Adding upstream version 115.7.0esr.upstream/115.7.0esr

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

59 files changed, 11236 insertions, 0 deletions

diff --git a/toolkit/actors/AboutHttpsOnlyErrorChild.sys.mjs b/toolkit/actors/AboutHttpsOnlyErrorChild.sys.mjs
new file mode 100644
index 0000000000..20313d30c0
--- /dev/null
+++ b/toolkit/actors/AboutHttpsOnlyErrorChild.sys.mjs
@@ -0,0 +1,83 @@
+/* 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 { RemotePageChild } from "resource://gre/actors/RemotePageChild.sys.mjs";
+
+const lazy = {};
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "@mozilla.org/network/serialization-helper;1",
+  "nsISerializationHelper"
+);
+
+export class AboutHttpsOnlyErrorChild extends RemotePageChild {
+  actorCreated() {
+    super.actorCreated();
+
+    // If you add a new function, remember to add it to RemotePageAccessManager.sys.mjs
+    // to allow content-privileged about:httpsonlyerror to use it.
+    const exportableFunctions = [
+      "RPMTryPingSecureWWWLink",
+      "RPMOpenSecureWWWLink",
+    ];
+    this.exportFunctions(exportableFunctions);
+  }
+
+  RPMTryPingSecureWWWLink() {
+    // try if the page can be reached with www prefix
+    // if so send message to the parent to send message to the error page to display
+    // suggestion button for www
+
+    const httpsOnlySuggestionPref = Services.prefs.getBoolPref(
+      "dom.security.https_only_mode_error_page_user_suggestions"
+    );
+
+    // only check if pref is true otherwise return
+    if (!httpsOnlySuggestionPref) {
+      return;
+    }
+
+    // get the host url without the path with www in front
+    const wwwURL = "https://www." + this.contentWindow.location.host;
+    fetch(wwwURL, {
+      credentials: "omit",
+      cache: "no-store",
+    })
+      .then(data => {
+        if (data.status === 200) {
+          this.contentWindow.dispatchEvent(
+            new this.contentWindow.CustomEvent("pingSecureWWWLinkSuccess")
+          );
+        }
+      })
+      .catch(() => {
+        dump("No secure www suggestion possible for " + wwwURL);
+      });
+  }
+
+  RPMOpenSecureWWWLink() {
+    // if user wants to visit suggested secure www page: visit page with www prefix and delete errorpage from history
+    const context = this.manager.browsingContext;
+    const docShell = context.docShell;
+    const httpChannel = docShell.failedChannel.QueryInterface(
+      Ci.nsIHttpChannel
+    );
+    const webNav = docShell.QueryInterface(Ci.nsIWebNavigation);
+    const triggeringPrincipal =
+      docShell.failedChannel.loadInfo.triggeringPrincipal;
+    const oldURI = httpChannel.URI;
+    const newWWWURI = oldURI
+      .mutate()
+      .setHost("www." + oldURI.host)
+      .finalize();
+
+    webNav.loadURI(newWWWURI, {
+      triggeringPrincipal,
+      loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_REPLACE_HISTORY,
+    });
+  }
+}
diff --git a/toolkit/actors/AboutHttpsOnlyErrorParent.sys.mjs b/toolkit/actors/AboutHttpsOnlyErrorParent.sys.mjs
new file mode 100644
index 0000000000..a7a4b23cfa
--- /dev/null
+++ b/toolkit/actors/AboutHttpsOnlyErrorParent.sys.mjs
@@ -0,0 +1,48 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const { HomePage } = ChromeUtils.import("resource:///modules/HomePage.jsm");
+import { PrivateBrowsingUtils } from "resource://gre/modules/PrivateBrowsingUtils.sys.mjs";
+
+export class AboutHttpsOnlyErrorParent extends JSWindowActorParent {
+  get browser() {
+    return this.browsingContext.top.embedderElement;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "goBack":
+        this.goBackFromErrorPage(this.browser);
+        break;
+    }
+  }
+
+  goBackFromErrorPage(aBrowser) {
+    if (!aBrowser.canGoBack) {
+      // If the unsafe page is the first or the only one in history, go to the
+      // start page.
+      aBrowser.fixupAndLoadURIString(
+        this.getDefaultHomePage(aBrowser.ownerGlobal),
+        {
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+        }
+      );
+    } else {
+      aBrowser.goBack();
+    }
+  }
+
+  getDefaultHomePage(win) {
+    if (PrivateBrowsingUtils.isWindowPrivate(win)) {
+      return win.BROWSER_NEW_TAB_URL || "about:blank";
+    }
+    let url = HomePage.getDefault();
+    // If url is a pipe-delimited set of pages, just take the first one.
