relation !== "==" && relation != "!=") {
+      throw new error.InvalidArgumentError(
+        "Reftest operator should be '==' or '!='"
+      );
+    }
+
+    let lhsIter, lhsCount, rhsIter, rhsCount;
+    if (!this.isPrint) {
+      // Take the reference screenshot first so that if we pause
+      // we see the test rendering
+      rhsIter = [await this.screenshot(win, rhsUrl, timeout)].values();
+      lhsIter = [await this.screenshot(win, lhsUrl, timeout)].values();
+      lhsCount = rhsCount = 1;
+    } else {
+      [rhsIter, rhsCount] = await this.screenshotPaginated(
+        win,
+        rhsUrl,
+        timeout,
+        pageRanges
+      );
+      [lhsIter, lhsCount] = await this.screenshotPaginated(
+        win,
+        lhsUrl,
+        timeout,
+        pageRanges
+      );
+    }
+
+    let passed = null;
+    let error = null;
+    let pixelsDifferent = null;
+    let maxDifferences = {};
+    let msg = null;
+
+    if (lhsCount != rhsCount) {
+      passed = relation == "!=";
+      if (!passed) {
+        msg = `Got different numbers of pages; test has ${lhsCount}, ref has ${rhsCount}`;
+      }
+    }
+
+    let lhs = null;
+    let rhs = null;
+    lazy.logger.debug(`Comparing ${lhsCount} pages`);
+    if (passed === null) {
+      for (let i = 0; i < lhsCount; i++) {
+        lhs = (await lhsIter.next()).value;
+        rhs = (await rhsIter.next()).value;
+        lazy.logger.debug(
+          `lhs canvas size ${lhs.canvas.width}x${lhs.canvas.height}`
+        );
+        lazy.logger.debug(
+          `rhs canvas size ${rhs.canvas.width}x${rhs.canvas.height}`
+        );
+        try {
+          pixelsDifferent = this.windowUtils.compareCanvases(
+            lhs.canvas,
+            rhs.canvas,
+            maxDifferences
+          );
+        } catch (e) {
+          error = e;
+          passed = false;
+          break;
+        }
+
+        let areEqual = this.isAcceptableDifference(
+          maxDifferences.value,
+          pixelsDifferent,
+          extras.fuzzy
+        );
+        lazy.logger.debug(
+          `Page ${i + 1} maxDifferences: ${maxDifferences.value} ` +
+            `pixelsDifferent: ${pixelsDifferent}`
+        );
+        lazy.logger.debug(
+          `Page ${i + 1} ${areEqual ? "compare equal" : "compare unequal"}`
+        );
+        if (!areEqual) {
+          if (relation == "==") {
+            passed = false;
+            msg =
+              `Found ${pixelsDifferent} pixels different, ` +
+              `maximum difference per channel ${maxDifferences.value}`;
+            if (this.isPrint) {
+              msg += ` on page ${i + 1}`;
+            }
+          } else {
+            passed = true;
+          }
+          break;
+        }
+      }
+    }
+
+    // If passed isn't set we got to the end without finding differences
+    if (passed === null) {
+      if (relation == "==") {
+        passed = true;
+      } else {
+        msg = `mismatch reftest has no differences`;
+        passed = false;
+      }
+    }
+    return { lhs, rhs, passed, error, msg };
+  }
+
+  isAcceptableDifference(maxDifference, pixelsDifferent, allowed) {
+    if (!allowed) {
+      lazy.logger.info(`No differences allowed`);
+      return pixelsDifferent === 0;
+    }
+    let [allowedDiff, allowedPixels] = allowed;
+    lazy.logger.info(
+      `Allowed ${allowedPixels.join("-")} pixels different, ` +
+        `maximum difference per channel ${allowedDiff.join("-")}`
+    );
+    return (
+      (pixelsDifferent === 0 && allowedPixels[0] == 0) ||
+      (maxDifference === 0 && allowedDiff[0] == 0) ||
+      (maxDifference >= allowedDiff[0] &&
+        maxDifference <= allowedDiff[1] &&
+        (pixelsDifferent >= allowedPixels[0] ||
+          pixelsDifferent <= allowedPixels[1]))
+    );
+  }
+
+  ensureFocus(win) {
+    const focusManager = Services.focus;
+    if (focusManager.activeWindow != win) {
+      win.focus();
+    }
+    this.driver.curBrowser.contentBrowser.focus();
+  }
+
+  updateBrowserRemotenessByURL(browser, url) {
+    // We don't use remote tabs on Android.
+    if (lazy.AppInfo.isAndroid) {
+      return;
+    }
+    let oa = lazy.E10SUtils.predictOriginAttributes({ browser });
+    let remoteType = lazy.E10SUtils.getRemoteTypeForURI(
+      url,
+      this.useRemoteTabs,
+      this.useRemoteSubframes,
+      lazy.E10SUtils.DEFAULT_REMOTE_TYPE,
+      null,
+      oa
+    );
+
+    // Only re-construct the browser if its remote type needs to change.
+    if (browser.remoteType !== remoteType) {
+      if (remoteType === lazy.E10SUtils.NOT_REMOTE) {
+        browser.removeAttribute("remote");
+        browser.removeAttribute("remoteType");
+      } else {
+        browser.setAttribute("remote", "true");
+        browser.setAttribute("remoteType", remoteType);
+      }
+
+      browser.changeRemoteness({ remoteType });
+      browser.construct();
+    }
+  }
+
+  async loadTestUrl(win, url, timeout) {
+    const browsingContext = this.driver.getBrowsingContext({ top: true });
+    const webProgress = browsingContext.webProgress;
+
+    lazy.logger.debug(`Starting load of ${url}`);
+    if (this.lastURL === url) {
+      lazy.logger.debug(`Refreshing page`);
+      await lazy.navigate.waitForNavigationCompleted(this.driver, () => {
+        lazy.navigate.refresh(browsingContext);
+      });
+    } else {
+      // HACK: DocumentLoadListener currently doesn't know how to
+      // process-switch loads in a non-tabbed <browser>. We need to manually
+      // set the browser's remote type in order to ensure that the load
+      // happens in the correct process.
+      //
+      // See bug 1636169.
+      this.updateBrowserRemotenessByURL(win.gBrowser, url);
+      lazy.navigate.navigateTo(browsingContext, url);
+
+      this.lastURL = url;
+    }
+
+    this.ensureFocus(win);
+
+    // TODO: Move all the wait logic into the parent process (bug 1669787)
+    let isReftestReady = false;
+    while (!isReftestReady) {
+      // Note: We cannot compare the URL here. Before the navigation is complete
+      // currentWindowGlobal.documentURI.spec will still point to the old URL.
+      const actor =
+        webProgress.browsingContext.currentWindowGlobal.getActor(
+          "MarionetteReftest"
+        );
+      isReftestReady = await actor.reftestWait(url, this.useRemoteTabs);
+    }
+  }
+
+  async screenshot(win, url, timeout) {
+    // On windows the above doesn't *actually* set the window to be the
+    // reftest size; but *does* set the content area to be the right size;
+    // the window is given some extra borders that aren't explicable from CSS
+    let browserRect = win.gBrowser.getBoundingClientRect();
+    let canvas = null;
+    let remainingCount = this.urlCount.get(url) || 1;
+    let cache = remainingCount > 1;
+    let cacheKey = browserRect.width + "x" + browserRect.height;
+    lazy.logger.debug(
+      `screenshot ${url} remainingCount: ` +
+        `${remainingCount} cache: ${cache} cacheKey: ${cacheKey}`
+    );
+    let reuseCanvas = false;
+    let sizedCache = this.canvasCache.get(cacheKey);
+    if (sizedCache.has(url)) {
+      lazy.logger.debug(`screenshot ${url} taken from cache`);
+      canvas = sizedCache.get(url);
+      if (!cache) {
+        sizedCache.delete(url);
+      }
+    } else {
+      let canvasPool = sizedCache.get(null);
+      if (canvasPool.length) {
+        lazy.logger.debug("reusing canvas from canvas pool");
+        canvas = canvasPool.pop();
+      } else {
+        lazy.logger.debug("using new canvas");
+        canvas = null;
+      }
+      reuseCanvas = !cache;
+
+      let ctxInterface = win.CanvasRenderingContext2D;
+      let flags =
+        ctxInterface.DRAWWINDOW_DRAW_CARET |
+        ctxInterface.DRAWWINDOW_DRAW_VIEW |
+        ctxInterface.DRAWWINDOW_USE_WIDGET_LAYERS;
+
+      if (
+        !(
+          0 <= browserRect.left &&
+          0 <= browserRect.top &&
+          win.innerWidth >= browserRect.width &&
+          win.innerHeight >= browserRect.height
+        )
+      ) {
+        lazy.logger.error(`Invalid window dimensions:
+browserRect.left: ${browserRect.left}
+browserRect.top: ${browserRect.top}
+win.innerWidth: ${win.innerWidth}
+browserRect.width: ${browserRect.width}
+win.innerHeight: ${win.innerHeight}
+browserRect.height: ${browserRect.height}`);
+        throw new Error("Window has incorrect dimensions");
+      }
+
+      url = new URL(url).href; // normalize the URL
+
+      await this.loadTestUrl(win, url, timeout);
+
+      canvas = await lazy.capture.canvas(
+        win,
+        win.docShell.browsingContext,
+        0, // left
+        0, // top
+        browserRect.width,
+        browserRect.height,
+        { canvas, flags, readback: true }
+      );
+    }
+    if (
+      canvas.width !== browserRect.width ||
+      canvas.height !== browserRect.height
+    ) {
+      lazy.logger.warn(
+        `Canvas dimensions changed to ${canvas.width}x${canvas.height}`
+      );
+      reuseCanvas = false;
+      cache = false;
+    }
+    if (cache) {
+      sizedCache.set(url, canvas);
+    }
+    this.urlCount.set(url, remainingCount - 1);
+    return { canvas, reuseCanvas };
+  }
+
+  async screenshotPaginated(win, url, timeout, pageRanges) {
+    url = new URL(url).href; // normalize the URL
+    await this.loadTestUrl(win, url, timeout);
+
+    const [width, height] = [DEFAULT_PAGE_WIDTH, DEFAULT_PAGE_HEIGHT];
+    const margin = DEFAULT_PAGE_MARGIN;
+    const settings = lazy.print.addDefaultSettings({
+      page: {
+        width,
+        height,
+      },
+      margin: {
+        left: margin,
+        right: margin,
+        top: margin,
+        bottom: margin,
+      },
+      shrinkToFit: false,
+      background: true,
+    });
+    const printSettings = lazy.print.getPrintSettings(settings);
+
+    const binaryString = await lazy.print.printToBinaryString(
+      win.gBrowser.browsingContext,
+      printSettings
+    );
+
+    try {
+      const pdf = await this.loadPdf(binaryString);
+      let pages = this.getPages(pageRanges, url, pdf.numPages);
+      return [this.renderPages(pdf, pages), pages.size];
+    } catch (e) {
+      lazy.logger.warn(`Loading of pdf failed`);
+      throw e;
+    }
+  }
+
+  async loadPdfJs() {
+    // Ensure pdf.js is loaded in the opener window
+    await new Promise((resolve, reject) => {
+      const doc = this.parentWindow.document;
+      const script = doc.createElement("script");
+      script.src = "resource://pdf.js/build/pdf.js";
+      script.onload = resolve;
+      script.onerror = () => reject(new Error("pdfjs load failed"));
+      doc.documentElement.appendChild(script);
+    });
+    this.parentWindow.pdfjsLib.GlobalWorkerOptions.workerSrc =
+      "resource://pdf.js/build/pdf.worker.js";
+  }
+
+  async loadPdf(data) {
+    return this.parentWindow.pdfjsLib.getDocument({ data }).promise;
+  }
+
+  async *renderPages(pdf, pages) {
+    let canvas = null;
+    for (let pageNumber = 1; pageNumber <= pdf.numPages; pageNumber++) {
+      if (!pages.has(pageNumber)) {
+        lazy.logger.info(`Skipping page ${pageNumber}/${pdf.numPages}`);
+        continue;
+      }
+      lazy.logger.info(`Rendering page ${pageNumber}/${pdf.numPages}`);
+      let page = await pdf.getPage(pageNumber);
+      let viewport = page.getViewport({ scale: DEFAULT_PDF_RESOLUTION });
+      // Prepare canvas using PDF page dimensions
+      if (canvas === null) {
+        canvas = this.parentWindow.document.createElementNS(XHTML_NS, "canvas");
+        canvas.height = viewport.height;
+        canvas.width = viewport.width;
+      }
+
+      // Render PDF page into canvas context
+      let context = canvas.getContext("2d");
+      let renderContext = {
+        canvasContext: context,
+        viewport,
+      };
+      await page.render(renderContext).promise;
+      yield { canvas, reuseCanvas: false };
+    }
+  }
+
+  getPages(pageRanges, url, totalPages) {
+    // Extract test id from URL without parsing
+    let afterHost = url.slice(url.indexOf(":") + 3);
+    afterHost = afterHost.slice(afterHost.indexOf("/"));
+    const ranges = pageRanges[afterHost];
+    let rv = new Set();
+
+    if (!ranges) {
+      for (let i = 1; i <= totalPages; i++) {
+        rv.add(i);
+      }
+      return rv;
+    }
+
+    for (let rangePart of ranges) {
+      if (rangePart.length === 1) {
+        rv.add(rangePart[0]);
+      } else {
+        if (rangePart.length !== 2) {
+          throw new Error(
+            `Page ranges must be <int> or <int> '-' <int>, got ${rangePart}`
+          );
+        }
+        let [lower, upper] = rangePart;
+        if (lower === null) {
+          lower = 1;
+        }
+        if (upper === null) {
+          upper = totalPages;
+        }
+        for (let i = lower; i <= upper; i++) {
+          rv.add(i);
+        }
+      }
+    }
+    return rv;
+  }
+};
+
+class DefaultMap extends Map {
+  constructor(iterable, defaultFactory) {
+    super(iterable);
+    this.defaultFactory = defaultFactory;
+  }
+
+  get(key) {
+    if (this.has(key)) {
+      return super.get(key);
+    }
+
+    let v = this.defaultFactory();
+    this.set(key, v);
+    return v;
+  }
+}
diff --git a/remote/marionette/server.sys.mjs b/remote/marionette/server.sys.mjs
new file mode 100644
index 0000000000..49d8055ea6
--- /dev/null
+++ b/remote/marionette/server.sys.mjs
@@ -0,0 +1,462 @@
+/* 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, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  Command: "chrome://remote/content/marionette/message.sys.mjs",
+  DebuggerTransport: "chrome://remote/content/marionette/transport.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  GeckoDriver: "chrome://remote/content/marionette/driver.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  MarionettePrefs: "chrome://remote/content/marionette/prefs.sys.mjs",
+  Message: "chrome://remote/content/marionette/message.sys.mjs",
+  PollPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  Response: "chrome://remote/content/marionette/message.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+XPCOMUtils.defineLazyGetter(lazy, "ServerSocket", () => {
+  return Components.Constructor(
+    "@mozilla.org/network/server-socket;1",
+    "nsIServerSocket",
+    "initSpecialConnection"
+  );
+});
+
+const { KeepWhenOffline, LoopbackOnly } = Ci.nsIServerSocket;
+
+const PROTOCOL_VERSION = 3;
+
+/**
+ * Bootstraps Marionette and handles incoming client connections.
+ *
+ * Starting the Marionette server will open a TCP socket sporting the
+ * debugger transport interface on the provided `port`.  For every
+ * new connection, a {@link TCPConnection} is created.
+ */
+export class TCPListener {
+  /**
+   * @param {number} port
+   *     Port for server to listen to.
+   */
+  constructor(port) {
+    this.port = port;
+    this.socket = null;
+    this.conns = new Set();
+    this.nextConnID = 0;
+    this.alive = false;
+  }
+
+  /**
+   * Function produces a {@link GeckoDriver}.
+   *
+   * Determines the application to initialise the driver with.
+   *
+   * @returns {GeckoDriver}
+   *     A driver instance.
+   */
+  driverFactory() {
+    return new lazy.GeckoDriver(this);
+  }
+
+  async setAcceptConnections(value) {
+    if (value) {
+      if (!this.socket) {
+        await lazy.PollPromise(
+          (resolve, reject) => {
+            try {
+              const flags = KeepWhenOffline | LoopbackOnly;
+              const backlog = 1;
+              this.socket = new lazy.ServerSocket(this.port, flags, backlog);
+              resolve();
+            } catch (e) {
+              lazy.logger.debug(
+                `Could not bind to port ${this.port} (${e.name})`
+              );
+              reject();
+            }
+          },
+          { interval: 250, timeout: 5000 }
+        );
+
+        // Since PollPromise doesn't throw when timeout expires,
+        // we can end up in the situation when the socket is undefined.
+        if (!this.socket) {
+          throw new Error(`Could not bind to port ${this.port}`);
+        }
+
+        this.port = this.socket.port;
+
+        this.socket.asyncListen(this);
+        lazy.logger.info(`Listening on port ${this.port}`);
+      }
+    } else if (this.socket) {
+      // Note that closing the server socket will not close currently active
+      // connections.
+      this.socket.close();
+      this.socket = null;
+      lazy.logger.info(`Stopped listening on port ${this.port}`);
+    }
+  }
+
+  /**
+   * Bind this listener to {@link #port} and start accepting incoming
+   * socket connections on {@link #onSocketAccepted}.
+   *
+   * The marionette.port preference will be populated with the value
+   * of {@link #port}.
+   */
+  async start() {
+    if (this.alive) {
+      return;
+    }
+
+    // Start socket server and listening for connection attempts
+    await this.setAcceptConnections(true);
+    lazy.MarionettePrefs.port = this.port;
+    this.alive = true;
+  }
+
+  async stop() {
+    if (!this.alive) {
+      return;
+    }
+
+    // Shutdown server socket, and no longer listen for new connections
+    await this.setAcceptConnections(false);
+    this.alive = false;
+  }
+
+  onSocketAccepted(serverSocket, clientSocket) {
+    let input = clientSocket.openInputStream(0, 0, 0);
+    let output = clientSocket.openOutputStream(0, 0, 0);
+    let transport = new lazy.DebuggerTransport(input, output);
+
+    // Only allow a single active WebDriver session at a time
+    const hasActiveSession = [...this.conns].find(
+      conn => !!conn.driver.currentSession
+    );
+    if (hasActiveSession) {
+      lazy.logger.warn(
+        "Connection attempt denied because an active session has been found"
+      );
+
+      // Ideally we should stop the server to listen for new connection
+      // attempts, but the current architecture doesn't allow us to do that.