+    if (url.includes("|")) {
+      url = url.split("|")[0];
+    }
+    return url;
+  }
+}
diff --git a/toolkit/actors/AudioPlaybackChild.sys.mjs b/toolkit/actors/AudioPlaybackChild.sys.mjs
new file mode 100644
index 0000000000..a392a73464
--- /dev/null
+++ b/toolkit/actors/AudioPlaybackChild.sys.mjs
@@ -0,0 +1,20 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class AudioPlaybackChild extends JSWindowActorChild {
+  observe(subject, topic, data) {
+    if (topic === "audio-playback") {
+      let name = "AudioPlayback:";
+      if (data === "activeMediaBlockStart") {
+        name += "ActiveMediaBlockStart";
+      } else if (data === "activeMediaBlockStop") {
+        name += "ActiveMediaBlockStop";
+      } else {
+        name += data === "active" ? "Start" : "Stop";
+      }
+      this.sendAsyncMessage(name);
+    }
+  }
+}
diff --git a/toolkit/actors/AudioPlaybackParent.sys.mjs b/toolkit/actors/AudioPlaybackParent.sys.mjs
new file mode 100644
index 0000000000..db682fd90b
--- /dev/null
+++ b/toolkit/actors/AudioPlaybackParent.sys.mjs
@@ -0,0 +1,42 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class AudioPlaybackParent extends JSWindowActorParent {
+  constructor() {
+    super();
+    this._hasAudioPlayback = false;
+    this._hasBlockMedia = false;
+  }
+  receiveMessage(aMessage) {
+    const browser = this.browsingContext.top.embedderElement;
+    switch (aMessage.name) {
+      case "AudioPlayback:Start":
+        this._hasAudioPlayback = true;
+        browser.audioPlaybackStarted();
+        break;
+      case "AudioPlayback:Stop":
+        this._hasAudioPlayback = false;
+        browser.audioPlaybackStopped();
+        break;
+      case "AudioPlayback:ActiveMediaBlockStart":
+        this._hasBlockMedia = true;
+        browser.activeMediaBlockStarted();
+        break;
+      case "AudioPlayback:ActiveMediaBlockStop":
+        this._hasBlockMedia = false;
+        browser.activeMediaBlockStopped();
+        break;
+    }
+  }
+  didDestroy() {
+    const browser = this.browsingContext.top.embedderElement;
+    if (browser && this._hasAudioPlayback) {
+      browser.audioPlaybackStopped();
+    }
+    if (browser && this._hasBlockMedia) {
+      browser.activeMediaBlockStopped();
+    }
+  }
+}
diff --git a/toolkit/actors/AutoCompleteChild.sys.mjs b/toolkit/actors/AutoCompleteChild.sys.mjs
new file mode 100644
index 0000000000..a405834ca9
--- /dev/null
+++ b/toolkit/actors/AutoCompleteChild.sys.mjs
@@ -0,0 +1,197 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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 no-unused-vars: ["error", {args: "none"}] */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ContentDOMReference: "resource://gre/modules/ContentDOMReference.sys.mjs",
+  LayoutUtils: "resource://gre/modules/LayoutUtils.sys.mjs",
+  LoginHelper: "resource://gre/modules/LoginHelper.sys.mjs",
+});
+
+let autoCompleteListeners = new Set();
+
+export class AutoCompleteChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    this._input = null;
+    this._popupOpen = false;
+  }
+
+  static addPopupStateListener(listener) {
+    autoCompleteListeners.add(listener);
+  }
+
+  static removePopupStateListener(listener) {
+    autoCompleteListeners.delete(listener);
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "FormAutoComplete:HandleEnter": {
+        this.selectedIndex = message.data.selectedIndex;
+
+        let controller = Cc[
+          "@mozilla.org/autocomplete/controller;1"
+        ].getService(Ci.nsIAutoCompleteController);
+        controller.handleEnter(message.data.isPopupSelection);
+        break;
+      }
+
+      case "FormAutoComplete:PopupClosed": {
+        this._popupOpen = false;
+        this.notifyListeners(message.name, message.data);
+        break;
+      }
+
+      case "FormAutoComplete:PopupOpened": {
+        this._popupOpen = true;
+        this.notifyListeners(message.name, message.data);
+        break;
+      }
+
+      case "FormAutoComplete:Focus": {
+        // XXX See bug 1582722
+        // Before bug 1573836, the messages here didn't match
+        // ("FormAutoComplete:Focus" versus "FormAutoComplete:RequestFocus")
+        // so this was never called. However this._input is actually a
+        // nsIAutoCompleteInput, which doesn't have a focus() method, so it
+        // wouldn't have worked anyway. So for now, I have just disabled this.
+        /*
+        if (this._input) {
+          this._input.focus();
+        }
+        */
+        break;
+      }
+    }
+  }
+
+  notifyListeners(messageName, data) {
+    for (let listener of autoCompleteListeners) {
+      try {
+        listener.popupStateChanged(messageName, data, this.contentWindow);
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  get input() {
+    return this._input;
+  }
+
+  set selectedIndex(index) {
+    this.sendAsyncMessage("FormAutoComplete:SetSelectedIndex", { index });
+  }
+
+  get selectedIndex() {
+    // selectedIndex getter must be synchronous because we need the
+    // correct value when the controller is in controller::HandleEnter.
+    // We can't easily just let the parent inform us the new value every
+    // time it changes because not every action that can change the
+    // selectedIndex is trivial to catch (e.g. moving the mouse over the
+    // list).
+    let selectedIndexResult = Services.cpmm.sendSyncMessage(
+      "FormAutoComplete:GetSelectedIndex",
+      {
+        browsingContext: this.browsingContext,
+      }
+    );
+
+    if (
+      selectedIndexResult.length != 1 ||
+      !Number.isInteger(selectedIndexResult[0])
+    ) {
+      throw new Error("Invalid autocomplete selectedIndex");
+    }
+    return selectedIndexResult[0];
+  }
+
+  get popupOpen() {
+    return this._popupOpen;
+  }
+
+  openAutocompletePopup(input, element) {
+    if (this._popupOpen || !input) {
+      return;
+    }
+
+    let rect = lazy.LayoutUtils.getElementBoundingScreenRect(element);
+    let window = element.ownerGlobal;
+    let dir = window.getComputedStyle(element).direction;
+    let results = this.getResultsFromController(input);
+    let formOrigin = lazy.LoginHelper.getLoginOrigin(
+      element.ownerDocument.documentURI
+    );
+    let inputElementIdentifier = lazy.ContentDOMReference.get(element);
+
+    this.sendAsyncMessage("FormAutoComplete:MaybeOpenPopup", {
+      results,
+      rect,
+      dir,
+      inputElementIdentifier,
+      formOrigin,
+    });
+
+    this._input = input;
+  }
+
+  closePopup() {
+    // We set this here instead of just waiting for the
+    // PopupClosed message to do it so that we don't end
+    // up in a state where the content thinks that a popup
+    // is open when it isn't (or soon won't be).
+    this._popupOpen = false;
+    this.sendAsyncMessage("FormAutoComplete:ClosePopup", {});
+  }
+
+  invalidate() {
+    if (this._popupOpen) {
+      let results = this.getResultsFromController(this._input);
+      this.sendAsyncMessage("FormAutoComplete:Invalidate", { results });
+    }
+  }
+
+  selectBy(reverse, page) {
+    Services.cpmm.sendSyncMessage("FormAutoComplete:SelectBy", {
+      browsingContext: this.browsingContext,
+      reverse,
+      page,
+    });
+  }
+
+  getResultsFromController(inputField) {
+    let results = [];
+
+    if (!inputField) {
+      return results;
+    }
+
+    let controller = inputField.controller;
+    if (!(controller instanceof Ci.nsIAutoCompleteController)) {
+      return results;
+    }
+
+    for (let i = 0; i < controller.matchCount; ++i) {
+      let result = {};
+      result.value = controller.getValueAt(i);
+      result.label = controller.getLabelAt(i);
+      result.comment = controller.getCommentAt(i);
+      result.style = controller.getStyleAt(i);
+      result.image = controller.getImageAt(i);
+      results.push(result);
+    }
+
+    return results;
+  }
+}
+
+AutoCompleteChild.prototype.QueryInterface = ChromeUtils.generateQI([
+  "nsIAutoCompletePopup",
+]);
diff --git a/toolkit/actors/AutoCompleteParent.sys.mjs b/toolkit/actors/AutoCompleteParent.sys.mjs
new file mode 100644
index 0000000000..82ebf22dcf
--- /dev/null
+++ b/toolkit/actors/AutoCompleteParent.sys.mjs
@@ -0,0 +1,516 @@
+/* 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 = {};
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  GeckoViewAutocomplete: "resource://gre/modules/GeckoViewAutocomplete.jsm",
+});
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "DELEGATE_AUTOCOMPLETE",
+  "toolkit.autocomplete.delegate",
+  false
+);
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+const PREF_SECURITY_DELAY = "security.notification_enable_delay";
+
+// Stores the actor that has the active popup, used by formfill
+let currentActor = null;
+
+let autoCompleteListeners = new Set();
+
+function compareContext(message) {
+  if (
+    !currentActor ||
+    (currentActor.browsingContext != message.data.browsingContext &&
+      currentActor.browsingContext.top != message.data.browsingContext)
+  ) {
+    return false;
+  }
+
+  return true;
+}
+
+// These are two synchronous messages sent by the child.
+// The browsingContext within the message data is either the one that has
+// the active autocomplete popup or the top-level of the one that has
+// the active autocomplete popup.
+Services.ppmm.addMessageListener(
+  "FormAutoComplete:GetSelectedIndex",
+  message => {
+    if (compareContext(message)) {
+      let actor = currentActor;
+      if (actor && actor.openedPopup) {
+        return actor.openedPopup.selectedIndex;
+      }
+    }
+
+    return -1;
+  }
+);
+
+Services.ppmm.addMessageListener("FormAutoComplete:SelectBy", message => {
+  if (compareContext(message)) {
+    let actor = currentActor;
+    if (actor && actor.openedPopup) {
+      actor.openedPopup.selectBy(message.data.reverse, message.data.page);
+    }
+  }
+});
+
+// AutoCompleteResultView is an abstraction around a list of results.
+// It implements enough of nsIAutoCompleteController and
+// nsIAutoCompleteInput to make the richlistbox popup work. Since only
+// one autocomplete popup should be open at a time, this is a singleton.
+var AutoCompleteResultView = {
+  // nsISupports
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIAutoCompleteController",
+    "nsIAutoCompleteInput",
+  ]),
+
+  // Private variables
+  results: [],
+
+  // The AutoCompleteParent currently showing results or null otherwise.
+  currentActor: null,
+
+  // nsIAutoCompleteController
+  get matchCount() {
+    return this.results.length;
+  },
+
+  getValueAt(index) {
+    return this.results[index].value;
+  },
+
+  getFinalCompleteValueAt(index) {
+    return this.results[index].value;
+  },
+
+  getLabelAt(index) {
+    // Backwardly-used by richlist autocomplete - see getCommentAt.
+    // The label is used for secondary information.
+    return this.results[index].comment;
+  },
+
+  getCommentAt(index) {
+    // The richlist autocomplete popup uses comment for its main
+    // display of an item, which is why we're returning the label
+    // here instead.