+      // As such just close the transport if no further connections are allowed.
+      transport.close();
+      return;
+    }
+
+    let conn = new TCPConnection(
+      this.nextConnID++,
+      transport,
+      this.driverFactory.bind(this)
+    );
+    conn.onclose = this.onConnectionClosed.bind(this);
+    this.conns.add(conn);
+
+    lazy.logger.debug(
+      `Accepted connection ${conn.id} ` +
+        `from ${clientSocket.host}:${clientSocket.port}`
+    );
+    conn.sayHello();
+    transport.ready();
+  }
+
+  onConnectionClosed(conn) {
+    lazy.logger.debug(`Closed connection ${conn.id}`);
+    this.conns.delete(conn);
+  }
+}
+
+/**
+ * Marionette client connection.
+ *
+ * Dispatches packets received to their correct service destinations
+ * and sends back the service endpoint's return values.
+ *
+ * @param {number} connID
+ *     Unique identifier of the connection this dispatcher should handle.
+ * @param {DebuggerTransport} transport
+ *     Debugger transport connection to the client.
+ * @param {function(): GeckoDriver} driverFactory
+ *     Factory function that produces a {@link GeckoDriver}.
+ */
+export class TCPConnection {
+  constructor(connID, transport, driverFactory) {
+    this.id = connID;
+    this.conn = transport;
+
+    // transport hooks are TCPConnection#onPacket
+    // and TCPConnection#onClosed
+    this.conn.hooks = this;
+
+    // callback for when connection is closed
+    this.onclose = null;
+
+    // last received/sent message ID
+    this.lastID = 0;
+
+    this.driver = driverFactory();
+  }
+
+  #log(msg) {
+    let dir = msg.origin == lazy.Message.Origin.Client ? "->" : "<-";
+    lazy.logger.debug(`${this.id} ${dir} ${msg.toString()}`);
+  }
+
+  /**
+   * Debugger transport callback that cleans up
+   * after a connection is closed.
+   */
+  onClosed() {
+    this.driver.deleteSession();
+    if (this.onclose) {
+      this.onclose(this);
+    }
+  }
+
+  /**
+   * Callback that receives data packets from the client.
+   *
+   * If the message is a Response, we look up the command previously
+   * issued to the client and run its callback, if any.  In case of
+   * a Command, the corresponding is executed.
+   *
+   * @param {Array.<number, number, ?, ?>} data
+   *     A four element array where the elements, in sequence, signifies
+   *     message type, message ID, method name or error, and parameters
+   *     or result.
+   */
+  onPacket(data) {
+    // unable to determine how to respond
+    if (!Array.isArray(data)) {
+      let e = new TypeError(
+        "Unable to unmarshal packet data: " + JSON.stringify(data)
+      );
+      lazy.error.report(e);
+      return;
+    }
+
+    // return immediately with any error trying to unmarshal message
+    let msg;
+    try {
+      msg = lazy.Message.fromPacket(data);
+      msg.origin = lazy.Message.Origin.Client;
+      this.#log(msg);
+    } catch (e) {
+      let resp = this.createResponse(data[1]);
+      resp.sendError(e);
+      return;
+    }
+
+    // execute new command
+    if (msg instanceof lazy.Command) {
+      (async () => {
+        await this.execute(msg);
+      })();
+    } else {
+      lazy.logger.fatal("Cannot process messages other than Command");
+    }
+  }
+
+  /**
+   * Executes a Marionette command and sends back a response when it
+   * has finished executing.
+   *
+   * If the command implementation sends the response itself by calling
+   * <code>resp.send()</code>, the response is guaranteed to not be
+   * sent twice.
+   *
+   * Errors thrown in commands are marshaled and sent back, and if they
+   * are not {@link WebDriverError} instances, they are additionally
+   * propagated and reported to {@link Components.utils.reportError}.
+   *
+   * @param {Command} cmd
+   *     Command to execute.
+   */
+  async execute(cmd) {
+    let resp = this.createResponse(cmd.id);
+    let sendResponse = () => resp.sendConditionally(resp => !resp.sent);
+    let sendError = resp.sendError.bind(resp);
+
+    await this.despatch(cmd, resp)
+      .then(sendResponse, sendError)
+      .catch(lazy.error.report);
+  }
+
+  /**
+   * Despatches command to appropriate Marionette service.
+   *
+   * @param {Command} cmd
+   *     Command to run.
+   * @param {Response} resp
+   *     Mutable response where the command's return value will be
+   *     assigned.
+   *
+   * @throws {Error}
+   *     A command's implementation may throw at any time.
+   */
+  async despatch(cmd, resp) {
+    const startTime = Cu.now();
+
+    let fn = this.driver.commands[cmd.name];
+    if (typeof fn == "undefined") {
+      throw new lazy.error.UnknownCommandError(cmd.name);
+    }
+
+    if (cmd.name != "WebDriver:NewSession") {
+      lazy.assert.session(this.driver.currentSession);
+    }
+
+    let rv = await fn.bind(this.driver)(cmd);
+
+    // Bug 1819029: Some older commands cannot return a response wrapped within
+    // a value field because it would break compatibility with geckodriver and
+    // Marionette client. It's unlikely that we are going to fix that.
+    //
+    // Warning: No more commands should be added to this list!
+    const commandsNoValueResponse = [
+      "Marionette:Quit",
+      "WebDriver:FindElements",
+      "WebDriver:FindElementsFromShadowRoot",
+      "WebDriver:CloseChromeWindow",
+      "WebDriver:CloseWindow",
+      "WebDriver:FullscreenWindow",
+      "WebDriver:GetCookies",
+      "WebDriver:GetElementRect",
+      "WebDriver:GetTimeouts",
+      "WebDriver:GetWindowHandles",
+      "WebDriver:GetWindowRect",
+      "WebDriver:MaximizeWindow",
+      "WebDriver:MinimizeWindow",
+      "WebDriver:NewSession",
+      "WebDriver:NewWindow",
+      "WebDriver:SetWindowRect",
+    ];
+
+    if (rv != null) {
+      // By default the Response' constructor sets the body to `{ value: null }`.
+      // As such we only want to override the value if it's neither `null` nor
+      // `undefined`.
+      if (commandsNoValueResponse.includes(cmd.name)) {
+        resp.body = rv;
+      } else {
+        resp.body.value = rv;
+      }
+    }
+
+    if (Services.profiler?.IsActive()) {
+      ChromeUtils.addProfilerMarker(
+        "Marionette: Command",
+        { startTime, category: "Remote-Protocol" },
+        `${cmd.name} (${cmd.id})`
+      );
+    }
+  }
+
+  /**
+   * Fail-safe creation of a new instance of {@link Response}.
+   *
+   * @param {number} msgID
+   *     Message ID to respond to.  If it is not a number, -1 is used.
+   *
+   * @returns {Response}
+   *     Response to the message with `msgID`.
+   */
+  createResponse(msgID) {
+    if (typeof msgID != "number") {
+      msgID = -1;
+    }
+    return new lazy.Response(msgID, this.send.bind(this));
+  }
+
+  sendError(err, cmdID) {
+    let resp = new lazy.Response(cmdID, this.send.bind(this));
+    resp.sendError(err);
+  }
+
+  /**
+   * When a client connects we send across a JSON Object defining the
+   * protocol level.
+   *
+   * This is the only message sent by Marionette that does not follow
+   * the regular message format.
+   */
+  sayHello() {
+    let whatHo = {
+      applicationType: "gecko",
+      marionetteProtocol: PROTOCOL_VERSION,
+    };
+    this.sendRaw(whatHo);
+  }
+
+  /**
+   * Delegates message to client based on the provided  {@code cmdID}.
+   * The message is sent over the debugger transport socket.
+   *
+   * The command ID is a unique identifier assigned to the client's request
+   * that is used to distinguish the asynchronous responses.
+   *
+   * Whilst responses to commands are synchronous and must be sent in the
+   * correct order.
+   *
+   * @param {Message} msg
+   *     The command or response to send.
+   */
+  send(msg) {
+    msg.origin = lazy.Message.Origin.Server;
+    if (msg instanceof lazy.Response) {
+      this.sendToClient(msg);
+    } else {
+      lazy.logger.fatal("Cannot send messages other than Response");
+    }
+  }
+
+  // Low-level methods:
+
+  /**
+   * Send given response to the client over the debugger transport socket.
+   *
+   * @param {Response} resp
+   *     The response to send back to the client.
+   */
+  sendToClient(resp) {
+    this.sendMessage(resp);
+  }
+
+  /**
+   * Marshal message to the Marionette message format and send it.
+   *
+   * @param {Message} msg
+   *     The message to send.
+   */
+  sendMessage(msg) {
+    this.#log(msg);
+    let payload = msg.toPacket();
+    this.sendRaw(payload);
+  }
+
+  /**
+   * Send the given payload over the debugger transport socket to the
+   * connected client.
+   *
+   * @param {Object<string, ?>} payload
+   *     The payload to ship.
+   */
+  sendRaw(payload) {
+    this.conn.send(payload);
+  }
+
+  toString() {
+    return `[object TCPConnection ${this.id}]`;
+  }
+}
diff --git a/remote/marionette/stream-utils.sys.mjs b/remote/marionette/stream-utils.sys.mjs
new file mode 100644
index 0000000000..9a4437966c
--- /dev/null
+++ b/remote/marionette/stream-utils.sys.mjs
@@ -0,0 +1,256 @@
+/* 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, {
+  EventEmitter: "resource://gre/modules/EventEmitter.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "IOUtil",
+  "@mozilla.org/io-util;1",
+  "nsIIOUtil"
+);
+
+XPCOMUtils.defineLazyGetter(lazy, "ScriptableInputStream", () => {
+  return Components.Constructor(
+    "@mozilla.org/scriptableinputstream;1",
+    "nsIScriptableInputStream",
+    "init"
+  );
+});
+
+const BUFFER_SIZE = 0x8000;
+
+/**
+ * This helper function (and its companion object) are used by bulk
+ * senders and receivers to read and write data in and out of other streams.
+ * Functions that make use of this tool are passed to callers when it is
+ * time to read or write bulk data.  It is highly recommended to use these
+ * copier functions instead of the stream directly because the copier
+ * enforces the agreed upon length. Since bulk mode reuses an existing
+ * stream, the sender and receiver must write and read exactly the agreed
+ * upon amount of data, or else the entire transport will be left in a
+ * invalid state.  Additionally, other methods of stream copying (such as
+ * NetUtil.asyncCopy) close the streams involved, which would terminate
+ * the debugging transport, and so it is avoided here.
+ *
+ * Overall, this *works*, but clearly the optimal solution would be
+ * able to just use the streams directly.  If it were possible to fully
+ * implement nsIInputStream/nsIOutputStream in JS, wrapper streams could
+ * be created to enforce the length and avoid closing, and consumers could
+ * use familiar stream utilities like NetUtil.asyncCopy.
+ *
+ * The function takes two async streams and copies a precise number
+ * of bytes from one to the other.  Copying begins immediately, but may
+ * complete at some future time depending on data size.  Use the returned
+ * promise to know when it's complete.
+ *
+ * @param {nsIAsyncInputStream} input
+ *     Stream to copy from.
+ * @param {nsIAsyncOutputStream} output
+ *        Stream to copy to.
+ * @param {number} length
+ *        Amount of data that needs to be copied.
+ *
+ * @returns {Promise}
+ *     Promise is resolved when copying completes or rejected if any
+ *     (unexpected) errors occur.
+ */
+function copyStream(input, output, length) {
+  let copier = new StreamCopier(input, output, length);
+  return copier.copy();
+}
+
+/** @class */
+function StreamCopier(input, output, length) {
+  lazy.EventEmitter.decorate(this);
+  this._id = StreamCopier._nextId++;
+  this.input = input;
+  // Save off the base output stream, since we know it's async as we've
+  // required
+  this.baseAsyncOutput = output;
+  if (lazy.IOUtil.outputStreamIsBuffered(output)) {
+    this.output = output;
+  } else {
+    this.output = Cc[
+      "@mozilla.org/network/buffered-output-stream;1"
+    ].createInstance(Ci.nsIBufferedOutputStream);
+    this.output.init(output, BUFFER_SIZE);
+  }
+  this._length = length;
+  this._amountLeft = length;
+  this._deferred = {
+    promise: new Promise((resolve, reject) => {
+      this._deferred.resolve = resolve;
+      this._deferred.reject = reject;
+    }),
+  };
+
+  this._copy = this._copy.bind(this);
+  this._flush = this._flush.bind(this);
+  this._destroy = this._destroy.bind(this);
+
+  // Copy promise's then method up to this object.
+  //
+  // Allows the copier to offer a promise interface for the simple succeed
+  // or fail scenarios, but also emit events (due to the EventEmitter)
+  // for other states, like progress.
+  this.then = this._deferred.promise.then.bind(this._deferred.promise);
+  this.then(this._destroy, this._destroy);
+
+  // Stream ready callback starts as |_copy|, but may switch to |_flush|
+  // at end if flushing would block the output stream.
+  this._streamReadyCallback = this._copy;
+}
+StreamCopier._nextId = 0;
+
+StreamCopier.prototype = {
+  copy() {
+    // Dispatch to the next tick so that it's possible to attach a progress
+    // event listener, even for extremely fast copies (like when testing).
+    Services.tm.currentThread.dispatch(() => {
+      try {
+        this._copy();
+      } catch (e) {
+        this._deferred.reject(e);
+      }
+    }, 0);
+    return this;
+  },
+
+  _copy() {
+    let bytesAvailable = this.input.available();
+    let amountToCopy = Math.min(bytesAvailable, this._amountLeft);
+    this._debug("Trying to copy: " + amountToCopy);
+
+    let bytesCopied;
+    try {
+      bytesCopied = this.output.writeFrom(this.input, amountToCopy);
+    } catch (e) {
+      if (e.result == Cr.NS_BASE_STREAM_WOULD_BLOCK) {
+        this._debug("Base stream would block, will retry");
+        this._debug("Waiting for output stream");
+        this.baseAsyncOutput.asyncWait(this, 0, 0, Services.tm.currentThread);
+        return;
+      }
+      throw e;
+    }
+
+    this._amountLeft -= bytesCopied;
+    this._debug("Copied: " + bytesCopied + ", Left: " + this._amountLeft);
+    this._emitProgress();
+
+    if (this._amountLeft === 0) {
+      this._debug("Copy done!");
+      this._flush();
+      return;
+    }
+
+    this._debug("Waiting for input stream");
+    this.input.asyncWait(this, 0, 0, Services.tm.currentThread);
+  },
+
+  _emitProgress() {
+    this.emit("progress", {
+      bytesSent: this._length - this._amountLeft,
+      totalBytes: this._length,
+    });
+  },
+
+  _flush() {
+    try {
+      this.output.flush();
+    } catch (e) {
+      if (
+        e.result == Cr.NS_BASE_STREAM_WOULD_BLOCK ||
+        e.result == Cr.NS_ERROR_FAILURE
+      ) {
+        this._debug("Flush would block, will retry");
+        this._streamReadyCallback = this._flush;
+        this._debug("Waiting for output stream");
+        this.baseAsyncOutput.asyncWait(this, 0, 0, Services.tm.currentThread);
+        return;
+      }
+      throw e;
+    }
+    this._deferred.resolve();
+  },
+
+  _destroy() {
+    this._destroy = null;
+    this._copy = null;
+    this._flush = null;
+    this.input = null;
+    this.output = null;
+  },
+
+  // nsIInputStreamCallback
+  onInputStreamReady() {
+    this._streamReadyCallback();
+  },
+
+  // nsIOutputStreamCallback
+  onOutputStreamReady() {
+    this._streamReadyCallback();
+  },
+
+  _debug() {},
+};
+
+/**
+ * Read from a stream, one byte at a time, up to the next
+ * <var>delimiter</var> character, but stopping if we've read |count|
+ * without finding it.  Reading also terminates early if there are less
+ * than <var>count</var> bytes available on the stream.  In that case,
+ * we only read as many bytes as the stream currently has to offer.
+ *
+ * @param {nsIInputStream} stream
+ *     Input stream to read from.
+ * @param {string} delimiter
+ *     Character we're trying to find.
+ * @param {number} count
+ *     Max number of characters to read while searching.
+ *
+ * @returns {string}
+ *     Collected data.  If the delimiter was found, this string will
+ *     end with it.
+ */
+// TODO: This implementation could be removed if bug 984651 is fixed,
+// which provides a native version of the same idea.
+function delimitedRead(stream, delimiter, count) {
+  let scriptableStream;
+  if (stream instanceof Ci.nsIScriptableInputStream) {
+    scriptableStream = stream;
+  } else {
+    scriptableStream = new lazy.ScriptableInputStream(stream);
+  }
+
+  let data = "";
+
+  // Don't exceed what's available on the stream
+  count = Math.min(count, stream.available());
+
+  if (count <= 0) {
+    return data;
+  }
+
+  let char;
+  while (char !== delimiter && count > 0) {
+    char = scriptableStream.readBytes(1);
+    count--;
+    data += char;
+  }
+
+  return data;
+}
+
+export const StreamUtils = {
+  copyStream,
+  delimitedRead,
+};
diff --git a/remote/marionette/sync.sys.mjs b/remote/marionette/sync.sys.mjs
new file mode 100644
index 0000000000..55d56702e6
--- /dev/null
+++ b/remote/marionette/sync.sys.mjs
@@ -0,0 +1,540 @@
+/* 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 { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+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, "logger", () =>
+  lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
+);
+
+const { TYPE_ONE_SHOT, TYPE_REPEATING_SLACK } = Ci.nsITimer;
+
+const PROMISE_TIMEOUT = AppConstants.DEBUG ? 4500 : 1500;
+
+/**
+ * Dispatch a function to be executed on the main thread.
+ *
+ * @param {Function} func
+ *     Function to be executed.
+ */
+export function executeSoon(func) {
+  if (typeof func != "function") {
+    throw new TypeError();
+  }
+
+  Services.tm.dispatchToMainThread(func);
+}
+
+/**
+ * Runs a Promise-like function off the main thread until it is resolved
+ * through ``resolve`` or ``rejected`` callbacks.  The function is
+ * guaranteed to be run at least once, irregardless of the timeout.