+    return this.results[index].label;
+  },
+
+  getStyleAt(index) {
+    return this.results[index].style;
+  },
+
+  getImageAt(index) {
+    return this.results[index].image;
+  },
+
+  handleEnter(aIsPopupSelection) {
+    if (this.currentActor) {
+      this.currentActor.handleEnter(aIsPopupSelection);
+    }
+  },
+
+  stopSearch() {},
+
+  searchString: "",
+
+  // nsIAutoCompleteInput
+  get controller() {
+    return this;
+  },
+
+  get popup() {
+    return null;
+  },
+
+  _focus() {
+    if (this.currentActor) {
+      this.currentActor.requestFocus();
+    }
+  },
+
+  // Internal JS-only API
+  clearResults() {
+    this.currentActor = null;
+    this.results = [];
+  },
+
+  setResults(actor, results) {
+    this.currentActor = actor;
+    this.results = results;
+  },
+};
+
+export class AutoCompleteParent extends JSWindowActorParent {
+  didDestroy() {
+    if (this.openedPopup) {
+      this.openedPopup.closePopup();
+    }
+  }
+
+  static getCurrentActor() {
+    return currentActor;
+  }
+
+  static addPopupStateListener(listener) {
+    autoCompleteListeners.add(listener);
+  }
+
+  static removePopupStateListener(listener) {
+    autoCompleteListeners.delete(listener);
+  }
+
+  handleEvent(evt) {
+    switch (evt.type) {
+      case "popupshowing": {
+        this.sendAsyncMessage("FormAutoComplete:PopupOpened", {});
+        break;
+      }
+
+      case "popuphidden": {
+        let selectedIndex = this.openedPopup.selectedIndex;
+        let selectedRowComment =
+          selectedIndex != -1
+            ? AutoCompleteResultView.getCommentAt(selectedIndex)
+            : "";
+        let selectedRowStyle =
+          selectedIndex != -1
+            ? AutoCompleteResultView.getStyleAt(selectedIndex)
+            : "";
+        this.sendAsyncMessage("FormAutoComplete:PopupClosed", {
+          selectedRowComment,
+          selectedRowStyle,
+        });
+        AutoCompleteResultView.clearResults();
+        // adjustHeight clears the height from the popup so that
+        // we don't have a big shrink effect if we closed with a
+        // large list, and then open on a small one.
+        this.openedPopup.adjustHeight();
+        this.openedPopup = null;
+        currentActor = null;
+        evt.target.removeEventListener("popuphidden", this);
+        evt.target.removeEventListener("popupshowing", this);
+        break;
+      }
+    }
+  }
+
+  showPopupWithResults({ rect, dir, results }) {
+    if (!results.length || this.openedPopup) {
+      // We shouldn't ever be showing an empty popup, and if we
+      // already have a popup open, the old one needs to close before
+      // we consider opening a new one.
+      return;
+    }
+
+    let browser = this.browsingContext.top.embedderElement;
+    let window = browser.ownerGlobal;
+    // Also check window top in case this is a sidebar.
+    if (
+      Services.focus.activeWindow !== window.top &&
+      Services.focus.focusedWindow.top !== window.top
+    ) {
+      // We were sent a message from a window or tab that went into the
+      // background, so we'll ignore it for now.
+      return;
+    }
+
+    // Non-empty result styles
+    let resultStyles = new Set(results.map(r => r.style).filter(r => !!r));
+    currentActor = this;
+    this.openedPopup = browser.autoCompletePopup;
+    // the layout varies according to different result type
+    this.openedPopup.setAttribute("resultstyles", [...resultStyles].join(" "));
+    this.openedPopup.hidden = false;
+    // don't allow the popup to become overly narrow
+    this.openedPopup.style.setProperty(
+      "--panel-width",
+      Math.max(100, rect.width) + "px"
+    );
+    this.openedPopup.style.direction = dir;
+
+    AutoCompleteResultView.setResults(this, results);
+    this.openedPopup.view = AutoCompleteResultView;
+    this.openedPopup.selectedIndex = -1;
+
+    // Reset fields that were set from the last time the search popup was open
+    this.openedPopup.mInput = AutoCompleteResultView;
+    // Temporarily increase the maxRows as we don't want to show
+    // the scrollbar in login or form autofill popups.
+    if (
+      resultStyles.size &&
+      (resultStyles.has("autofill-profile") || resultStyles.has("loginsFooter"))
+    ) {
+      this.openedPopup._normalMaxRows = this.openedPopup.maxRows;
+      this.openedPopup.mInput.maxRows = 10;
+    }
+    browser.constrainPopup(this.openedPopup);
+    this.openedPopup.addEventListener("popuphidden", this);
+    this.openedPopup.addEventListener("popupshowing", this);
+    this.openedPopup.openPopupAtScreenRect(
+      "after_start",
+      rect.left,
+      rect.top,
+      rect.width,
+      rect.height,
+      false,
+      false
+    );
+    this.openedPopup.invalidate();
+    this._maybeRecordTelemetryEvents(results);
+
+    // This is a temporary solution. We should replace it with
+    // proper meta information about the popup once such field
+    // becomes available.
+    let isCreditCard = results.some(result =>
+      result?.comment?.includes("cc-number")
+    );
+
+    if (isCreditCard) {
+      this.delayPopupInput();
+    }
+  }
+
+  /**
+   * @param {object[]} results - Non-empty array of autocomplete results.