+ *
+ * The ``func`` is evaluated every ``interval`` for as long as its
+ * runtime duration does not exceed ``interval``.  Evaluations occur
+ * sequentially, meaning that evaluations of ``func`` are queued if
+ * the runtime evaluation duration of ``func`` is greater than ``interval``.
+ *
+ * ``func`` is given two arguments, ``resolve`` and ``reject``,
+ * of which one must be called for the evaluation to complete.
+ * Calling ``resolve`` with an argument indicates that the expected
+ * wait condition was met and will return the passed value to the
+ * caller.  Conversely, calling ``reject`` will evaluate ``func``
+ * again until the ``timeout`` duration has elapsed or ``func`` throws.
+ * The passed value to ``reject`` will also be returned to the caller
+ * once the wait has expired.
+ *
+ * Usage::
+ *
+ *     let els = new PollPromise((resolve, reject) => {
+ *       let res = document.querySelectorAll("p");
+ *       if (res.length > 0) {
+ *         resolve(Array.from(res));
+ *       } else {
+ *         reject([]);
+ *       }
+ *     }, {timeout: 1000});
+ *
+ * @param {Condition} func
+ *     Function to run off the main thread.
+ * @param {object=} options
+ * @param {number=} options.timeout
+ *     Desired timeout if wanted.  If 0 or less than the runtime evaluation
+ *     time of ``func``, ``func`` is guaranteed to run at least once.
+ *     Defaults to using no timeout.
+ * @param {number=} options.interval
+ *     Duration between each poll of ``func`` in milliseconds.
+ *     Defaults to 10 milliseconds.
+ *
+ * @returns {Promise.<*>}
+ *     Yields the value passed to ``func``'s
+ *     ``resolve`` or ``reject`` callbacks.
+ *
+ * @throws {*}
+ *     If ``func`` throws, its error is propagated.
+ * @throws {TypeError}
+ *     If `timeout` or `interval`` are not numbers.
+ * @throws {RangeError}
+ *     If `timeout` or `interval` are not unsigned integers.
+ */
+export function PollPromise(func, { timeout = null, interval = 10 } = {}) {
+  const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+
+  if (typeof func != "function") {
+    throw new TypeError();
+  }
+  if (timeout != null && typeof timeout != "number") {
+    throw new TypeError();
+  }
+  if (typeof interval != "number") {
+    throw new TypeError();
+  }
+  if (
+    (timeout && (!Number.isInteger(timeout) || timeout < 0)) ||
+    !Number.isInteger(interval) ||
+    interval < 0
+  ) {
+    throw new RangeError();
+  }
+
+  return new Promise((resolve, reject) => {
+    let start, end;
+
+    if (Number.isInteger(timeout)) {
+      start = new Date().getTime();
+      end = start + timeout;
+    }
+
+    let evalFn = () => {
+      new Promise(func)
+        .then(resolve, rejected => {
+          if (lazy.error.isError(rejected)) {
+            throw rejected;
+          }
+
+          // return if there is a timeout and set to 0,
+          // allowing |func| to be evaluated at least once
+          if (
+            typeof end != "undefined" &&
+            (start == end || new Date().getTime() >= end)
+          ) {
+            resolve(rejected);
+          }
+        })
+        .catch(reject);
+    };
+
+    // the repeating slack timer waits |interval|
+    // before invoking |evalFn|
+    evalFn();
+
+    timer.init(evalFn, interval, TYPE_REPEATING_SLACK);
+  }).then(
+    res => {
+      timer.cancel();
+      return res;
+    },
+    err => {
+      timer.cancel();
+      throw err;
+    }
+  );
+}
+
+/**
+ * Represents the timed, eventual completion (or failure) of an
+ * asynchronous operation, and its resulting value.
+ *
+ * In contrast to a regular Promise, it times out after ``timeout``.
+ *
+ * @param {Function} fn
+ *     Function to run, which will have its ``reject``
+ *     callback invoked after the ``timeout`` duration is reached.
+ *     It is given two callbacks: ``resolve(value)`` and
+ *     ``reject(error)``.
+ * @param {object=} options
+ * @param {string} options.errorMessage
+ *     Message to use for the thrown error.
+ * @param {number=} options.timeout
+ *     ``condition``'s ``reject`` callback will be called
+ *     after this timeout, given in milliseconds.
+ *     By default 1500 ms in an optimised build and 4500 ms in
+ *     debug builds.
+ * @param {Error=} options.throws
+ *     When the ``timeout`` is hit, this error class will be
+ *     thrown.  If it is null, no error is thrown and the promise is
+ *     instead resolved on timeout with a TimeoutError.
+ *
+ * @returns {Promise.<*>}
+ *     Timed promise.
+ *
+ * @throws {TypeError}
+ *     If `timeout` is not a number.
+ * @throws {RangeError}
+ *     If `timeout` is not an unsigned integer.
+ */
+export function TimedPromise(fn, options = {}) {
+  const {
+    errorMessage = "TimedPromise timed out",
+    timeout = PROMISE_TIMEOUT,
+    throws = lazy.error.TimeoutError,
+  } = options;
+
+  const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+
+  if (typeof fn != "function") {
+    throw new TypeError();
+  }
+  if (typeof timeout != "number") {
+    throw new TypeError();
+  }
+  if (!Number.isInteger(timeout) || timeout < 0) {
+    throw new RangeError();
+  }
+
+  return new Promise((resolve, reject) => {
+    let trace;
+
+    // Reject only if |throws| is given.  Otherwise it is assumed that
+    // the user is OK with the promise timing out.
+    let bail = () => {
+      const message = `${errorMessage} after ${timeout} ms`;
+      if (throws !== null) {
+        let err = new throws(message);
+        reject(err);
+      } else {
+        lazy.logger.warn(message, trace);
+        resolve();
+      }
+    };
+
+    trace = lazy.error.stack();
+    timer.initWithCallback({ notify: bail }, timeout, TYPE_ONE_SHOT);
+
+    try {
+      fn(resolve, reject);
+    } catch (e) {
+      reject(e);
+    }
+  }).then(
+    res => {
+      timer.cancel();
+      return res;
+    },
+    err => {
+      timer.cancel();
+      throw err;
+    }
+  );
+}
+
+/**
+ * Pauses for the given duration.
+ *
+ * @param {number} timeout
+ *     Duration to wait before fulfilling promise in milliseconds.
+ *
+ * @returns {Promise}
+ *     Promise that fulfills when the `timeout` is elapsed.
+ *
+ * @throws {TypeError}
+ *     If `timeout` is not a number.
+ * @throws {RangeError}
+ *     If `timeout` is not an unsigned integer.
+ */
+export function Sleep(timeout) {
+  if (typeof timeout != "number") {
+    throw new TypeError();
+  }
+  if (!Number.isInteger(timeout) || timeout < 0) {
+    throw new RangeError();
+  }
+
+  return new Promise(resolve => {
+    const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    timer.init(
+      () => {
+        // Bug 1663880 - Explicitely cancel the timer for now to prevent a hang
+        timer.cancel();
+        resolve();
+      },
+      timeout,
+      TYPE_ONE_SHOT
+    );
+  });
+}
+
+/**
+ * Detects when the specified message manager has been destroyed.
+ *
+ * One can observe the removal and detachment of a content browser
+ * (`<xul:browser>`) or a chrome window by its message manager
+ * disconnecting.
+ *
+ * When a browser is associated with a tab, this is safer than only
+ * relying on the event `TabClose` which signalises the _intent to_
+ * remove a tab and consequently would lead to the destruction of
+ * the content browser and its browser message manager.
+ *
+ * When closing a chrome window it is safer than only relying on
+ * the event 'unload' which signalises the _intent to_ close the
+ * chrome window and consequently would lead to the destruction of
+ * the window and its window message manager.
+ *
+ * @param {MessageListenerManager} messageManager
+ *     The message manager to observe for its disconnect state.
+ *     Use the browser message manager when closing a content browser,
+ *     and the window message manager when closing a chrome window.
+ *
+ * @returns {Promise}
+ *     A promise that resolves when the message manager has been destroyed.
+ */
+export function MessageManagerDestroyedPromise(messageManager) {
+  return new Promise(resolve => {
+    function observe(subject, topic) {
+      lazy.logger.trace(`Received observer notification ${topic}`);
+
+      if (subject == messageManager) {
+        Services.obs.removeObserver(this, "message-manager-disconnect");
+        resolve();
+      }
+    }
+
+    Services.obs.addObserver(observe, "message-manager-disconnect");
+  });
+}
+
+/**
+ * Throttle until the main thread is idle and `window` has performed
+ * an animation frame (in that order).
+ *
+ * @param {ChromeWindow} win
+ *     Window to request the animation frame from.
+ *
+ * @returns {Promise}
+ */
+export function IdlePromise(win) {
+  const animationFramePromise = new Promise(resolve => {
+    executeSoon(() => {
+      win.requestAnimationFrame(resolve);
+    });
+  });
+
+  // Abort if the underlying window gets closed
+  const windowClosedPromise = new PollPromise(resolve => {
+    if (win.closed) {
+      resolve();
+    }
+  });
+
+  return Promise.race([animationFramePromise, windowClosedPromise]);
+}
+
+/**
+ * Wraps a callback function, that, as long as it continues to be
+ * invoked, will not be triggered.  The given function will be
+ * called after the timeout duration is reached, after no more
+ * events fire.
+ *
+ * This class implements the {@link EventListener} interface,
+ * which means it can be used interchangably with `addEventHandler`.
+ *
+ * Debouncing events can be useful when dealing with e.g. DOM events
+ * that fire at a high rate.  It is generally advisable to avoid
+ * computationally expensive operations such as DOM modifications
+ * under these circumstances.
+ *
+ * One such high frequenecy event is `resize` that can fire multiple
+ * times before the window reaches its final dimensions.  In order
+ * to delay an operation until the window has completed resizing,
+ * it is possible to use this technique to only invoke the callback
+ * after the last event has fired::
+ *
+ *     let cb = new DebounceCallback(event => {
+ *       // fires after the final resize event
+ *       console.log("resize", event);
+ *     });
+ *     window.addEventListener("resize", cb);
+ *
+ * Note that it is not possible to use this synchronisation primitive
+ * with `addEventListener(..., {once: true})`.
+ *
+ * @param {function(Event)} fn
+ *     Callback function that is guaranteed to be invoked once only,
+ *     after `timeout`.
+ * @param {number=} [timeout = 250] timeout
+ *     Time since last event firing, before `fn` will be invoked.
+ */
+export class DebounceCallback {
+  constructor(fn, { timeout = 250 } = {}) {
+    if (typeof fn != "function" || typeof timeout != "number") {
+      throw new TypeError();
+    }
+    if (!Number.isInteger(timeout) || timeout < 0) {
+      throw new RangeError();
+    }
+
+    this.fn = fn;
+    this.timeout = timeout;
+    this.timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  }
+
+  handleEvent(ev) {
+    this.timer.cancel();
+    this.timer.initWithCallback(
+      () => {
+        this.timer.cancel();
+        this.fn(ev);
+      },
+      this.timeout,
+      TYPE_ONE_SHOT
+    );
+  }
+}
+
+/**
+ * Wait for a message to be fired from a particular message manager.
+ *
+ * This method has been duplicated from BrowserTestUtils.sys.mjs.
+ *
+ * @param {nsIMessageManager} messageManager
+ *     The message manager that should be used.
+ * @param {string} messageName
+ *     The message to wait for.
+ * @param {object=} options
+ *     Extra options.
+ * @param {function(Message)=} options.checkFn
+ *     Called with the ``Message`` object as argument, should return ``true``
+ *     if the message is the expected one, or ``false`` if it should be
+ *     ignored and listening should continue. If not specified, the first
+ *     message with the specified name resolves the returned promise.
+ *
+ * @returns {Promise.<object>}
+ *     Promise which resolves to the data property of the received
+ *     ``Message``.
+ */
+export function waitForMessage(
+  messageManager,
+  messageName,
+  { checkFn = undefined } = {}
+) {
+  if (messageManager == null || !("addMessageListener" in messageManager)) {
+    throw new TypeError();
+  }
+  if (typeof messageName != "string") {
+    throw new TypeError();
+  }
+  if (checkFn && typeof checkFn != "function") {
+    throw new TypeError();
+  }
+
+  return new Promise(resolve => {
+    messageManager.addMessageListener(messageName, function onMessage(msg) {
+      lazy.logger.trace(`Received ${messageName} for ${msg.target}`);
+      if (checkFn && !checkFn(msg)) {
+        return;
+      }
+      messageManager.removeMessageListener(messageName, onMessage);
+      resolve(msg.data);
+    });
+  });
+}
+
+/**
+ * Wait for the specified observer topic to be observed.
+ *
+ * This method has been duplicated from TestUtils.sys.mjs.
+ *
+ * Because this function is intended for testing, any error in checkFn
+ * will cause the returned promise to be rejected instead of waiting for
+ * the next notification, since this is probably a bug in the test.
+ *
+ * @param {string} topic
+ *     The topic to observe.
+ * @param {object=} options
+ *     Extra options.
+ * @param {function(string, object)=} options.checkFn
+ *     Called with ``subject``, and ``data`` as arguments, should return true
+ *     if the notification is the expected one, or false if it should be
+ *     ignored and listening should continue. If not specified, the first
+ *     notification for the specified topic resolves the returned promise.
+ * @param {number=} options.timeout
+ *     Timeout duration in milliseconds, if provided.
+ *     If specified, then the returned promise will be rejected with
+ *     TimeoutError, if not already resolved, after this duration has elapsed.
+ *     If not specified, then no timeout is used. Defaults to null.
+ *
+ * @returns {Promise.<Array<string, object>>}
+ *     Promise which is either resolved to an array of ``subject``, and ``data``
+ *     from the observed notification, or rejected with TimeoutError after
+ *     options.timeout milliseconds if specified.
+ *
+ * @throws {TypeError}
+ * @throws {RangeError}
+ */
+export function waitForObserverTopic(topic, options = {}) {
+  const { checkFn = null, timeout = null } = options;
+  if (typeof topic != "string") {
+    throw new TypeError();
+  }
+  if (
+    (checkFn != null && typeof checkFn != "function") ||
+    (timeout !== null && typeof timeout != "number")
+  ) {
+    throw new TypeError();
+  }
+  if (timeout && (!Number.isInteger(timeout) || timeout < 0)) {
+    throw new RangeError();
+  }
+
+  return new Promise((resolve, reject) => {
+    let timer;
+
+    function cleanUp() {
+      Services.obs.removeObserver(observer, topic);
+      timer?.cancel();
+    }
+
+    function observer(subject, topic, data) {
+      lazy.logger.trace(`Received observer notification ${topic}`);
+      try {
+        if (checkFn && !checkFn(subject, data)) {
+          return;
+        }
+        cleanUp();
+        resolve({ subject, data });
+      } catch (ex) {
+        cleanUp();
+        reject(ex);
+      }
+    }
+
+    Services.obs.addObserver(observer, topic);
+
+    if (timeout !== null) {
+      timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+      timer.init(
+        () => {
+          cleanUp();
+          reject(
+            new lazy.error.TimeoutError(
+              `waitForObserverTopic timed out after ${timeout} ms`
+            )
+          );
+        },
+        timeout,
+        TYPE_ONE_SHOT
+      );
+    }
+  });
+}
diff --git a/remote/marionette/test/README b/remote/marionette/test/README
new file mode 100644
index 0000000000..9305b92cab
--- /dev/null
+++ b/remote/marionette/test/README
@@ -0,0 +1 @@
+See ../doc/Testing.md
\ No newline at end of file
diff --git a/remote/marionette/test/xpcshell/.eslintrc.js b/remote/marionette/test/xpcshell/.eslintrc.js
new file mode 100644
index 0000000000..2ef179ab5e
--- /dev/null
+++ b/remote/marionette/test/xpcshell/.eslintrc.js
@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  rules: {
+    camelcase: "off",
+  },
+};
diff --git a/remote/marionette/test/xpcshell/README b/remote/marionette/test/xpcshell/README
new file mode 100644
index 0000000000..ce516d17ca
--- /dev/null
+++ b/remote/marionette/test/xpcshell/README
@@ -0,0 +1,16 @@
+To run the tests in this directory, from the top source directory,
+either invoke the test despatcher in mach:
+
+	% ./mach test remote/marionette/test/xpcshell
+
+Or call out the harness specifically:
+
+	% ./mach xpcshell-test remote/marionette/test/xpcshell
+
+The latter gives you the --sequential option which can be useful
+when debugging to prevent tests from running in parallel.
+
+When adding new tests you must make sure they are listed in
+xpcshell.ini, otherwise they will not run on try.
+
+See also ../../doc/Testing.md for more advice on our other types of tests.
diff --git a/remote/marionette/test/xpcshell/head.js b/remote/marionette/test/xpcshell/head.js
new file mode 100644
index 0000000000..2e7cf578d3
--- /dev/null
+++ b/remote/marionette/test/xpcshell/head.js
@@ -0,0 +1,3 @@
+const SVG_NS = "http://www.w3.org/2000/svg";
+const XHTML_NS = "http://www.w3.org/1999/xhtml";
+const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
diff --git a/remote/marionette/test/xpcshell/test_actors.js b/remote/marionette/test/xpcshell/test_actors.js
new file mode 100644
index 0000000000..9b24d1d10f
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_actors.js
@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  getMarionetteCommandsActorProxy,
+  registerCommandsActor,
+  unregisterCommandsActor,
+} = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/actors/MarionetteCommandsParent.sys.mjs"
+);
+const { enableEventsActor, disableEventsActor } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/actors/MarionetteEventsParent.sys.mjs"
+);
+
+registerCleanupFunction(function () {
+  unregisterCommandsActor();
+  disableEventsActor();
+});
+
+add_task(function test_commandsActor_register() {
+  registerCommandsActor();
+  unregisterCommandsActor();
+
+  registerCommandsActor();
+  registerCommandsActor();
+  unregisterCommandsActor();
+});
+
+add_task(async function test_commandsActor_getActorProxy_noBrowsingContext() {
+  registerCommandsActor();
+
+  try {
+    await getMarionetteCommandsActorProxy(() => null).sendQuery("foo", "bar");
+    ok(false, "Expected NoBrowsingContext error not raised");
+  } catch (e) {
+    ok(
+      e.message.includes("No BrowsingContext found"),
+      "Expected default error message found"
+    );
+  }
+
+  unregisterCommandsActor();
+});
+
+add_task(function test_eventsActor_enable_disable() {
+  enableEventsActor();
+  disableEventsActor();
+
+  enableEventsActor();
+  enableEventsActor();
+  disableEventsActor();
+});
diff --git a/remote/marionette/test/xpcshell/test_browser.js b/remote/marionette/test/xpcshell/test_browser.js
new file mode 100644
index 0000000000..fdd83ba7e3
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_browser.js
@@ -0,0 +1,21 @@
+const { Context } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/browser.sys.mjs"
+);
+
+add_task(function test_Context() {
+  ok(Context.hasOwnProperty("Chrome"));
+  ok(Context.hasOwnProperty("Content"));
+  equal(typeof Context.Chrome, "string");
+  equal(typeof Context.Content, "string");
+  equal(Context.Chrome, "chrome");
+  equal(Context.Content, "content");
+});
+
+add_task(function test_Context_fromString() {
+  equal(Context.fromString("chrome"), Context.Chrome);
+  equal(Context.fromString("content"), Context.Content);
+
+  for (let typ of ["", "foo", true, 42, [], {}, null, undefined]) {
+    Assert.throws(() => Context.fromString(typ), /TypeError/);
+  }
+});
diff --git a/remote/marionette/test/xpcshell/test_cookie.js b/remote/marionette/test/xpcshell/test_cookie.js
new file mode 100644
index 0000000000..b5ce5e9008
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_cookie.js
@@ -0,0 +1,362 @@
+/* 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 { cookie } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/cookie.sys.mjs"
+);
+
+/* eslint-disable mozilla/use-chromeutils-generateqi */
+
+cookie.manager = {
+  cookies: [],
+
+  add(
+    domain,
+    path,
+    name,
+    value,
+    secure,
+    httpOnly,
+    session,
+    expiry,
+    originAttributes,
+    sameSite
+  ) {
+    if (name === "fail") {
+      throw new Error("An error occurred while adding cookie");
+    }
+    let newCookie = {
+      host: domain,
+      path,
+      name,
+      value,
+      isSecure: secure,
+      isHttpOnly: httpOnly,
+      isSession: session,
+      expiry,
+      originAttributes,
+      sameSite,
+    };
+    cookie.manager.cookies.push(newCookie);
+  },
+
+  remove(host, name, path) {
+    for (let i = 0; i < this.cookies.length; ++i) {
+      let candidate = this.cookies[i];
+      if (
+        candidate.host === host &&
+        candidate.name === name &&
+        candidate.path === path
+      ) {
+        return this.cookies.splice(i, 1);
+      }
+    }
+    return false;
+  },
+
+  getCookiesFromHost(host) {
+    let hostCookies = this.cookies.filter(
+      c => c.host === host || c.host === "." + host
+    );
+
+    return hostCookies;
+  },
+};
+
+add_task(function test_fromJSON() {
+  // object
+  for (let invalidType of ["foo", 42, true, [], null, undefined]) {
+    Assert.throws(() => cookie.fromJSON(invalidType), /Expected cookie object/);
+  }
+
+  // name and value
+  for (let invalidType of [42, true, [], {}, null, undefined]) {
+    Assert.throws(
+      () => cookie.fromJSON({ name: invalidType }),
+      /Cookie name must be string/
+    );
+    Assert.throws(
+      () => cookie.fromJSON({ name: "foo", value: invalidType }),
+      /Cookie value must be string/
+    );
+  }
+
+  // domain
+  for (let invalidType of [42, true, [], {}, null]) {
+    let domainTest = {
+      name: "foo",
+      value: "bar",
+      domain: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(domainTest),
+      /Cookie domain must be string/
+    );
+  }
+  let domainTest = {
+    name: "foo",
+    value: "bar",
+    domain: "domain",
+  };
+  let parsedCookie = cookie.fromJSON(domainTest);
+  equal(parsedCookie.domain, "domain");
+
+  // path
+  for (let invalidType of [42, true, [], {}, null]) {
+    let pathTest = {
+      name: "foo",
+      value: "bar",
+      path: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(pathTest),
+      /Cookie path must be string/
+    );
+  }
+
+  // secure
+  for (let invalidType of ["foo", 42, [], {}, null]) {
+    let secureTest = {
+      name: "foo",
+      value: "bar",
+      secure: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(secureTest),
+      /Cookie secure flag must be boolean/
+    );
+  }
+
+  // httpOnly
+  for (let invalidType of ["foo", 42, [], {}, null]) {
+    let httpOnlyTest = {
+      name: "foo",
+      value: "bar",
+      httpOnly: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(httpOnlyTest),
+      /Cookie httpOnly flag must be boolean/
+    );
+  }
+
+  // expiry
+  for (let invalidType of [
+    -1,
+    Number.MAX_SAFE_INTEGER + 1,
+    "foo",
+    true,
+    [],
+    {},
+    null,
+  ]) {
+    let expiryTest = {
+      name: "foo",
+      value: "bar",
+      expiry: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(expiryTest),
+      /Cookie expiry must be a positive integer/
+    );
+  }
+
+  // sameSite
+  for (let invalidType of ["foo", 42, [], {}, null]) {
+    const sameSiteTest = {
+      name: "foo",
+      value: "bar",
+      sameSite: invalidType,
+    };
+    Assert.throws(
+      () => cookie.fromJSON(sameSiteTest),
+      /Cookie SameSite flag must be one of None, Lax, or Strict/
+    );
+  }
+
+  // bare requirements
+  let bare = cookie.fromJSON({ name: "name", value: "value" });
+  equal("name", bare.name);
+  equal("value", bare.value);
+  for (let missing of [
+    "path",
+    "secure",
+    "httpOnly",
+    "session",
+    "expiry",
+    "sameSite",
+  ]) {
+    ok(!bare.hasOwnProperty(missing));
+  }
+
+  // everything
+  let full = cookie.fromJSON({
+    name: "name",
+    value: "value",
+    domain: ".domain",
+    path: "path",
+    secure: true,
+    httpOnly: true,
+    expiry: 42,
+    sameSite: "Lax",
+  });
+  equal("name", full.name);
+  equal("value", full.value);
+  equal(".domain", full.domain);
+  equal("path", full.path);
+  equal(true, full.secure);
+  equal(true, full.httpOnly);
+  equal(42, full.expiry);
+  equal("Lax", full.sameSite);
+});
+
+add_task(function test_add() {
+  cookie.manager.cookies = [];
+
+  for (let invalidType of [42, true, [], {}, null, undefined]) {
+    Assert.throws(
+      () => cookie.add({ name: invalidType }),
+      /Cookie name must be string/
+    );
+    Assert.throws(
+      () => cookie.add({ name: "name", value: invalidType }),
+      /Cookie value must be string/
+    );
+    Assert.throws(
+      () => cookie.add({ name: "name", value: "value", domain: invalidType }),
+      /Cookie domain must be string/
+    );
+  }
+
+  cookie.add({
+    name: "name",
+    value: "value",
+    domain: "domain",
+  });
+  equal(1, cookie.manager.cookies.length);
+  equal("name", cookie.manager.cookies[0].name);
+  equal("value", cookie.manager.cookies[0].value);
+  equal(".domain", cookie.manager.cookies[0].host);
+  equal("/", cookie.manager.cookies[0].path);
+  ok(cookie.manager.cookies[0].expiry > new Date(Date.now()).getTime() / 1000);
+
+  cookie.add({
+    name: "name2",
+    value: "value2",
+    domain: "domain2",
+  });
+  equal(2, cookie.manager.cookies.length);
+
+  Assert.throws(() => {
+    let biscuit = { name: "name3", value: "value3", domain: "domain3" };
+    cookie.add(biscuit, { restrictToHost: "other domain" });
+  }, /Cookies may only be set for the current domain/);
+
+  cookie.add({
+    name: "name4",
+    value: "value4",
+    domain: "my.domain:1234",
+  });
+  equal(".my.domain", cookie.manager.cookies[2].host);
+
+  cookie.add({
+    name: "name5",
+    value: "value5",
+    domain: "domain5",
+    path: "/foo/bar",
+  });
+  equal("/foo/bar", cookie.manager.cookies[3].path);
+
+  cookie.add({
+    name: "name6",
+    value: "value",
+    domain: ".domain",
+  });
+  equal(".domain", cookie.manager.cookies[4].host);
+
+  const sameSiteMap = new Map([
+    ["None", Ci.nsICookie.SAMESITE_NONE],
+    ["Lax", Ci.nsICookie.SAMESITE_LAX],
+    ["Strict", Ci.nsICookie.SAMESITE_STRICT],
+  ]);
+
+  Array.from(sameSiteMap.keys()).forEach((entry, index) => {
+    cookie.add({
+      name: "name" + index,
+      value: "value",
+      domain: ".domain",
+      sameSite: entry,
+    });
+    equal(sameSiteMap.get(entry), cookie.manager.cookies[5 + index].sameSite);
+  });
+
+  Assert.throws(() => {
+    cookie.add({ name: "fail", value: "value6", domain: "domain6" });
+  }, /UnableToSetCookieError/);
+});
+
+add_task(function test_remove() {
+  cookie.manager.cookies = [];
+
+  let crumble = {
+    name: "test_remove",
+    value: "value",
+    domain: "domain",
+    path: "/custom/path",
+  };
+
+  equal(0, cookie.manager.cookies.length);
+  cookie.add(crumble);
+  equal(1, cookie.manager.cookies.length);
+
+  cookie.remove(crumble);
+  equal(0, cookie.manager.cookies.length);
+  equal(undefined, cookie.manager.cookies[0]);
+});
+
+add_task(function test_iter() {
+  cookie.manager.cookies = [];
+  let tomorrow = new Date();
+  tomorrow.setHours(tomorrow.getHours() + 24);
+
+  cookie.add({
+    expiry: tomorrow,
+    name: "0",
+    value: "",
+    domain: "foo.example.com",
+  });
+  cookie.add({
+    expiry: tomorrow,
+    name: "1",
+    value: "",
+    domain: "bar.example.com",
+  });
+
+  let fooCookies = [...cookie.iter("foo.example.com")];
+  equal(1, fooCookies.length);
+  equal(".foo.example.com", fooCookies[0].domain);
+  equal(true, fooCookies[0].hasOwnProperty("expiry"));
+
+  cookie.add({
+    name: "aSessionCookie",
+    value: "",
+    domain: "session.com",
+  });
+
+  let sessionCookies = [...cookie.iter("session.com")];
+  equal(1, sessionCookies.length);
+  equal("aSessionCookie", sessionCookies[0].name);
+  equal(false, sessionCookies[0].hasOwnProperty("expiry"));
+
+  cookie.add({
+    name: "2",
+    value: "",
+    domain: "samesite.example.com",
+    sameSite: "Lax",
+  });
+
+  let sameSiteCookies = [...cookie.iter("samesite.example.com")];
+  equal(1, sameSiteCookies.length);
+  equal("Lax", sameSiteCookies[0].sameSite);
+});
diff --git a/remote/marionette/test/xpcshell/test_element.js b/remote/marionette/test/xpcshell/test_element.js
new file mode 100644
index 0000000000..7b2379b1ab
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_element.js
@@ -0,0 +1,789 @@
+/* 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 { element, ShadowRoot, WebElement, WebFrame, WebReference, WebWindow } =
+  ChromeUtils.importESModule(
+    "chrome://remote/content/marionette/element.sys.mjs"
+  );
+const { NodeCache } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/NodeCache.sys.mjs"
+);
+
+const MemoryReporter = Cc["@mozilla.org/memory-reporter-manager;1"].getService(
+  Ci.nsIMemoryReporterManager
+);
+
+class MockElement {
+  constructor(tagName, attrs = {}) {
+    this.tagName = tagName;
+    this.localName = tagName;
+
+    this.isConnected = false;
+    this.ownerGlobal = {
+      document: {
+        isActive() {
+          return true;
+        },
+      },
+    };
+
+    for (let attr in attrs) {
+      this[attr] = attrs[attr];
+    }
+  }
+
+  get nodeType() {
+    return 1;
+  }
+
+  get ELEMENT_NODE() {
+    return 1;
+  }
+
+  // this is a severely limited CSS selector
+  // that only supports lists of tag names
+  matches(selector) {
+    let tags = selector.split(",");
+    return tags.includes(this.localName);
+  }
+}
+
+class MockXULElement extends MockElement {
+  constructor(tagName, attrs = {}) {
+    super(tagName, attrs);
+    this.namespaceURI = XUL_NS;
+
+    if (typeof this.ownerDocument == "undefined") {
+      this.ownerDocument = {};
+    }
+    if (typeof this.ownerDocument.documentElement == "undefined") {
+      this.ownerDocument.documentElement = { namespaceURI: XUL_NS };
+    }
+  }
+}
+
+const xulEl = new MockXULElement("text");
+
+const domElInPrivilegedDocument = new MockElement("input", {
+  nodePrincipal: { isSystemPrincipal: true },
+});
+const xulElInPrivilegedDocument = new MockXULElement("text", {
+  nodePrincipal: { isSystemPrincipal: true },
+});
+
+function setupTest() {
+  const browser = Services.appShell.createWindowlessBrowser(false);
+
+  browser.document.body.innerHTML = `
+    <div id="foo" style="margin: 50px">
+      <iframe></iframe>
+      <video></video>
+      <svg xmlns="http://www.w3.org/2000/svg"></svg>
+      <textarea></textarea>
+    </div>
+  `;
+
+  const divEl = browser.document.querySelector("div");
+  const svgEl = browser.document.querySelector("svg");
+  const textareaEl = browser.document.querySelector("textarea");
+  const videoEl = browser.document.querySelector("video");
+
+  const iframeEl = browser.document.querySelector("iframe");
+  const childEl = iframeEl.contentDocument.createElement("div");
+  iframeEl.contentDocument.body.appendChild(childEl);
+
+  const shadowRoot = videoEl.openOrClosedShadowRoot;
+
+  return {
+    browser,
+    nodeCache: new NodeCache(),
+    childEl,
+    divEl,
+    iframeEl,
+    shadowRoot,
+    svgEl,
+    textareaEl,
+    videoEl,
+  };
+}
+
+add_task(function test_findClosest() {
+  const { divEl, videoEl } = setupTest();
+
+  equal(element.findClosest(divEl, "foo"), null);
+  equal(element.findClosest(videoEl, "div"), divEl);
+});
+
+add_task(function test_isSelected() {
+  const { browser, divEl } = setupTest();
+
+  const checkbox = browser.document.createElement("input");
+  checkbox.setAttribute("type", "checkbox");
+
+  ok(!element.isSelected(checkbox));
+  checkbox.checked = true;
+  ok(element.isSelected(checkbox));
+
+  // selected is not a property of <input type=checkbox>
+  checkbox.selected = true;
+  checkbox.checked = false;
+  ok(!element.isSelected(checkbox));
+
+  const option = browser.document.createElement("option");
+
+  ok(!element.isSelected(option));
+  option.selected = true;
+  ok(element.isSelected(option));
+
+  // checked is not a property of <option>
+  option.checked = true;
+  option.selected = false;
+  ok(!element.isSelected(option));
+
+  // anything else should not be selected
+  for (const type of [undefined, null, "foo", true, [], {}, divEl]) {
+    ok(!element.isSelected(type));
+  }
+});
+
+add_task(function test_isElement() {
+  const { divEl, iframeEl, shadowRoot, svgEl } = setupTest();
+
+  ok(element.isElement(divEl));
+  ok(element.isElement(svgEl));
+  ok(element.isElement(xulEl));
+  ok(element.isElement(domElInPrivilegedDocument));
+  ok(element.isElement(xulElInPrivilegedDocument));
+
+  ok(!element.isElement(shadowRoot));
+  ok(!element.isElement(divEl.ownerGlobal));
+  ok(!element.isElement(iframeEl.contentWindow));
+
+  for (const type of [true, 42, {}, [], undefined, null]) {
+    ok(!element.isElement(type));
+  }
+});
+
+add_task(function test_isDOMElement() {
+  const { divEl, iframeEl, shadowRoot, svgEl } = setupTest();
+
+  ok(element.isDOMElement(divEl));
+  ok(element.isDOMElement(svgEl));
+  ok(element.isDOMElement(domElInPrivilegedDocument));
+
+  ok(!element.isDOMElement(shadowRoot));
+  ok(!element.isDOMElement(divEl.ownerGlobal));
+  ok(!element.isDOMElement(iframeEl.contentWindow));
+  ok(!element.isDOMElement(xulEl));
+  ok(!element.isDOMElement(xulElInPrivilegedDocument));
+
+  for (const type of [true, 42, "foo", {}, [], undefined, null]) {
+    ok(!element.isDOMElement(type));
+  }
+});
+
+add_task(function test_isXULElement() {
+  const { divEl, iframeEl, shadowRoot, svgEl } = setupTest();
+
+  ok(element.isXULElement(xulEl));
+  ok(element.isXULElement(xulElInPrivilegedDocument));
+
+  ok(!element.isXULElement(divEl));
+  ok(!element.isXULElement(domElInPrivilegedDocument));
+  ok(!element.isXULElement(svgEl));
+  ok(!element.isXULElement(shadowRoot));