+   */
+  _maybeRecordTelemetryEvents(results) {
+    let actor =
+      this.browsingContext.currentWindowGlobal.getActor("LoginManager");
+    actor.maybeRecordPasswordGenerationShownTelemetryEvent(results);
+
+    // Assume the result with the start time (loginsFooter) is last.
+    let lastResult = results[results.length - 1];
+    if (lastResult.style != "loginsFooter") {
+      return;
+    }
+
+    // The comment field of `loginsFooter` results have many additional pieces of
+    // information for telemetry purposes. After bug 1555209, this information
+    // can be passed to the parent process outside of nsIAutoCompleteResult APIs
+    // so we won't need this hack.
+    let rawExtraData = JSON.parse(lastResult.comment).telemetryEventData;
+    if (!rawExtraData.searchStartTimeMS) {
+      throw new Error("Invalid autocomplete search start time");
+    }
+
+    if (rawExtraData.stringLength > 1) {
+      // To reduce event volume, only record for lengths 0 and 1.
+      return;
+    }
+
+    let duration =
+      Services.telemetry.msSystemNow() - rawExtraData.searchStartTimeMS;
+    delete rawExtraData.searchStartTimeMS;
+
+    // Add counts by result style to rawExtraData.
+    results.reduce((accumulated, r) => {
+      // Ignore learn more as it is only added after importable logins.
+      // Do not track generic items in the telemetry.
+      if (r.style === "importableLearnMore" || r.style === "generic") {
+        return accumulated;
+      }
+
+      // Keys can be a maximum of 15 characters and values must be strings.
+      // Also treat both "loginWithOrigin" and "login" as "login" as extra_keys
+      // is limited to 10.
+      let truncatedStyle = r.style.substring(
+        0,
+        r.style === "loginWithOrigin" ? 5 : 15
+      );
+      accumulated[truncatedStyle] = (accumulated[truncatedStyle] || 0) + 1;
+      return accumulated;
+    }, rawExtraData);
+
+    // Convert extra values to strings since recordEvent requires that.
+    let extraStrings = Object.fromEntries(
+      Object.entries(rawExtraData).map(([key, val]) => {
+        let stringVal = "";
+        if (typeof val == "boolean") {
+          stringVal += val ? "1" : "0";
+        } else {
+          stringVal += val;
+        }
+        return [key, stringVal];
+      })
+    );
+
+    Services.telemetry.recordEvent(
+      "form_autocomplete",
+      "show",
+      "logins",
+      // Convert to a string
+      duration + "",
+      extraStrings
+    );
+  }
+
+  invalidate(results) {
+    if (!this.openedPopup) {
+      return;
+    }
+
+    if (!results.length) {
+      this.closePopup();
+    } else {
+      AutoCompleteResultView.setResults(this, results);
+      this.openedPopup.invalidate();
+      this._maybeRecordTelemetryEvents(results);
+    }
+  }
+
+  closePopup() {
+    if (this.openedPopup) {
+      // Note that hidePopup() closes the popup immediately,
+      // so popuphiding or popuphidden events will be fired
+      // and handled during this call.
+      this.openedPopup.hidePopup();
+    }
+  }
+
+  receiveMessage(message) {
+    let browser = this.browsingContext.top.embedderElement;
+
+    if (
+      !browser ||
+      (!lazy.DELEGATE_AUTOCOMPLETE && !browser.autoCompletePopup)
+    ) {
+      // If there is no browser or popup, just make sure that the popup has been closed.
+      if (this.openedPopup) {
+        this.openedPopup.closePopup();
+      }
+
+      // Returning false to pacify ESLint, but this return value is
+      // ignored by the messaging infrastructure.
+      return false;
+    }
+
+    switch (message.name) {
+      case "FormAutoComplete:SetSelectedIndex": {
+        let { index } = message.data;
+        if (this.openedPopup) {
+          this.openedPopup.selectedIndex = index;
+        }
+        break;
+      }
+
+      case "FormAutoComplete:MaybeOpenPopup": {
+        let { results, rect, dir, inputElementIdentifier, formOrigin } =
+          message.data;
+        if (lazy.DELEGATE_AUTOCOMPLETE) {
+          lazy.GeckoViewAutocomplete.delegateSelection({
+            browsingContext: this.browsingContext,
+            options: results,
+            inputElementIdentifier,
+            formOrigin,
+          });
+        } else {
+          this.showPopupWithResults({ results, rect, dir });
+          this.notifyListeners();
+        }
+        break;
+      }
+
+      case "FormAutoComplete:Invalidate": {
+        let { results } = message.data;
+        this.invalidate(results);
+        break;
+      }
+
+      case "FormAutoComplete:ClosePopup": {
+        if (lazy.DELEGATE_AUTOCOMPLETE) {
+          lazy.GeckoViewAutocomplete.delegateDismiss();
+          break;
+        }
+        this.closePopup();
+        break;
+      }
+    }
+    // Returning false to pacify ESLint, but this return value is
+    // ignored by the messaging infrastructure.
+    return false;
+  }
+
+  // Imposes a brief period during which the popup will not respond to
+  // a click, so as to reduce the chances of a successful clickjacking
+  // attempt
+  delayPopupInput() {
+    if (!this.openedPopup) {
+      return;
+    }
+    const popupDelay = Services.prefs.getIntPref(PREF_SECURITY_DELAY);
+
+    // Mochitests set this to 0, and many will fail on integration
+    // if we make the popup items inactive, even briefly.