+  ok(!element.isXULElement(divEl.ownerGlobal));
+  ok(!element.isXULElement(iframeEl.contentWindow));
+
+  for (const type of [true, 42, "foo", {}, [], undefined, null]) {
+    ok(!element.isXULElement(type));
+  }
+});
+
+add_task(function test_isDOMWindow() {
+  const { divEl, iframeEl, shadowRoot, svgEl } = setupTest();
+
+  ok(element.isDOMWindow(divEl.ownerGlobal));
+  ok(element.isDOMWindow(iframeEl.contentWindow));
+
+  ok(!element.isDOMWindow(divEl));
+  ok(!element.isDOMWindow(svgEl));
+  ok(!element.isDOMWindow(shadowRoot));
+  ok(!element.isDOMWindow(domElInPrivilegedDocument));
+  ok(!element.isDOMWindow(xulEl));
+  ok(!element.isDOMWindow(xulElInPrivilegedDocument));
+
+  for (const type of [true, 42, {}, [], undefined, null]) {
+    ok(!element.isDOMWindow(type));
+  }
+});
+
+add_task(function test_isShadowRoot() {
+  const { browser, divEl, iframeEl, shadowRoot, svgEl } = setupTest();
+
+  ok(element.isShadowRoot(shadowRoot));
+
+  ok(!element.isShadowRoot(divEl));
+  ok(!element.isShadowRoot(svgEl));
+  ok(!element.isShadowRoot(divEl.ownerGlobal));
+  ok(!element.isShadowRoot(iframeEl.contentWindow));
+  ok(!element.isShadowRoot(xulEl));
+  ok(!element.isShadowRoot(domElInPrivilegedDocument));
+  ok(!element.isShadowRoot(xulElInPrivilegedDocument));
+
+  for (const type of [true, 42, "foo", {}, [], undefined, null]) {
+    ok(!element.isShadowRoot(type));
+  }
+
+  const documentFragment = browser.document.createDocumentFragment();
+  ok(!element.isShadowRoot(documentFragment));
+});
+
+add_task(function test_isReadOnly() {
+  const { browser, divEl, textareaEl } = setupTest();
+
+  const input = browser.document.createElement("input");
+  input.readOnly = true;
+  ok(element.isReadOnly(input));
+
+  textareaEl.readOnly = true;
+  ok(element.isReadOnly(textareaEl));
+
+  ok(!element.isReadOnly(divEl));
+  divEl.readOnly = true;
+  ok(!element.isReadOnly(divEl));
+
+  ok(!element.isReadOnly(null));
+});
+
+add_task(function test_isDisabled() {
+  const { browser, divEl, svgEl } = setupTest();
+
+  const select = browser.document.createElement("select");
+  const option = browser.document.createElement("option");
+  select.appendChild(option);
+  select.disabled = true;
+  ok(element.isDisabled(option));
+
+  const optgroup = browser.document.createElement("optgroup");
+  option.parentNode = optgroup;
+  ok(element.isDisabled(option));
+
+  optgroup.parentNode = select;
+  ok(element.isDisabled(option));
+
+  select.disabled = false;
+  ok(!element.isDisabled(option));
+
+  for (const type of ["button", "input", "select", "textarea"]) {
+    const elem = browser.document.createElement(type);
+    ok(!element.isDisabled(elem));
+    elem.disabled = true;
+    ok(element.isDisabled(elem));
+  }
+
+  ok(!element.isDisabled(divEl));
+
+  svgEl.disabled = true;
+  ok(!element.isDisabled(svgEl));
+
+  ok(!element.isDisabled(new MockXULElement("browser", { disabled: true })));
+});
+
+add_task(function test_isEditingHost() {
+  const { browser, divEl, svgEl } = setupTest();
+
+  ok(!element.isEditingHost(null));
+
+  ok(!element.isEditingHost(divEl));
+  divEl.contentEditable = true;
+  ok(element.isEditingHost(divEl));
+
+  ok(!element.isEditingHost(svgEl));
+  browser.document.designMode = "on";
+  ok(element.isEditingHost(svgEl));
+});
+
+add_task(function test_isEditable() {
+  const { browser, divEl, svgEl, textareaEl } = setupTest();
+
+  ok(!element.isEditable(null));
+
+  for (let type of [
+    "checkbox",
+    "radio",
+    "hidden",
+    "submit",
+    "button",
+    "image",
+  ]) {
+    const input = browser.document.createElement("input");
+    input.setAttribute("type", type);
+
+    ok(!element.isEditable(input));
+  }
+
+  const input = browser.document.createElement("input");
+  ok(element.isEditable(input));
+  input.setAttribute("type", "text");
+  ok(element.isEditable(input));
+
+  ok(element.isEditable(textareaEl));
+
+  const textareaDisabled = browser.document.createElement("textarea");
+  textareaDisabled.disabled = true;
+  ok(!element.isEditable(textareaDisabled));
+
+  const textareaReadOnly = browser.document.createElement("textarea");
+  textareaReadOnly.readOnly = true;
+  ok(!element.isEditable(textareaReadOnly));
+
+  ok(!element.isEditable(divEl));
+  divEl.contentEditable = true;
+  ok(element.isEditable(divEl));
+
+  ok(!element.isEditable(svgEl));
+  browser.document.designMode = "on";
+  ok(element.isEditable(svgEl));
+});
+
+add_task(function test_isMutableFormControlElement() {
+  const { browser, divEl, textareaEl } = setupTest();
+
+  ok(!element.isMutableFormControl(null));
+
+  ok(element.isMutableFormControl(textareaEl));
+
+  const textareaDisabled = browser.document.createElement("textarea");
+  textareaDisabled.disabled = true;
+  ok(!element.isMutableFormControl(textareaDisabled));
+
+  const textareaReadOnly = browser.document.createElement("textarea");
+  textareaReadOnly.readOnly = true;
+  ok(!element.isMutableFormControl(textareaReadOnly));
+
+  const mutableStates = new Set([
+    "color",
+    "date",
+    "datetime-local",
+    "email",
+    "file",
+    "month",
+    "number",
+    "password",
+    "range",
+    "search",
+    "tel",
+    "text",
+    "url",
+    "week",
+  ]);
+  for (const type of mutableStates) {
+    const input = browser.document.createElement("input");
+    input.setAttribute("type", type);
+    ok(element.isMutableFormControl(input));
+  }
+
+  const inputHidden = browser.document.createElement("input");
+  inputHidden.setAttribute("type", "hidden");
+  ok(!element.isMutableFormControl(inputHidden));
+
+  ok(!element.isMutableFormControl(divEl));
+  divEl.contentEditable = true;
+  ok(!element.isMutableFormControl(divEl));
+  browser.document.designMode = "on";
+  ok(!element.isMutableFormControl(divEl));
+});
+
+add_task(function test_coordinates() {
+  const { divEl } = setupTest();
+
+  let coords = element.coordinates(divEl);
+  ok(coords.hasOwnProperty("x"));
+  ok(coords.hasOwnProperty("y"));
+  equal(typeof coords.x, "number");
+  equal(typeof coords.y, "number");
+
+  deepEqual(element.coordinates(divEl), { x: 0, y: 0 });
+  deepEqual(element.coordinates(divEl, 10, 10), { x: 10, y: 10 });
+  deepEqual(element.coordinates(divEl, -5, -5), { x: -5, y: -5 });
+
+  Assert.throws(() => element.coordinates(null), /node is null/);
+
+  Assert.throws(
+    () => element.coordinates(divEl, "string", undefined),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, undefined, "string"),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, "string", "string"),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, {}, undefined),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, undefined, {}),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, {}, {}),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, [], undefined),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, undefined, []),
+    /Offset must be a number/
+  );
+  Assert.throws(
+    () => element.coordinates(divEl, [], []),
+    /Offset must be a number/
+  );
+});
+
+add_task(function test_isNodeReferenceKnown() {
+  const { browser, nodeCache, childEl, iframeEl, videoEl } = setupTest();
+
+  // Unknown node reference
+  ok(!element.isNodeReferenceKnown(browser.browsingContext, "foo", nodeCache));
+
+  // Known node reference
+  const videoElRef = nodeCache.getOrCreateNodeReference(videoEl);
+  ok(
+    element.isNodeReferenceKnown(browser.browsingContext, videoElRef, nodeCache)
+  );
+
+  // Different top-level browsing context
+  const browser2 = Services.appShell.createWindowlessBrowser(false);
+  ok(
+    !element.isNodeReferenceKnown(
+      browser2.browsingContext,
+      videoElRef,
+      nodeCache
+    )
+  );
+
+  // Different child browsing context
+  const childElRef = nodeCache.getOrCreateNodeReference(childEl);
+  const childBrowsingContext = iframeEl.contentWindow.browsingContext;
+  ok(element.isNodeReferenceKnown(childBrowsingContext, childElRef, nodeCache));
+
+  const iframeEl2 = browser2.document.createElement("iframe");
+  browser2.document.body.appendChild(iframeEl2);
+  const childBrowsingContext2 = iframeEl2.contentWindow.browsingContext;
+  ok(
+    !element.isNodeReferenceKnown(childBrowsingContext2, childElRef, nodeCache)
+  );
+});
+
+add_task(function test_getKnownElement() {
+  const { browser, nodeCache, shadowRoot, videoEl } = setupTest();
+
+  // Unknown element reference
+  Assert.throws(() => {
+    element.getKnownElement(browser.browsingContext, "foo", nodeCache);
+  }, /NoSuchElementError/);
+
+  // With a ShadowRoot reference
+  const shadowRootRef = nodeCache.getOrCreateNodeReference(shadowRoot);
+  Assert.throws(() => {
+    element.getKnownElement(browser.browsingContext, shadowRootRef, nodeCache);
+  }, /NoSuchElementError/);
+
+  // Deleted element (eg. garbage collected)
+  let detachedEl = browser.document.createElement("div");
+  const detachedElRef = nodeCache.getOrCreateNodeReference(detachedEl);
+
+  // ... not connected to the DOM
+  Assert.throws(() => {
+    element.getKnownElement(browser.browsingContext, detachedElRef, nodeCache);
+  }, /StaleElementReferenceError/);
+
+  // ... element garbage collected
+  detachedEl = null;
+  MemoryReporter.minimizeMemoryUsage(() => {
+    Assert.throws(() => {
+      element.getKnownElement(
+        browser.browsingContext,
+        detachedElRef,
+        nodeCache
+      );
+    }, /StaleElementReferenceError/);
+  });
+
+  // Known element reference
+  const videoElRef = nodeCache.getOrCreateNodeReference(videoEl);
+  equal(
+    element.getKnownElement(browser.browsingContext, videoElRef, nodeCache),
+    videoEl
+  );
+});
+
+add_task(function test_getKnownShadowRoot() {
+  const { browser, nodeCache, shadowRoot, videoEl } = setupTest();
+
+  const videoElRef = nodeCache.getOrCreateNodeReference(videoEl);
+
+  // Unknown ShadowRoot reference
+  Assert.throws(() => {
+    element.getKnownShadowRoot(browser.browsingContext, "foo", nodeCache);
+  }, /NoSuchShadowRootError/);
+
+  // With a HTMLElement reference
+  Assert.throws(() => {
+    element.getKnownShadowRoot(browser.browsingContext, videoElRef, nodeCache);
+  }, /NoSuchShadowRootError/);
+
+  // Known ShadowRoot reference
+  const shadowRootRef = nodeCache.getOrCreateNodeReference(shadowRoot);
+  equal(
+    element.getKnownShadowRoot(
+      browser.browsingContext,
+      shadowRootRef,
+      nodeCache
+    ),
+    shadowRoot
+  );
+
+  // Detached ShadowRoot host
+  let el = browser.document.createElement("div");
+  let detachedShadowRoot = el.attachShadow({ mode: "open" });
+  detachedShadowRoot.innerHTML = "<input></input>";
+
+  const detachedShadowRootRef =
+    nodeCache.getOrCreateNodeReference(detachedShadowRoot);
+
+  // ... not connected to the DOM
+  Assert.throws(() => {
+    element.getKnownShadowRoot(
+      browser.browsingContext,
+      detachedShadowRootRef,
+      nodeCache
+    );
+  }, /DetachedShadowRootError/);
+
+  // ... host and shadow root garbage collected
+  el = null;
+  detachedShadowRoot = null;
+  MemoryReporter.minimizeMemoryUsage(() => {
+    Assert.throws(() => {
+      element.getKnownShadowRoot(
+        browser.browsingContext,
+        detachedShadowRootRef,
+        nodeCache
+      );
+    }, /DetachedShadowRootError/);
+  });
+});
+
+add_task(function test_isDetached() {
+  const { childEl, iframeEl } = setupTest();
+
+  let detachedShadowRoot = childEl.attachShadow({ mode: "open" });
+  detachedShadowRoot.innerHTML = "<input></input>";
+
+  // Connected to the DOM
+  ok(!element.isDetached(detachedShadowRoot));
+
+  // Node document (ownerDocument) is not the active document
+  iframeEl.remove();
+  ok(element.isDetached(detachedShadowRoot));
+
+  // host element is stale (eg. not connected)
+  detachedShadowRoot.host.remove();
+  equal(childEl.isConnected, false);
+  ok(element.isDetached(detachedShadowRoot));
+});
+
+add_task(function test_isStale() {
+  const { childEl, iframeEl } = setupTest();
+
+  // Connected to the DOM
+  ok(!element.isStale(childEl));
+
+  // Not part of the active document
+  iframeEl.remove();
+  ok(element.isStale(childEl));
+
+  // Not connected to the DOM
+  childEl.remove();
+  ok(element.isStale(childEl));
+});
+
+add_task(function test_WebReference_ctor() {
+  const el = new WebReference("foo");
+  equal(el.uuid, "foo");
+
+  for (let t of [42, true, [], {}, null, undefined]) {
+    Assert.throws(() => new WebReference(t), /to be a string/);
+  }
+});
+
+add_task(function test_WebElemenet_is() {
+  const a = new WebReference("a");
+  const b = new WebReference("b");
+
+  ok(a.is(a));
+  ok(b.is(b));
+  ok(!a.is(b));
+  ok(!b.is(a));
+
+  ok(!a.is({}));
+});
+
+add_task(function test_WebReference_from() {
+  const { divEl, iframeEl } = setupTest();
+
+  ok(WebReference.from(divEl) instanceof WebElement);
+  ok(WebReference.from(xulEl) instanceof WebElement);
+  ok(WebReference.from(divEl.ownerGlobal) instanceof WebWindow);
+  ok(WebReference.from(iframeEl.contentWindow) instanceof WebFrame);
+  ok(WebReference.from(domElInPrivilegedDocument) instanceof WebElement);
+  ok(WebReference.from(xulElInPrivilegedDocument) instanceof WebElement);
+
+  Assert.throws(() => WebReference.from({}), /InvalidArgumentError/);
+});
+
+add_task(function test_WebReference_fromJSON_WebElement() {
+  const { Identifier } = WebElement;
+
+  const ref = { [Identifier]: "foo" };
+  const webEl = WebReference.fromJSON(ref);
+  ok(webEl instanceof WebElement);
+  equal(webEl.uuid, "foo");
+
+  let identifierPrecedence = {
+    [Identifier]: "identifier-uuid",
+  };
+  const precedenceEl = WebReference.fromJSON(identifierPrecedence);
+  ok(precedenceEl instanceof WebElement);
+  equal(precedenceEl.uuid, "identifier-uuid");
+});
+
+add_task(function test_WebReference_fromJSON_WebWindow() {
+  const ref = { [WebWindow.Identifier]: "foo" };
+  const win = WebReference.fromJSON(ref);
+
+  ok(win instanceof WebWindow);
+  equal(win.uuid, "foo");
+});
+
+add_task(function test_WebReference_fromJSON_WebFrame() {
+  const ref = { [WebFrame.Identifier]: "foo" };
+  const frame = WebReference.fromJSON(ref);
+  ok(frame instanceof WebFrame);
+  equal(frame.uuid, "foo");
+});
+
+add_task(function test_WebReference_fromJSON_malformed() {
+  Assert.throws(() => WebReference.fromJSON({}), /InvalidArgumentError/);
+  Assert.throws(() => WebReference.fromJSON(null), /InvalidArgumentError/);
+});
+
+add_task(function test_WebReference_isReference() {
+  for (let t of [42, true, "foo", [], {}]) {
+    ok(!WebReference.isReference(t));
+  }
+
+  ok(WebReference.isReference({ [WebElement.Identifier]: "foo" }));
+  ok(WebReference.isReference({ [WebWindow.Identifier]: "foo" }));
+  ok(WebReference.isReference({ [WebFrame.Identifier]: "foo" }));
+});
+
+add_task(function test_WebElement_toJSON() {
+  const { Identifier } = WebElement;
+
+  const el = new WebElement("foo");
+  const json = el.toJSON();
+
+  ok(Identifier in json);
+  equal(json[Identifier], "foo");
+});
+
+add_task(function test_WebElement_fromJSON() {
+  const { Identifier } = WebElement;
+
+  const el = WebElement.fromJSON({ [Identifier]: "foo" });
+  ok(el instanceof WebElement);
+  equal(el.uuid, "foo");
+
+  Assert.throws(() => WebElement.fromJSON({}), /InvalidArgumentError/);
+});
+
+add_task(function test_WebElement_fromUUID() {
+  const domWebEl = WebElement.fromUUID("bar");
+
+  ok(domWebEl instanceof WebElement);
+  equal(domWebEl.uuid, "bar");
+
+  Assert.throws(() => WebElement.fromUUID(), /InvalidArgumentError/);
+});
+
+add_task(function test_ShadowRoot_toJSON() {
+  const { Identifier } = ShadowRoot;
+
+  const shadowRoot = new ShadowRoot("foo");
+  const json = shadowRoot.toJSON();
+
+  ok(Identifier in json);
+  equal(json[Identifier], "foo");
+});
+
+add_task(function test_ShadowRoot_fromJSON() {
+  const { Identifier } = ShadowRoot;
+
+  const shadowRoot = ShadowRoot.fromJSON({ [Identifier]: "foo" });
+  ok(shadowRoot instanceof ShadowRoot);
+  equal(shadowRoot.uuid, "foo");
+
+  Assert.throws(() => ShadowRoot.fromJSON({}), /InvalidArgumentError/);
+});
+
+add_task(function test_ShadowRoot_fromUUID() {
+  const shadowRoot = ShadowRoot.fromUUID("baz");
+
+  ok(shadowRoot instanceof ShadowRoot);
+  equal(shadowRoot.uuid, "baz");
+
+  Assert.throws(() => ShadowRoot.fromUUID(), /InvalidArgumentError/);
+});
+
+add_task(function test_WebWindow_toJSON() {
+  const win = new WebWindow("foo");
+  const json = win.toJSON();
+
+  ok(WebWindow.Identifier in json);
+  equal(json[WebWindow.Identifier], "foo");
+});
+
+add_task(function test_WebWindow_fromJSON() {
+  const ref = { [WebWindow.Identifier]: "foo" };
+  const win = WebWindow.fromJSON(ref);
+
+  ok(win instanceof WebWindow);
+  equal(win.uuid, "foo");
+});
+
+add_task(function test_WebFrame_toJSON() {
+  const frame = new WebFrame("foo");
+  const json = frame.toJSON();
+
+  ok(WebFrame.Identifier in json);
+  equal(json[WebFrame.Identifier], "foo");
+});
+
+add_task(function test_WebFrame_fromJSON() {
+  const ref = { [WebFrame.Identifier]: "foo" };
+  const win = WebFrame.fromJSON(ref);
+
+  ok(win instanceof WebFrame);
+  equal(win.uuid, "foo");
+});
diff --git a/remote/marionette/test/xpcshell/test_json.js b/remote/marionette/test/xpcshell/test_json.js
new file mode 100644
index 0000000000..479e1fe473
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_json.js
@@ -0,0 +1,275 @@
+const { json } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/json.sys.mjs"
+);
+const { NodeCache } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/NodeCache.sys.mjs"
+);
+const { ShadowRoot, WebElement, WebReference } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/element.sys.mjs"
+);
+
+function setupTest() {
+  const browser = Services.appShell.createWindowlessBrowser(false);
+  const nodeCache = new NodeCache();
+
+  const htmlEl = browser.document.createElement("video");
+  browser.document.body.appendChild(htmlEl);
+
+  const svgEl = browser.document.createElementNS(
+    "http://www.w3.org/2000/svg",
+    "rect"
+  );
+  browser.document.body.appendChild(svgEl);
+
+  const shadowRoot = htmlEl.openOrClosedShadowRoot;
+
+  const iframeEl = browser.document.createElement("iframe");
+  browser.document.body.appendChild(iframeEl);
+  const childEl = iframeEl.contentDocument.createElement("div");
+
+  return { browser, nodeCache, childEl, iframeEl, htmlEl, shadowRoot, svgEl };
+}
+
+add_task(function test_clone_generalTypes() {
+  const { nodeCache } = setupTest();
+
+  // null
+  equal(json.clone(undefined, nodeCache), null);
+  equal(json.clone(null, nodeCache), null);
+
+  // primitives
+  equal(json.clone(true, nodeCache), true);
+  equal(json.clone(42, nodeCache), 42);
+  equal(json.clone("foo", nodeCache), "foo");
+
+  // toJSON
+  equal(
+    json.clone({
+      toJSON() {
+        return "foo";
+      },
+    }),
+    "foo"
+  );
+});
+
+add_task(function test_clone_ShadowRoot() {
+  const { nodeCache, shadowRoot } = setupTest();
+
+  const shadowRootRef = nodeCache.getOrCreateNodeReference(shadowRoot);
+  deepEqual(
+    json.clone(shadowRoot, nodeCache),
+    WebReference.from(shadowRoot, shadowRootRef).toJSON()
+  );
+});
+
+add_task(function test_clone_WebElement() {
+  const { htmlEl, nodeCache, svgEl } = setupTest();
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+  deepEqual(
+    json.clone(htmlEl, nodeCache),
+    WebReference.from(htmlEl, htmlElRef).toJSON()
+  );
+
+  // Check an element with a different namespace
+  const svgElRef = nodeCache.getOrCreateNodeReference(svgEl);
+  deepEqual(
+    json.clone(svgEl, nodeCache),
+    WebReference.from(svgEl, svgElRef).toJSON()
+  );
+});
+
+add_task(function test_clone_Sequences() {
+  const { htmlEl, nodeCache } = setupTest();
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+
+  const input = [
+    null,
+    true,
+    [],
+    htmlEl,
+    {
+      toJSON() {
+        return "foo";
+      },
+    },
+    { bar: "baz" },
+  ];
+
+  const actual = json.clone(input, nodeCache);
+
+  equal(actual[0], null);
+  equal(actual[1], true);
+  deepEqual(actual[2], []);
+  deepEqual(actual[3], { [WebElement.Identifier]: htmlElRef });
+  equal(actual[4], "foo");
+  deepEqual(actual[5], { bar: "baz" });
+});
+
+add_task(function test_clone_objects() {
+  const { htmlEl, nodeCache } = setupTest();
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+
+  const input = {
+    null: null,
+    boolean: true,
+    array: [42],
+    element: htmlEl,
+    toJSON: {
+      toJSON() {
+        return "foo";
+      },
+    },
+    object: { bar: "baz" },
+  };
+
+  const actual = json.clone(input, nodeCache);
+
+  equal(actual.null, null);
+  equal(actual.boolean, true);
+  deepEqual(actual.array, [42]);
+  deepEqual(actual.element, { [WebElement.Identifier]: htmlElRef });
+  equal(actual.toJSON, "foo");
+  deepEqual(actual.object, { bar: "baz" });
+});
+
+add_task(function test_clone_сyclicReference() {
+  const { nodeCache } = setupTest();
+
+  // object
+  Assert.throws(() => {
+    const obj = {};
+    obj.reference = obj;
+    json.clone(obj, nodeCache);
+  }, /JavaScriptError/);
+
+  // array
+  Assert.throws(() => {
+    const array = [];
+    array.push(array);
+    json.clone(array, nodeCache);
+  }, /JavaScriptError/);
+
+  // array in object
+  Assert.throws(() => {
+    const array = [];
+    array.push(array);
+    json.clone({ array }, nodeCache);
+  }, /JavaScriptError/);
+
+  // object in array
+  Assert.throws(() => {
+    const obj = {};
+    obj.reference = obj;
+    json.clone([obj], nodeCache);
+  }, /JavaScriptError/);
+});
+
+add_task(function test_deserialize_generalTypes() {
+  const { browser, nodeCache } = setupTest();
+  const win = browser.document.ownerGlobal;
+
+  // null
+  equal(json.deserialize(undefined, nodeCache, win), undefined);
+  equal(json.deserialize(null, nodeCache, win), null);
+
+  // primitives
+  equal(json.deserialize(true, nodeCache, win), true);
+  equal(json.deserialize(42, nodeCache, win), 42);
+  equal(json.deserialize("foo", nodeCache, win), "foo");
+});
+
+add_task(function test_deserialize_ShadowRoot() {
+  const { browser, nodeCache, shadowRoot } = setupTest();
+  const win = browser.document.ownerGlobal;
+
+  // Fails to resolve for unknown elements
+  const unknownShadowRootId = { [ShadowRoot.Identifier]: "foo" };
+  Assert.throws(() => {
+    json.deserialize(unknownShadowRootId, nodeCache, win);
+  }, /NoSuchShadowRootError/);
+
+  const shadowRootRef = nodeCache.getOrCreateNodeReference(shadowRoot);
+  const shadowRootEl = { [ShadowRoot.Identifier]: shadowRootRef };
+
+  // Fails to resolve for missing window reference
+  Assert.throws(() => json.deserialize(shadowRootEl, nodeCache), /TypeError/);
+
+  // Previously seen element is associated with original web element reference
+  const root = json.deserialize(shadowRootEl, nodeCache, win);
+  deepEqual(root, shadowRoot);
+  deepEqual(root, nodeCache.getNode(browser.browsingContext, shadowRootRef));
+});
+
+add_task(function test_deserialize_WebElement() {
+  const { browser, htmlEl, nodeCache } = setupTest();
+  const win = browser.document.ownerGlobal;
+
+  // Fails to resolve for unknown elements
+  const unknownWebElId = { [WebElement.Identifier]: "foo" };
+  Assert.throws(() => {
+    json.deserialize(unknownWebElId, nodeCache, win);
+  }, /NoSuchElementError/);
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+  const htmlWebEl = { [WebElement.Identifier]: htmlElRef };
+
+  // Fails to resolve for missing window reference
+  Assert.throws(() => json.deserialize(htmlWebEl, nodeCache), /TypeError/);
+
+  // Previously seen element is associated with original web element reference
+  const el = json.deserialize(htmlWebEl, nodeCache, win);
+  deepEqual(el, htmlEl);
+  deepEqual(el, nodeCache.getNode(browser.browsingContext, htmlElRef));
+});
+
+add_task(function test_deserialize_Sequences() {
+  const { browser, htmlEl, nodeCache } = setupTest();
+  const win = browser.document.ownerGlobal;
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+
+  const input = [
+    null,
+    true,
+    [42],
+    { [WebElement.Identifier]: htmlElRef },
+    { bar: "baz" },
+  ];
+
+  const actual = json.deserialize(input, nodeCache, win);
+
+  equal(actual[0], null);
+  equal(actual[1], true);
+  deepEqual(actual[2], [42]);
+  deepEqual(actual[3], htmlEl);
+  deepEqual(actual[4], { bar: "baz" });
+});
+
+add_task(function test_deserialize_objects() {
+  const { browser, htmlEl, nodeCache } = setupTest();
+  const win = browser.document.ownerGlobal;
+
+  const htmlElRef = nodeCache.getOrCreateNodeReference(htmlEl);
+
+  const input = {
+    null: null,
+    boolean: true,
+    array: [42],
+    element: { [WebElement.Identifier]: htmlElRef },
+    object: { bar: "baz" },
+  };
+
+  const actual = json.deserialize(input, nodeCache, win);
+
+  equal(actual.null, null);
+  equal(actual.boolean, true);
+  deepEqual(actual.array, [42]);
+  deepEqual(actual.element, htmlEl);
+  deepEqual(actual.object, { bar: "baz" });
+
+  nodeCache.clear({ all: true });
+});
diff --git a/remote/marionette/test/xpcshell/test_message.js b/remote/marionette/test/xpcshell/test_message.js
new file mode 100644
index 0000000000..9926aea191
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_message.js
@@ -0,0 +1,245 @@
+/* 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/webdriver/Errors.sys.mjs"
+);
+const { Command, Message, Response } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/message.sys.mjs"
+);
+
+add_task(function test_Message_Origin() {
+  equal(0, Message.Origin.Client);
+  equal(1, Message.Origin.Server);
+});
+
+add_task(function test_Message_fromPacket() {
+  let cmd = new Command(4, "foo");
+  let resp = new Response(5, () => {});
+  resp.error = "foo";
+
+  ok(Message.fromPacket(cmd.toPacket()) instanceof Command);
+  ok(Message.fromPacket(resp.toPacket()) instanceof Response);
+  Assert.throws(
+    () => Message.fromPacket([3, 4, 5, 6]),
+    /Unrecognised message type in packet/
+  );
+});
+
+add_task(function test_Command() {
+  let cmd = new Command(42, "foo", { bar: "baz" });
+  equal(42, cmd.id);
+  equal("foo", cmd.name);
+  deepEqual({ bar: "baz" }, cmd.parameters);
+  equal(null, cmd.onerror);
+  equal(null, cmd.onresult);
+  equal(Message.Origin.Client, cmd.origin);
+  equal(false, cmd.sent);
+});
+
+add_task(function test_Command_onresponse() {
+  let onerrorOk = false;
+  let onresultOk = false;
+
+  let cmd = new Command(7, "foo");
+  cmd.onerror = () => (onerrorOk = true);
+  cmd.onresult = () => (onresultOk = true);
+
+  let errorResp = new Response(8, () => {});
+  errorResp.error = new error.WebDriverError("foo");
+
+  let bodyResp = new Response(9, () => {});
+  bodyResp.body = "bar";
+
+  cmd.onresponse(errorResp);
+  equal(true, onerrorOk);
+  equal(false, onresultOk);
+
+  cmd.onresponse(bodyResp);
+  equal(true, onresultOk);
+});
+
+add_task(function test_Command_ctor() {
+  let cmd = new Command(42, "bar", { bar: "baz" });
+  let msg = cmd.toPacket();
+
+  equal(Command.Type, msg[0]);
+  equal(cmd.id, msg[1]);
+  equal(cmd.name, msg[2]);
+  equal(cmd.parameters, msg[3]);
+});
+
+add_task(function test_Command_toString() {
+  let cmd = new Command(42, "foo", { bar: "baz" });
+  equal(JSON.stringify(cmd.toPacket()), cmd.toString());
+});
+
+add_task(function test_Command_fromPacket() {
+  let c1 = new Command(42, "foo", { bar: "baz" });
+
+  let msg = c1.toPacket();
+  let c2 = Command.fromPacket(msg);
+
+  equal(c1.id, c2.id);
+  equal(c1.name, c2.name);
+  equal(c1.parameters, c2.parameters);
+
+  Assert.throws(
+    () => Command.fromPacket([null, 2, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Command.fromPacket([1, 2, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Command.fromPacket([0, null, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Command.fromPacket([0, 2, null, {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Command.fromPacket([0, 2, "foo", false]),
+    /InvalidArgumentError/
+  );
+
+  let nullParams = Command.fromPacket([0, 2, "foo", null]);
+  equal(
+    "[object Object]",
+    Object.prototype.toString.call(nullParams.parameters)
+  );
+});
+
+add_task(function test_Command_Type() {
+  equal(0, Command.Type);
+});
+
+add_task(function test_Response_ctor() {
+  let handler = () => {
+    throw new Error("foo");
+  };
+
+  let resp = new Response(42, handler);
+  equal(42, resp.id);
+  equal(null, resp.error);
+  ok("origin" in resp);
+  equal(Message.Origin.Server, resp.origin);
+  equal(false, resp.sent);
+  equal(handler, resp.respHandler_);
+});
+
+add_task(function test_Response_sendConditionally() {
+  let fired = false;
+  let resp = new Response(42, () => (fired = true));
+  resp.sendConditionally(() => false);
+  equal(false, resp.sent);
+  equal(false, fired);
+  resp.sendConditionally(() => true);
+  equal(true, resp.sent);
+  equal(true, fired);
+});
+
+add_task(function test_Response_send() {
+  let fired = false;
+  let resp = new Response(42, () => (fired = true));
+  resp.send();
+  equal(true, resp.sent);
+  equal(true, fired);
+});
+
+add_task(function test_Response_sendError_sent() {
+  let resp = new Response(42, r => equal(false, r.sent));
+  resp.sendError(new error.WebDriverError());
+  ok(resp.sent);
+  Assert.throws(() => resp.send(), /already been sent/);
+});
+
+add_task(function test_Response_sendError_body() {
+  let resp = new Response(42, r => equal(null, r.body));
+  resp.sendError(new error.WebDriverError());
+});
+
+add_task(function test_Response_sendError_errorSerialisation() {
+  let err1 = new error.WebDriverError();
+  let resp1 = new Response(42);
+  resp1.sendError(err1);
+  equal(err1.status, resp1.error.error);
+  deepEqual(err1.toJSON(), resp1.error);
+
+  let err2 = new error.InvalidArgumentError();
+  let resp2 = new Response(43);
+  resp2.sendError(err2);
+  equal(err2.status, resp2.error.error);
+  deepEqual(err2.toJSON(), resp2.error);
+});
+
+add_task(function test_Response_sendError_wrapInternalError() {
+  let err = new ReferenceError("foo");
+
+  // errors that originate from JavaScript (i.e. Marionette implementation
+  // issues) should be converted to UnknownError for transport
+  let resp = new Response(42, r => {
+    equal("unknown error", r.error.error);
+    equal(false, resp.sent);
+  });
+
+  // they should also throw after being sent
+  Assert.throws(() => resp.sendError(err), /foo/);
+  equal(true, resp.sent);
+});
+
+add_task(function test_Response_toPacket() {
+  let resp = new Response(42, () => {});
+  let msg = resp.toPacket();
+
+  equal(Response.Type, msg[0]);
+  equal(resp.id, msg[1]);
+  equal(resp.error, msg[2]);
+  equal(resp.body, msg[3]);
+});
+
+add_task(function test_Response_toString() {
+  let resp = new Response(42, () => {});
+  resp.error = "foo";
+  resp.body = "bar";
+
+  equal(JSON.stringify(resp.toPacket()), resp.toString());
+});
+
+add_task(function test_Response_fromPacket() {
+  let r1 = new Response(42, () => {});
+  r1.error = "foo";
+  r1.body = "bar";
+
+  let msg = r1.toPacket();
+  let r2 = Response.fromPacket(msg);
+
+  equal(r1.id, r2.id);
+  equal(r1.error, r2.error);
+  equal(r1.body, r2.body);
+
+  Assert.throws(
+    () => Response.fromPacket([null, 2, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Response.fromPacket([0, 2, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Response.fromPacket([1, null, "foo", {}]),
+    /InvalidArgumentError/
+  );
+  Assert.throws(
+    () => Response.fromPacket([1, 2, null, {}]),
+    /InvalidArgumentError/
+  );
+  Response.fromPacket([1, 2, "foo", null]);
+});
+
+add_task(function test_Response_Type() {
+  equal(1, Response.Type);
+});
diff --git a/remote/marionette/test/xpcshell/test_modal.js b/remote/marionette/test/xpcshell/test_modal.js
new file mode 100644
index 0000000000..c36811f7ec
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_modal.js
@@ -0,0 +1,113 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { modal } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/modal.sys.mjs"
+);
+
+const chromeWindow = {};
+
+const mockModalDialog = {
+  docShell: {
+    chromeEventHandler: null,
+  },
+  opener: {
+    ownerGlobal: chromeWindow,
+  },
+  Dialog: {
+    args: {
+      modalType: Services.prompt.MODAL_TYPE_WINDOW,
+    },
+  },
+};
+
+const mockCurBrowser = {
+  window: chromeWindow,
+};
+
+add_task(function test_addCallback() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+  let cb1 = () => true;
+  let cb2 = () => false;
+
+  equal(observer.callbacks.size, 0);
+  observer.add(cb1);
+  equal(observer.callbacks.size, 1);
+  observer.add(cb1);
+  equal(observer.callbacks.size, 1);
+  observer.add(cb2);
+  equal(observer.callbacks.size, 2);
+});
+
+add_task(function test_removeCallback() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+  let cb1 = () => true;
+  let cb2 = () => false;
+
+  equal(observer.callbacks.size, 0);
+  observer.add(cb1);
+  observer.add(cb2);
+
+  equal(observer.callbacks.size, 2);
+  observer.remove(cb1);
+  equal(observer.callbacks.size, 1);
+  observer.remove(cb1);
+  equal(observer.callbacks.size, 1);
+  observer.remove(cb2);
+  equal(observer.callbacks.size, 0);
+});
+
+add_task(function test_registerDialogClosedEventHandler() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+  let mockChromeWindow = {
+    addEventListener(event, cb) {
+      equal(
+        event,
+        "DOMModalDialogClosed",
+        "registered event for closing modal"
+      );
+      equal(cb, observer, "set itself as handler");
+    },
+  };
+
+  observer.observe(mockChromeWindow, "domwindowopened");
+});
+
+add_task(function test_handleCallbackOpenModalDialog() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+
+  observer.add((action, dialog) => {
+    equal(action, modal.ACTION_OPENED, "'opened' action has been passed");
+    equal(dialog, mockModalDialog, "dialog has been passed");
+  });
+  observer.observe(mockModalDialog, "common-dialog-loaded");
+});
+
+add_task(function test_handleCallbackCloseModalDialog() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+
+  observer.add((action, dialog) => {
+    equal(action, modal.ACTION_CLOSED, "'closed' action has been passed");
+    equal(dialog, mockModalDialog, "dialog has been passed");
+  });
+  observer.handleEvent({
+    type: "DOMModalDialogClosed",
+    target: mockModalDialog,
+  });
+});
+
+add_task(async function test_dialogClosed() {
+  let observer = new modal.DialogObserver(() => mockCurBrowser);
+