+    if (!popupDelay) {
+      return;
+    }
+
+    const items = Array.from(
+      this.openedPopup.getElementsByTagName("richlistitem")
+    );
+    items.forEach(item => (item.disabled = true));
+
+    lazy.setTimeout(
+      () => items.forEach(item => (item.disabled = false)),
+      popupDelay
+    );
+  }
+
+  notifyListeners() {
+    let window = this.browsingContext.top.embedderElement.ownerGlobal;
+    for (let listener of autoCompleteListeners) {
+      try {
+        listener(window);
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  /**
+   * Despite its name, this handleEnter is only called when the user clicks on
+   * one of the items in the popup since the popup is rendered in the parent process.
+   * The real controller's handleEnter is called directly in the content process
+   * for other methods of completing a selection (e.g. using the tab or enter
+   * keys) since the field with focus is in that process.
+   * @param {boolean} aIsPopupSelection
+   */
+  handleEnter(aIsPopupSelection) {
+    if (this.openedPopup) {
+      this.sendAsyncMessage("FormAutoComplete:HandleEnter", {
+        selectedIndex: this.openedPopup.selectedIndex,
+        isPopupSelection: aIsPopupSelection,
+      });
+    }
+  }
+
+  stopSearch() {}
+
+  /**
+   * Sends a message to the browser that is requesting the input
+   * that the open popup should be focused.
+   */
+  requestFocus() {
+    // Bug 1582722 - See the response in AutoCompleteChild.jsm for why this disabled.
+    /*
+    if (this.openedPopup) {
+      this.sendAsyncMessage("FormAutoComplete:Focus");
+    }
+    */
+  }
+}
diff --git a/toolkit/actors/AutoScrollChild.sys.mjs b/toolkit/actors/AutoScrollChild.sys.mjs
new file mode 100644
index 0000000000..88fa078c28
--- /dev/null
+++ b/toolkit/actors/AutoScrollChild.sys.mjs
@@ -0,0 +1,445 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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, {
+  BrowserUtils: "resource://gre/modules/BrowserUtils.sys.mjs",
+});
+
+export class AutoScrollChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    this._scrollable = null;
+    this._scrolldir = "";
+    this._startX = null;
+    this._startY = null;
+    this._screenX = null;
+    this._screenY = null;
+    this._lastFrame = null;
+    this._autoscrollHandledByApz = false;
+    this._scrollId = null;
+
+    this.observer = new AutoScrollObserver(this);
+    this.autoscrollLoop = this.autoscrollLoop.bind(this);
+  }
+
+  isAutoscrollBlocker(event) {
+    let mmPaste = Services.prefs.getBoolPref("middlemouse.paste");
+    let mmScrollbarPosition = Services.prefs.getBoolPref(
+      "middlemouse.scrollbarPosition"
+    );
+    let node = event.originalTarget;
+    let content = node.ownerGlobal;
+
+    // If the node is in editable document or content, we don't want to start
+    // autoscroll.
+    if (mmPaste) {
+      if (node.ownerDocument?.designMode == "on") {
+        return true;
+      }
+      const element =
+        node.nodeType === content.Node.ELEMENT_NODE ? node : node.parentElement;
+      if (element.isContentEditable) {
+        return true;
+      }
+    }
+
+    // Don't start if we're on a link.
+    let [href] = lazy.BrowserUtils.hrefAndLinkNodeForClickEvent(event);
+    if (href) {
+      return true;
+    }
+
+    // Or if we're pasting into an input field of sorts.
+    let closestInput = mmPaste && node.closest("input,textarea");
+    if (
+      content.HTMLInputElement.isInstance(closestInput) ||
+      content.HTMLTextAreaElement.isInstance(closestInput)
+    ) {
+      return true;
+    }
+
+    // Or if we're on a scrollbar or XUL <tree>
+    if (
+      (mmScrollbarPosition &&
+        content.XULElement.isInstance(
+          node.closest("scrollbar,scrollcorner")
+        )) ||
+      content.XULElement.isInstance(node.closest("treechildren"))
+    ) {
+      return true;
+    }
+    return false;
+  }
+
+  isScrollableElement(aNode) {
+    let content = aNode.ownerGlobal;
+    if (content.HTMLElement.isInstance(aNode)) {
+      return !content.HTMLSelectElement.isInstance(aNode) || aNode.multiple;
+    }
+
+    return content.XULElement.isInstance(aNode);
+  }
+
+  computeWindowScrollDirection(global) {
+    if (!global.scrollbars.visible) {
+      return null;
+    }
+    if (global.scrollMaxX != global.scrollMinX) {
+      return global.scrollMaxY != global.scrollMinY ? "NSEW" : "EW";
+    }
+    if (global.scrollMaxY != global.scrollMinY) {
+      return "NS";
+    }
+    return null;
+  }
+
+  computeNodeScrollDirection(node) {
+    if (!this.isScrollableElement(node)) {
+      return null;
+    }
+
+    let global = node.ownerGlobal;
+