+  const dialogClosed = observer.dialogClosed();
+
+  observer.handleEvent({
+    type: "DOMModalDialogClosed",
+    target: mockModalDialog,
+  });
+
+  await dialogClosed;
+});
diff --git a/remote/marionette/test/xpcshell/test_navigate.js b/remote/marionette/test/xpcshell/test_navigate.js
new file mode 100644
index 0000000000..9b5e2a1bc7
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_navigate.js
@@ -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/. */
+
+const { navigate } = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/navigate.sys.mjs"
+);
+
+const mockTopContext = {
+  get children() {
+    return [mockNestedContext];
+  },
+  id: 7,
+  get top() {
+    return this;
+  },
+};
+
+const mockNestedContext = {
+  id: 8,
+  parent: mockTopContext,
+  top: mockTopContext,
+};
+
+add_task(function test_isLoadEventExpectedForCurrent() {
+  Assert.throws(
+    () => navigate.isLoadEventExpected(undefined),
+    /Expected at least one URL/
+  );
+
+  ok(navigate.isLoadEventExpected(new URL("http://a/")));
+});
+
+add_task(function test_isLoadEventExpectedForFuture() {
+  const data = [
+    { current: "http://a/", future: undefined, expected: true },
+    { current: "http://a/", future: "http://a/", expected: true },
+    { current: "http://a/", future: "http://a/#", expected: true },
+    { current: "http://a/#", future: "http://a/", expected: true },
+    { current: "http://a/#a", future: "http://a/#A", expected: true },
+    { current: "http://a/#a", future: "http://a/#a", expected: false },
+    { current: "http://a/", future: "javascript:whatever", expected: false },
+  ];
+
+  for (const entry of data) {
+    const current = new URL(entry.current);
+    const future = entry.future ? new URL(entry.future) : undefined;
+    equal(navigate.isLoadEventExpected(current, { future }), entry.expected);
+  }
+});
+
+add_task(function test_isLoadEventExpectedForTarget() {
+  for (const target of ["_parent", "_top"]) {
+    Assert.throws(
+      () => navigate.isLoadEventExpected(new URL("http://a"), { target }),
+      /Expected browsingContext when target is _parent or _top/
+    );
+  }
+
+  const data = [
+    { cur: "http://a/", target: "", expected: true },
+    { cur: "http://a/", target: "_blank", expected: false },
+    { cur: "http://a/", target: "_parent", bc: mockTopContext, expected: true },
+    {
+      cur: "http://a/",
+      target: "_parent",
+      bc: mockNestedContext,
+      expected: false,
+    },
+    { cur: "http://a/", target: "_self", expected: true },
+    { cur: "http://a/", target: "_top", bc: mockTopContext, expected: true },
+    {
+      cur: "http://a/",
+      target: "_top",
+      bc: mockNestedContext,
+      expected: false,
+    },
+  ];
+
+  for (const entry of data) {
+    const current = entry.cur ? new URL(entry.cur) : undefined;
+    equal(
+      navigate.isLoadEventExpected(current, {
+        target: entry.target,
+        browsingContext: entry.bc,
+      }),
+      entry.expected
+    );
+  }
+});
diff --git a/remote/marionette/test/xpcshell/test_prefs.js b/remote/marionette/test/xpcshell/test_prefs.js
new file mode 100644
index 0000000000..ac3432544b
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_prefs.js
@@ -0,0 +1,98 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { Branch, EnvironmentPrefs, MarionettePrefs } =
+  ChromeUtils.importESModule(
+    "chrome://remote/content/marionette/prefs.sys.mjs"
+  );
+
+function reset() {
+  Services.prefs.setBoolPref("test.bool", false);
+  Services.prefs.setStringPref("test.string", "foo");
+  Services.prefs.setIntPref("test.int", 777);
+}
+
+// Give us something to work with:
+reset();
+
+add_task(function test_Branch_get_root() {
+  let root = new Branch(null);
+  equal(false, root.get("test.bool"));
+  equal("foo", root.get("test.string"));
+  equal(777, root.get("test.int"));
+  Assert.throws(() => root.get("doesnotexist"), /TypeError/);
+});
+
+add_task(function test_Branch_get_branch() {
+  let test = new Branch("test.");
+  equal(false, test.get("bool"));
+  equal("foo", test.get("string"));
+  equal(777, test.get("int"));
+  Assert.throws(() => test.get("doesnotexist"), /TypeError/);
+});
+
+add_task(function test_Branch_set_root() {
+  let root = new Branch(null);
+
+  try {
+    root.set("test.string", "bar");
+    root.set("test.in", 777);
+    root.set("test.bool", true);
+
+    equal("bar", Services.prefs.getStringPref("test.string"));
+    equal(777, Services.prefs.getIntPref("test.int"));
+    equal(true, Services.prefs.getBoolPref("test.bool"));
+  } finally {
+    reset();
+  }
+});
+
+add_task(function test_Branch_set_branch() {
+  let test = new Branch("test.");
+
+  try {
+    test.set("string", "bar");
+    test.set("int", 888);
+    test.set("bool", true);
+
+    equal("bar", Services.prefs.getStringPref("test.string"));
+    equal(888, Services.prefs.getIntPref("test.int"));
+    equal(true, Services.prefs.getBoolPref("test.bool"));
+  } finally {
+    reset();
+  }
+});
+
+add_task(function test_EnvironmentPrefs_from() {
+  let prefsTable = {
+    "test.bool": true,
+    "test.int": 888,
+    "test.string": "bar",
+  };
+  Services.env.set("FOO", JSON.stringify(prefsTable));
+
+  try {
+    for (let [key, value] of EnvironmentPrefs.from("FOO")) {
+      equal(prefsTable[key], value);
+    }
+  } finally {
+    Services.env.set("FOO", null);
+  }
+});
+
+add_task(function test_MarionettePrefs_getters() {
+  equal(false, MarionettePrefs.clickToStart);
+  equal(2828, MarionettePrefs.port);
+});
+
+add_task(function test_MarionettePrefs_setters() {
+  try {
+    MarionettePrefs.port = 777;
+    equal(777, MarionettePrefs.port);
+  } finally {
+    Services.prefs.clearUserPref("marionette.port");
+  }
+});
diff --git a/remote/marionette/test/xpcshell/test_sync.js b/remote/marionette/test/xpcshell/test_sync.js
new file mode 100644
index 0000000000..87ec44e960
--- /dev/null
+++ b/remote/marionette/test/xpcshell/test_sync.js
@@ -0,0 +1,419 @@
+/* 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 {
+  DebounceCallback,
+  IdlePromise,
+  PollPromise,
+  Sleep,
+  TimedPromise,
+  waitForMessage,
+  waitForObserverTopic,
+} = ChromeUtils.importESModule(
+  "chrome://remote/content/marionette/sync.sys.mjs"
+);
+
+/**
+ * Mimic a message manager for sending messages.
+ */
+class MessageManager {
+  constructor() {
+    this.func = null;
+    this.message = null;
+  }
+
+  addMessageListener(message, func) {
+    this.func = func;
+    this.message = message;
+  }
+
+  removeMessageListener(message) {
+    this.func = null;
+    this.message = null;
+  }
+
+  send(message, data) {
+    if (this.func) {
+      this.func({
+        data,
+        message,
+        target: this,
+      });
+    }
+  }
+}
+
+/**
+ * Mimics nsITimer, but instead of using a system clock you can
+ * preprogram it to invoke the callback after a given number of ticks.
+ */
+class MockTimer {
+  constructor(ticksBeforeFiring) {
+    this.goal = ticksBeforeFiring;
+    this.ticks = 0;
+    this.cancelled = false;
+  }
+
+  initWithCallback(cb, timeout, type) {
+    this.ticks++;
+    if (this.ticks >= this.goal) {
+      cb();
+    }
+  }
+
+  cancel() {
+    this.cancelled = true;
+  }
+}
+
+add_task(function test_executeSoon_callback() {
+  // executeSoon() is already defined for xpcshell in head.js. As such import
+  // our implementation into a custom namespace.
+  let sync = ChromeUtils.importESModule(
+    "chrome://remote/content/marionette/sync.sys.mjs"
+  );
+
+  for (let func of ["foo", null, true, [], {}]) {
+    Assert.throws(() => sync.executeSoon(func), /TypeError/);
+  }
+
+  let a;
+  sync.executeSoon(() => {
+    a = 1;
+  });
+  executeSoon(() => equal(1, a));
+});
+
+add_task(function test_PollPromise_funcTypes() {
+  for (let type of ["foo", 42, null, undefined, true, [], {}]) {
+    Assert.throws(() => new PollPromise(type), /TypeError/);
+  }
+  new PollPromise(() => {});
+  new PollPromise(function () {});
+});
+
+add_task(function test_PollPromise_timeoutTypes() {
+  for (let timeout of ["foo", true, [], {}]) {
+    Assert.throws(() => new PollPromise(() => {}, { timeout }), /TypeError/);
+  }
+  for (let timeout of [1.2, -1]) {
+    Assert.throws(() => new PollPromise(() => {}, { timeout }), /RangeError/);
+  }
+  for (let timeout of [null, undefined, 42]) {
+    new PollPromise(resolve => resolve(1), { timeout });
+  }
+});
+
+add_task(function test_PollPromise_intervalTypes() {
+  for (let interval of ["foo", null, true, [], {}]) {
+    Assert.throws(() => new PollPromise(() => {}, { interval }), /TypeError/);
+  }
+  for (let interval of [1.2, -1]) {
+    Assert.throws(() => new PollPromise(() => {}, { interval }), /RangeError/);
+  }
+  new PollPromise(() => {}, { interval: 42 });
+});
+
+add_task(async function test_PollPromise_retvalTypes() {
+  for (let typ of [true, false, "foo", 42, [], {}]) {
+    strictEqual(typ, await new PollPromise(resolve => resolve(typ)));
+  }
+});
+
+add_task(async function test_PollPromise_rethrowError() {
+  let nevals = 0;
+  let err;
+  try {
+    await PollPromise(() => {
+      ++nevals;
+      throw new Error();
+    });
+  } catch (e) {
+    err = e;
+  }
+  equal(1, nevals);
+  ok(err instanceof Error);
+});
+
+add_task(async function test_PollPromise_noTimeout() {
+  let nevals = 0;
+  await new PollPromise((resolve, reject) => {
+    ++nevals;
+    nevals < 100 ? reject() : resolve();
+  });
+  equal(100, nevals);
+});
+
+add_task(async function test_PollPromise_zeroTimeout() {
+  // run at least once when timeout is 0
+  let nevals = 0;
+  let start = new Date().getTime();
+  await new PollPromise(
+    (resolve, reject) => {
+      ++nevals;
+      reject();
+    },
+    { timeout: 0 }
+  );
+  let end = new Date().getTime();
+  equal(1, nevals);
+  less(end - start, 500);
+});
+
+add_task(async function test_PollPromise_timeoutElapse() {
+  let nevals = 0;
+  let start = new Date().getTime();
+  await new PollPromise(
+    (resolve, reject) => {
+      ++nevals;
+      reject();
+    },
+    { timeout: 100 }
+  );
+  let end = new Date().getTime();
+  lessOrEqual(nevals, 11);
+  greaterOrEqual(end - start, 100);
+});
+
+add_task(async function test_PollPromise_interval() {
+  let nevals = 0;
+  await new PollPromise(
+    (resolve, reject) => {
+      ++nevals;
+      reject();
+    },
+    { timeout: 100, interval: 100 }
+  );
+  equal(2, nevals);
+});
+
+add_task(function test_TimedPromise_funcTypes() {
+  for (let type of ["foo", 42, null, undefined, true, [], {}]) {
+    Assert.throws(() => new TimedPromise(type), /TypeError/);
+  }
+  new TimedPromise(resolve => resolve());
+  new TimedPromise(function (resolve) {
+    resolve();
+  });
+});
+
+add_task(function test_TimedPromise_timeoutTypes() {
+  for (let timeout of ["foo", null, true, [], {}]) {
+    Assert.throws(
+      () => new TimedPromise(resolve => resolve(), { timeout }),
+      /TypeError/
+    );
+  }
+  for (let timeout of [1.2, -1]) {
+    Assert.throws(
+      () => new TimedPromise(resolve => resolve(), { timeout }),
+      /RangeError/
+    );
+  }
+  new TimedPromise(resolve => resolve(), { timeout: 42 });
+});
+
+add_task(async function test_TimedPromise_errorMessage() {
+  try {
+    await new TimedPromise(resolve => {}, { timeout: 0 });
+    ok(false, "Expected Timeout error not raised");
+  } catch (e) {
+    ok(
+      e.message.includes("TimedPromise timed out after"),
+      "Expected default error message found"
+    );
+  }
+
+  try {
+    await new TimedPromise(resolve => {}, {
+      errorMessage: "Not found",
+      timeout: 0,
+    });
+    ok(false, "Expected Timeout error not raised");
+  } catch (e) {
+    ok(
+      e.message.includes("Not found after"),
+      "Expected custom error message found"
+    );
+  }
+});
+
+add_task(async function test_Sleep() {
+  await Sleep(0);
+  for (let type of ["foo", true, null, undefined]) {
+    Assert.throws(() => new Sleep(type), /TypeError/);
+  }
+  Assert.throws(() => new Sleep(1.2), /RangeError/);
+  Assert.throws(() => new Sleep(-1), /RangeError/);
+});
+
+add_task(async function test_IdlePromise() {
+  let called = false;
+  let win = {
+    requestAnimationFrame(callback) {
+      called = true;
+      callback();
+    },
+  };
+  await IdlePromise(win);
+  ok(called);
+});
+
+add_task(async function test_IdlePromiseAbortWhenWindowClosed() {
+  let win = {
+    closed: true,
+    requestAnimationFrame() {},
+  };
+  await IdlePromise(win);
+});
+
+add_task(function test_DebounceCallback_constructor() {
+  for (let cb of [42, "foo", true, null, undefined, [], {}]) {
+    Assert.throws(() => new DebounceCallback(cb), /TypeError/);
+  }
+  for (let timeout of ["foo", true, [], {}, () => {}]) {
+    Assert.throws(
+      () => new DebounceCallback(() => {}, { timeout }),
+      /TypeError/
+    );
+  }
+  for (let timeout of [-1, 2.3, NaN]) {
+    Assert.throws(
+      () => new DebounceCallback(() => {}, { timeout }),
+      /RangeError/
+    );
+  }
+});
+
+add_task(async function test_DebounceCallback_repeatedCallback() {
+  let uniqueEvent = {};
+  let ncalls = 0;
+
+  let cb = ev => {
+    ncalls++;
+    equal(ev, uniqueEvent);
+  };
+  let debouncer = new DebounceCallback(cb);
+  debouncer.timer = new MockTimer(3);
+
+  // flood the debouncer with events,
+  // we only expect the last one to fire
+  debouncer.handleEvent(uniqueEvent);
+  debouncer.handleEvent(uniqueEvent);
+  debouncer.handleEvent(uniqueEvent);
+
+  equal(ncalls, 1);
+  ok(debouncer.timer.cancelled);
+});
+
+add_task(async function test_waitForMessage_messageManagerAndMessageTypes() {
+  let messageManager = new MessageManager();
+
+  for (let manager of ["foo", 42, null, undefined, true, [], {}]) {
+    Assert.throws(() => waitForMessage(manager, "message"), /TypeError/);
+  }
+
+  for (let message of [42, null, undefined, true, [], {}]) {
+    Assert.throws(() => waitForMessage(messageManager, message), /TypeError/);
+  }
+
+  let data = { foo: "bar" };
+  let sent = waitForMessage(messageManager, "message");
+  messageManager.send("message", data);
+  equal(data, await sent);
+});
+
+add_task(async function test_waitForMessage_checkFnTypes() {
+  let messageManager = new MessageManager();
+
+  for (let checkFn of ["foo", 42, true, [], {}]) {
+    Assert.throws(
+      () => waitForMessage(messageManager, "message", { checkFn }),
+      /TypeError/
+    );
+  }
+
+  let data1 = { fo: "bar" };
+  let data2 = { foo: "bar" };
+
+  for (let checkFn of [null, undefined, msg => "foo" in msg.data]) {
+    let expected_data = checkFn == null ? data1 : data2;
+
+    messageManager = new MessageManager();
+    let sent = waitForMessage(messageManager, "message", { checkFn });
+    messageManager.send("message", data1);
+    messageManager.send("message", data2);
+    equal(expected_data, await sent);
+  }
+});
+
+add_task(async function test_waitForObserverTopic_topicTypes() {
+  for (let topic of [42, null, undefined, true, [], {}]) {
+    Assert.throws(() => waitForObserverTopic(topic), /TypeError/);
+  }
+
+  let data = { foo: "bar" };
+  let sent = waitForObserverTopic("message");
+  Services.obs.notifyObservers(this, "message", data);
+  let result = await sent;
+  equal(this, result.subject);
+  equal(data, result.data);
+});
+
+add_task(async function test_waitForObserverTopic_checkFnTypes() {
+  for (let checkFn of ["foo", 42, true, [], {}]) {
+    Assert.throws(
+      () => waitForObserverTopic("message", { checkFn }),
+      /TypeError/
+    );
+  }
+
+  let data1 = { fo: "bar" };
+  let data2 = { foo: "bar" };
+
+  for (let checkFn of [null, undefined, (subject, data) => data == data2]) {
+    let expected_data = checkFn == null ? data1 : data2;
+
+    let sent = waitForObserverTopic("message");
+    Services.obs.notifyObservers(this, "message", data1);
+    Services.obs.notifyObservers(this, "message", data2);
+    let result = await sent;
+    equal(expected_data, result.data);
+  }
+});
+
+add_task(async function test_waitForObserverTopic_timeoutTypes() {
+  for (let timeout of ["foo", true, [], {}]) {
+    Assert.throws(
+      () => waitForObserverTopic("message", { timeout }),
+      /TypeError/
+    );
+  }
+  for (let timeout of [1.2, -1]) {
+    Assert.throws(
+      () => waitForObserverTopic("message", { timeout }),
+      /RangeError/
+    );
+  }
+  for (let timeout of [null, undefined, 42]) {
+    let data = { foo: "bar" };
+    let sent = waitForObserverTopic("message", { timeout });
+    Services.obs.notifyObservers(this, "message", data);
+    let result = await sent;
+    equal(this, result.subject);
+    equal(data, result.data);
+  }
+});
+
+add_task(async function test_waitForObserverTopic_timeoutElapse() {
+  try {
+    await waitForObserverTopic("message", { timeout: 0 });
+    ok(false, "Expected Timeout error not raised");
+  } catch (e) {
+    ok(
+      e.message.includes("waitForObserverTopic timed out after"),
+      "Expected error received"
+    );
+  }
+});
diff --git a/remote/marionette/test/xpcshell/xpcshell.ini b/remote/marionette/test/xpcshell/xpcshell.ini
new file mode 100644
index 0000000000..262d57ca39
--- /dev/null
+++ b/remote/marionette/test/xpcshell/xpcshell.ini
@@ -0,0 +1,18 @@
+# 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/.
+
+[DEFAULT]
+head = head.js
+skip-if = appname == "thunderbird"
+
+[test_actors.js]
+[test_browser.js]
+[test_cookie.js]
+[test_element.js]
+[test_json.js]
+[test_message.js]
+[test_modal.js]
+[test_navigate.js]
+[test_prefs.js]
+[test_sync.js]
diff --git a/remote/marionette/transport.sys.mjs b/remote/marionette/transport.sys.mjs
new file mode 100644
index 0000000000..98f6a524f6
--- /dev/null
+++ b/remote/marionette/transport.sys.mjs
@@ -0,0 +1,529 @@
+/* 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, {
+  EventEmitter: "resource://gre/modules/EventEmitter.sys.mjs",
+
+  BulkPacket: "chrome://remote/content/marionette/packets.sys.mjs",
+  executeSoon: "chrome://remote/content/marionette/sync.sys.mjs",
+  JSONPacket: "chrome://remote/content/marionette/packets.sys.mjs",
+  Packet: "chrome://remote/content/marionette/packets.sys.mjs",
+  StreamUtils: "chrome://remote/content/marionette/stream-utils.sys.mjs",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "ScriptableInputStream", () => {
+  return Components.Constructor(
+    "@mozilla.org/scriptableinputstream;1",
+    "nsIScriptableInputStream",
+    "init"
+  );
+});
+
+const flags = { wantVerbose: false, wantLogging: false };
+
+const dumpv = flags.wantVerbose
+  ? function (msg) {
+      dump(msg + "\n");
+    }
+  : function () {};
+
+const PACKET_HEADER_MAX = 200;
+
+/**
+ * An adapter that handles data transfers between the debugger client
+ * and server. It can work with both nsIPipe and nsIServerSocket
+ * transports so long as the properly created input and output streams
+ * are specified.  (However, for intra-process connections,
+ * LocalDebuggerTransport, below, is more efficient than using an nsIPipe
+ * pair with DebuggerTransport.)
+ *
+ * @param {nsIAsyncInputStream} input
+ *     The input stream.
+ * @param {nsIAsyncOutputStream} output
+ *     The output stream.
+ *
+ * Given a DebuggerTransport instance dt:
+ * 1) Set dt.hooks to a packet handler object (described below).
+ * 2) Call dt.ready() to begin watching for input packets.
+ * 3) Call dt.send() / dt.startBulkSend() to send packets.
+ * 4) Call dt.close() to close the connection, and disengage from
+ *    the event loop.
+ *
+ * A packet handler is an object with the following methods:
+ *
+ * - onPacket(packet) - called when we have received a complete packet.
+ *   |packet| is the parsed form of the packet --- a JavaScript value, not
+ *   a JSON-syntax string.
+ *
+ * - onBulkPacket(packet) - called when we have switched to bulk packet
+ *   receiving mode. |packet| is an object containing:
+ *   actor:  Name of actor that will receive the packet
+ *   type:   Name of actor's method that should be called on receipt
+ *   length: Size of the data to be read
+ *   stream: This input stream should only be used directly if you
+ *             can ensure that you will read exactly |length| bytes and
+ *             will not close the stream when reading is complete
+ *   done:   If you use the stream directly (instead of |copyTo|
+ *             below), you must signal completion by resolving/rejecting
+ *             this deferred.  If it's rejected, the transport will
+ *             be closed.  If an Error is supplied as a rejection value,
+ *             it will be logged via |dump|.  If you do use |copyTo|,
+ *             resolving is taken care of for you when copying completes.
+ *   copyTo: A helper function for getting your data out of the
+ *             stream that meets the stream handling requirements above,
+ *             and has the following signature:
+ *
+ *             - params
+ *                {nsIAsyncOutputStream} output
+ *                  The stream to copy to.
+ *             - returns {Promise}
+ *                  The promise is resolved when copying completes or
+ *                  rejected if any (unexpected) errors occur.  This object
+ *                  also emits "progress" events for each chunk that is
+ *                  copied.  See stream-utils.js.
+ *
+ * - onClosed(reason) - called when the connection is closed. |reason|
+ *   is an optional nsresult or object, typically passed when the
+ *   transport is closed due to some error in a underlying stream.
+ *
+ * See ./packets.js and the Remote Debugging Protocol specification for
+ * more details on the format of these packets.
+ *
+ * @class
+ */
+export function DebuggerTransport(input, output) {
+  lazy.EventEmitter.decorate(this);
+
+  this._input = input;
+  this._scriptableInput = new lazy.ScriptableInputStream(input);
+  this._output = output;
+
+  // The current incoming (possibly partial) header, which will determine
+  // which type of Packet |_incoming| below will become.
+  this._incomingHeader = "";
+  // The current incoming Packet object
+  this._incoming = null;
+  // A queue of outgoing Packet objects
+  this._outgoing = [];
+
+  this.hooks = null;
+  this.active = false;
+
+  this._incomingEnabled = true;
+  this._outgoingEnabled = true;
+
+  this.close = this.close.bind(this);
+}
+
+DebuggerTransport.prototype = {
+  /**
+   * Transmit an object as a JSON packet.
+   *
+   * This method returns immediately, without waiting for the entire
+   * packet to be transmitted, registering event handlers as needed to
+   * transmit the entire packet. Packets are transmitted in the order they
+   * are passed to this method.
+   */
+  send(object) {
+    this.emit("send", object);
+
+    let packet = new lazy.JSONPacket(this);
+    packet.object = object;
+    this._outgoing.push(packet);
+    this._flushOutgoing();
+  },
+
+  /**
+   * Transmit streaming data via a bulk packet.
+   *
+   * This method initiates the bulk send process by queuing up the header
+   * data.  The caller receives eventual access to a stream for writing.
+   *
+   * N.B.: Do *not* attempt to close the stream handed to you, as it
+   * will continue to be used by this transport afterwards.  Most users
+   * should instead use the provided |copyFrom| function instead.
+   *
+   * @param {object} header
+   *     This is modeled after the format of JSON packets above, but does
+   *     not actually contain the data, but is instead just a routing
+   *     header:
+   *
+   *       - actor:  Name of actor that will receive the packet
+   *       - type:   Name of actor's method that should be called on receipt
+   *       - length: Size of the data to be sent
+   *
+   * @returns {Promise}
+   *     The promise will be resolved when you are allowed to write to
+   *     the stream with an object containing:
+   *
+   *       - stream:   This output stream should only be used directly
+   *                   if you can ensure that you will write exactly
+   *                   |length| bytes and will not close the stream when
+   *                    writing is complete.
+   *       - done:     If you use the stream directly (instead of
+   *                   |copyFrom| below), you must signal completion by
+   *                   resolving/rejecting this deferred.  If it's
+   *                   rejected, the transport will be closed.  If an
+   *                   Error is supplied as a rejection value, it will
+   *                   be logged via |dump|.  If you do use |copyFrom|,
+   *                   resolving is taken care of for you when copying
+   *                   completes.
+   *       - copyFrom: A helper function for getting your data onto the
+   *                   stream that meets the stream handling requirements
+   *                   above, and has the following signature:
+   *
+   *                   - params
+   *                     {nsIAsyncInputStream} input
+   *                       The stream to copy from.
+   *                   - returns {Promise}
+   *                       The promise is resolved when copying completes
+   *                       or rejected if any (unexpected) errors occur.
+   *                       This object also emits "progress" events for
+   *                       each chunkthat is copied.  See stream-utils.js.
+   */
+  startBulkSend(header) {
+    this.emit("startbulksend", header);
+
+    let packet = new lazy.BulkPacket(this);
+    packet.header = header;
+    this._outgoing.push(packet);
+    this._flushOutgoing();
+    return packet.streamReadyForWriting;
+  },
+
+  /**
+   * Close the transport.
+   *
+   * @param {(nsresult|object)=} reason
+   *     The status code or error message that corresponds to the reason
+   *     for closing the transport (likely because a stream closed
+   *     or failed).
+   */
+  close(reason) {
+    this.emit("close", reason);
+
+    this.active = false;
+    this._input.close();
+    this._scriptableInput.close();
+    this._output.close();
+    this._destroyIncoming();
+    this._destroyAllOutgoing();
+    if (this.hooks) {
+      this.hooks.onClosed(reason);
+      this.hooks = null;
+    }
+    if (reason) {
+      dumpv("Transport closed: " + reason);
+    } else {
+      dumpv("Transport closed.");
+    }
+  },
+
+  /**
+   * The currently outgoing packet (at the top of the queue).
+   */
+  get _currentOutgoing() {
+    return this._outgoing[0];
+  },
+
+  /**
+   * Flush data to the outgoing stream.  Waits until the output
+   * stream notifies us that it is ready to be written to (via
+   * onOutputStreamReady).
+   */
+  _flushOutgoing() {
+    if (!this._outgoingEnabled || this._outgoing.length === 0) {
+      return;
+    }
+
+    // If the top of the packet queue has nothing more to send, remove it.
+    if (this._currentOutgoing.done) {
+      this._finishCurrentOutgoing();
+    }
+
+    if (this._outgoing.length) {
+      let threadManager = Cc["@mozilla.org/thread-manager;1"].getService();
+      this._output.asyncWait(this, 0, 0, threadManager.currentThread);
+    }
+  },
+
+  /**
+   * Pause this transport's attempts to write to the output stream.
+   * This is used when we've temporarily handed off our output stream for
+   * writing bulk data.
+   */
+  pauseOutgoing() {
+    this._outgoingEnabled = false;
+  },
+
+  /**
+   * Resume this transport's attempts to write to the output stream.
+   */
+  resumeOutgoing() {
+    this._outgoingEnabled = true;
+    this._flushOutgoing();
+  },
+
+  // nsIOutputStreamCallback
+  /**
+   * This is called when the output stream is ready for more data to
+   * be written.  The current outgoing packet will attempt to write some
+   * amount of data, but may not complete.
+   */
+  onOutputStreamReady(stream) {
+    if (!this._outgoingEnabled || this._outgoing.length === 0) {
+      return;
+    }
+
+    try {
+      this._currentOutgoing.write(stream);
+    } catch (e) {
+      if (e.result != Cr.NS_BASE_STREAM_WOULD_BLOCK) {
+        this.close(e.result);
+        return;
+      }
+      throw e;
+    }
+
+    this._flushOutgoing();
+  },
+
+  /**
+   * Remove the current outgoing packet from the queue upon completion.
+   */
+  _finishCurrentOutgoing() {
+    if (this._currentOutgoing) {
+      this._currentOutgoing.destroy();
+      this._outgoing.shift();
+    }
+  },
+
+  /**
+   * Clear the entire outgoing queue.
+   */
+  _destroyAllOutgoing() {
+    for (let packet of this._outgoing) {
+      packet.destroy();
+    }
+    this._outgoing = [];
+  },
+
+  /**
+   * Initialize the input stream for reading. Once this method has been
+   * called, we watch for packets on the input stream, and pass them to
+   * the appropriate handlers via this.hooks.
+   */
+  ready() {
+    this.active = true;
+    this._waitForIncoming();
+  },
+
+  /**
+   * Asks the input stream to notify us (via onInputStreamReady) when it is
+   * ready for reading.
+   */
+  _waitForIncoming() {
+    if (this._incomingEnabled) {
+      let threadManager = Cc["@mozilla.org/thread-manager;1"].getService();
+      this._input.asyncWait(this, 0, 0, threadManager.currentThread);
+    }
+  },
+
+  /**
+   * Pause this transport's attempts to read from the input stream.
+   * This is used when we've temporarily handed off our input stream for
+   * reading bulk data.
+   */
+  pauseIncoming() {
+    this._incomingEnabled = false;
+  },
+
+  /**
+   * Resume this transport's attempts to read from the input stream.
+   */
+  resumeIncoming() {
+    this._incomingEnabled = true;
+    this._flushIncoming();
+    this._waitForIncoming();
+  },
+
+  // nsIInputStreamCallback
+  /**
+   * Called when the stream is either readable or closed.
+   */
+  onInputStreamReady(stream) {
+    try {
+      while (
+        stream.available() &&
+        this._incomingEnabled &&
+        this._processIncoming(stream, stream.available())
+      ) {
+        // Loop until there is nothing more to process
+      }
+      this._waitForIncoming();
+    } catch (e) {
+      if (e.result != Cr.NS_BASE_STREAM_WOULD_BLOCK) {
+        this.close(e.result);
+      } else {
+        throw e;
+      }
+    }
+  },
+
+  /**
+   * Process the incoming data.  Will create a new currently incoming
+   * Packet if needed.  Tells the incoming Packet to read as much data
+   * as it can, but reading may not complete.  The Packet signals that
+   * its data is ready for delivery by calling one of this transport's
+   * _on*Ready methods (see ./packets.js and the _on*Ready methods below).
+   *
+   * @returns {boolean}
+   *     Whether incoming stream processing should continue for any
+   *     remaining data.
+   */
+  _processIncoming(stream, count) {
+    dumpv("Data available: " + count);
+
+    if (!count) {
+      dumpv("Nothing to read, skipping");
+      return false;
+    }
+
+    try {
+      if (!this._incoming) {
+        dumpv("Creating a new packet from incoming");
+
+        if (!this._readHeader(stream)) {
+          // Not enough data to read packet type
+          return false;
+        }
+
+        // Attempt to create a new Packet by trying to parse each possible
+        // header pattern.
+        this._incoming = lazy.Packet.fromHeader(this._incomingHeader, this);
+        if (!this._incoming) {
+          throw new Error(
+            "No packet types for header: " + this._incomingHeader
+          );
+        }
+      }
+
+      if (!this._incoming.done) {
+        // We have an incomplete packet, keep reading it.
+        dumpv("Existing packet incomplete, keep reading");
+        this._incoming.read(stream, this._scriptableInput);
+      }
+    } catch (e) {
+      dump(`Error reading incoming packet: (${e} - ${e.stack})\n`);
+
+      // Now in an invalid state, shut down the transport.
+      this.close();
+      return false;
+    }
+
+    if (!this._incoming.done) {
+      // Still not complete, we'll wait for more data.
+      dumpv("Packet not done, wait for more");
+      return true;
+    }
+
+    // Ready for next packet
+    this._flushIncoming();
+    return true;
+  },
+
+  /**
+   * Read as far as we can into the incoming data, attempting to build
+   * up a complete packet header (which terminates with ":").  We'll only
+   * read up to PACKET_HEADER_MAX characters.
+   *
+   * @returns {boolean}
+   *     True if we now have a complete header.
+   */
+  _readHeader() {
+    let amountToRead = PACKET_HEADER_MAX - this._incomingHeader.length;
+    this._incomingHeader += lazy.StreamUtils.delimitedRead(
+      this._scriptableInput,
+      ":",
+      amountToRead
+    );
+    if (flags.wantVerbose) {
+      dumpv("Header read: " + this._incomingHeader);
+    }
+
+    if (this._incomingHeader.endsWith(":")) {
+      if (flags.wantVerbose) {
+        dumpv("Found packet header successfully: " + this._incomingHeader);
+      }
+      return true;
+    }
+
+    if (this._incomingHeader.length >= PACKET_HEADER_MAX) {
+      throw new Error("Failed to parse packet header!");
+    }
+
+    // Not enough data yet.
+    return false;
+  },
+
+  /**
+   * If the incoming packet is done, log it as needed and clear the buffer.
+   */
+  _flushIncoming() {
+    if (!this._incoming.done) {
+      return;
+    }
+    if (flags.wantLogging) {
+      dumpv("Got: " + this._incoming);
+    }
+    this._destroyIncoming();
+  },
+
+  /**
+   * Handler triggered by an incoming JSONPacket completing it's |read|
+   * method.  Delivers the packet to this.hooks.onPacket.
+   */
+  _onJSONObjectReady(object) {
+    lazy.executeSoon(() => {
+      // Ensure the transport is still alive by the time this runs.
+      if (this.active) {
+        this.emit("packet", object);
+        this.hooks.onPacket(object);
+      }
+    });
+  },
+
+  /**
+   * Handler triggered by an incoming BulkPacket entering the |read|
+   * phase for the stream portion of the packet.  Delivers info about the
+   * incoming streaming data to this.hooks.onBulkPacket.  See the main
+   * comment on the transport at the top of this file for more details.
+   */
+  _onBulkReadReady(...args) {
+    lazy.executeSoon(() => {
+      // Ensure the transport is still alive by the time this runs.
+      if (this.active) {
+        this.emit("bulkpacket", ...args);
+        this.hooks.onBulkPacket(...args);
+      }
+    });
+  },
+
+  /**
+   * Remove all handlers and references related to the current incoming
+   * packet, either because it is now complete or because the transport
+   * is closing.
+   */
+  _destroyIncoming() {
+    if (this._incoming) {
+      this._incoming.destroy();
+    }
+    this._incomingHeader = "";
+    this._incoming = null;
+  },
+};