+    // this is a list of overflow property values that allow scrolling
+    const scrollingAllowed = ["scroll", "auto"];
+
+    let cs = global.getComputedStyle(node);
+    let overflowx = cs.getPropertyValue("overflow-x");
+    let overflowy = cs.getPropertyValue("overflow-y");
+    // we already discarded non-multiline selects so allow vertical
+    // scroll for multiline ones directly without checking for a
+    // overflow property
+    let scrollVert =
+      node.scrollTopMax &&
+      (global.HTMLSelectElement.isInstance(node) ||
+        scrollingAllowed.includes(overflowy));
+
+    // do not allow horizontal scrolling for select elements, it leads
+    // to visual artifacts and is not the expected behavior anyway
+    if (
+      !global.HTMLSelectElement.isInstance(node) &&
+      node.scrollLeftMin != node.scrollLeftMax &&
+      scrollingAllowed.includes(overflowx)
+    ) {
+      return scrollVert ? "NSEW" : "EW";
+    }
+
+    if (scrollVert) {
+      return "NS";
+    }
+
+    return null;
+  }
+
+  findNearestScrollableElement(aNode) {
+    // go upward in the DOM and find any parent element that has a overflow
+    // area and can therefore be scrolled
+    this._scrollable = null;
+    for (let node = aNode; node; node = node.flattenedTreeParentNode) {
+      // do not use overflow based autoscroll for <html> and <body>
+      // Elements or non-html/non-xul elements such as svg or Document nodes
+      // also make sure to skip select elements that are not multiline
+      let direction = this.computeNodeScrollDirection(node);
+      if (direction) {
+        this._scrolldir = direction;
+        this._scrollable = node;
+        break;
+      }
+    }
+
+    if (!this._scrollable) {
+      let direction = this.computeWindowScrollDirection(aNode.ownerGlobal);
+      if (direction) {
+        this._scrolldir = direction;
+        this._scrollable = aNode.ownerGlobal;
+      } else if (aNode.ownerGlobal.frameElement) {
+        // Note, in case of out of process iframes frameElement is null, and
+        // a caller is supposed to communicate to iframe's parent on its own to
+        // support cross process scrolling.
+        this.findNearestScrollableElement(aNode.ownerGlobal.frameElement);
+      }
+    }
+  }
+
+  async startScroll(event) {
+    this.findNearestScrollableElement(event.originalTarget);
+    if (!this._scrollable) {
+      this.sendAsyncMessage("Autoscroll:MaybeStartInParent", {
+        browsingContextId: this.browsingContext.id,
+        screenX: event.screenX,
+        screenY: event.screenY,
+      });
+      return;
+    }
+
+    let content = event.originalTarget.ownerGlobal;
+
+    // In some configurations like Print Preview, content.performance
+    // (which we use below) is null. Autoscrolling is broken in Print
+    // Preview anyways (see bug 1393494), so just don't start it at all.
+    if (!content.performance) {
+      return;
+    }
+
+    let domUtils = content.windowUtils;
+    let scrollable = this._scrollable;
+    if (scrollable instanceof Ci.nsIDOMWindow) {
+      // getViewId() needs an element to operate on.
+      scrollable = scrollable.document.documentElement;
+    }
+    this._scrollId = null;
+    try {
+      this._scrollId = domUtils.getViewId(scrollable);
+    } catch (e) {
+      // No view ID - leave this._scrollId as null. Receiving side will check.
+    }
+    let presShellId = domUtils.getPresShellId();
+    let { autoscrollEnabled, usingApz } = await this.sendQuery(
+      "Autoscroll:Start",
+      {
+        scrolldir: this._scrolldir,
+        screenXDevPx: event.screenX * content.devicePixelRatio,
+        screenYDevPx: event.screenY * content.devicePixelRatio,
+        scrollId: this._scrollId,
+        presShellId,
+        browsingContext: this.browsingContext,
+      }
+    );
+    if (!autoscrollEnabled) {
+      this._scrollable = null;
+      return;
+    }
+
+    Services.els.addSystemEventListener(this.document, "mousemove", this, true);
+    Services.els.addSystemEventListener(this.document, "mouseup", this, true);
+    this.document.addEventListener("pagehide", this, true);
+
+    this._startX = event.screenX;
+    this._startY = event.screenY;
+    this._screenX = event.screenX;
+    this._screenY = event.screenY;
+    this._scrollErrorX = 0;
+    this._scrollErrorY = 0;
+    this._autoscrollHandledByApz = usingApz;
+
+    if (!usingApz) {
+      // If the browser didn't hand the autoscroll off to APZ,
+      // scroll here in the main thread.
+      this.startMainThreadScroll();
+    } else {
+      // Even if the browser did hand the autoscroll to APZ,
+      // APZ might reject it in which case it will notify us
+      // and we need to take over.
+      Services.obs.addObserver(this.observer, "autoscroll-rejected-by-apz");
+    }
+
+    if (Cu.isInAutomation) {
+      Services.obs.notifyObservers(content, "autoscroll-start");
+    }
+  }
+
+  startMainThreadScroll() {
+    let content = this.document.defaultView;
+    this._lastFrame = content.performance.now();
+    content.requestAnimationFrame(this.autoscrollLoop);
+  }
+
+  stopScroll() {
+    if (this._scrollable) {
+      this._scrollable.mozScrollSnap();
+      this._scrollable = null;
+
+      Services.els.removeSystemEventListener(
+        this.document,
+        "mousemove",
+        this,
+        true
+      );
+      Services.els.removeSystemEventListener(
+        this.document,
+        "mouseup",
+        this,
+        true
+      );
+      this.document.removeEventListener("pagehide", this, true);
+      if (this._autoscrollHandledByApz) {
+        Services.obs.removeObserver(
+          this.observer,
+          "autoscroll-rejected-by-apz"
+        );
+      }
+    }
+  }
+
+  accelerate(curr, start) {
+    const speed = 12;
+    var val = (curr - start) / speed;
+
+    if (val > 1) {
+      return val * Math.sqrt(val) - 1;
+    }
+    if (val < -1) {
+      return val * Math.sqrt(-val) + 1;
+    }
+    return 0;
+  }
+
+  roundToZero(num) {
+    if (num > 0) {
+      return Math.floor(num);
+    }
+    return Math.ceil(num);
+  }
+
+  autoscrollLoop(timestamp) {
+    if (!this._scrollable) {
+      // Scrolling has been canceled
+      return;
+    }
+
+    // avoid long jumps when the browser hangs for more than
+    // |maxTimeDelta| ms
+    const maxTimeDelta = 100;
+    var timeDelta = Math.min(maxTimeDelta, timestamp - this._lastFrame);
+    // we used to scroll |accelerate()| pixels every 20ms (50fps)
+    var timeCompensation = timeDelta / 20;
+    this._lastFrame = timestamp;
+
+    var actualScrollX = 0;
+    var actualScrollY = 0;
+    // don't bother scrolling vertically when the scrolldir is only horizontal
+    // and the other way around
+    if (this._scrolldir != "EW") {
+      var y = this.accelerate(this._screenY, this._startY) * timeCompensation;
+      var desiredScrollY = this._scrollErrorY + y;
+      actualScrollY = this.roundToZero(desiredScrollY);
+      this._scrollErrorY = desiredScrollY - actualScrollY;
+    }
+    if (this._scrolldir != "NS") {
+      var x = this.accelerate(this._screenX, this._startX) * timeCompensation;
+      var desiredScrollX = this._scrollErrorX + x;
+      actualScrollX = this.roundToZero(desiredScrollX);
+      this._scrollErrorX = desiredScrollX - actualScrollX;
+    }
+
+    this._scrollable.scrollBy({
+      left: actualScrollX,
+      top: actualScrollY,
+      behavior: "instant",
+    });
+
+    this._scrollable.ownerGlobal.requestAnimationFrame(this.autoscrollLoop);
+  }
+
+  canStartAutoScrollWith(event) {
+    if (
+      !event.isTrusted ||
+      event.defaultPrevented ||
+      event.button !== 1 ||
+      event.clickEventPrevented()
+    ) {
+      return false;
+    }
+
+    for (const modifier of ["shift", "alt", "ctrl", "meta"]) {
+      if (
+        event[modifier + "Key"] &&
+        Services.prefs.getBoolPref(
+          `general.autoscroll.prevent_to_start.${modifier}Key`,
+          false
+        )
+      ) {
+        return false;
+      }
+    }
+    if (
+      event.getModifierState("OS") &&
+      Services.prefs.getBoolPref("general.autoscroll.prevent_to_start.osKey")
+    ) {
+      return false;
+    }
+    return true;
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "mousemove":
+        this._screenX = event.screenX;
+        this._screenY = event.screenY;
+        break;
+      case "mousedown":
+        if (
+          this.canStartAutoScrollWith(event) &&
+          !this._scrollable &&
+          !this.isAutoscrollBlocker(event)
+        ) {
+          this.startScroll(event);
+        }
+      // fallthrough
+      case "mouseup":
+        if (this._scrollable) {
+          // Middle mouse click event shouldn't be fired in web content for
+          // compatibility with Chrome.
+          event.preventClickEvent();
+        }
+        break;
+      case "pagehide":
+        if (this._scrollable) {
+          var doc = this._scrollable.ownerDocument || this._scrollable.document;
+          if (doc == event.target) {
+            this.sendAsyncMessage("Autoscroll:Cancel");
+            this.stopScroll();
+          }
+        }
+        break;
+    }
+  }
+
+  receiveMessage(msg) {
+    let data = msg.data;
+    switch (msg.name) {
+      case "Autoscroll:MaybeStart":
+        for (let child of this.browsingContext.children) {
+          if (data.browsingContextId == child.id) {
+            this.startScroll({
+              screenX: data.screenX,
+              screenY: data.screenY,
+              originalTarget: child.embedderElement,
+            });
+            break;
+          }
+        }
+        break;
+      case "Autoscroll:Stop": {
+        this.stopScroll();
+        break;
+      }
+    }
+  }
+
+  rejectedByApz(data) {
+    // The caller passes in the scroll id via 'data'.
+    if (data == this._scrollId) {
+      this._autoscrollHandledByApz = false;
+      this.startMainThreadScroll();
+      Services.obs.removeObserver(this.observer, "autoscroll-rejected-by-apz");
+    }
+  }
+}
+
+class AutoScrollObserver {
+  constructor(actor) {
+    this.actor = actor;
+  }
+
+  observe(subject, topic, data) {
+    if (topic === "autoscroll-rejected-by-apz") {
+      this.actor.rejectedByApz(data);
+    }
+  }
+}
diff --git a/toolkit/actors/AutoScrollParent.sys.mjs b/toolkit/actors/AutoScrollParent.sys.mjs
new file mode 100644
index 0000000000..1f9f780902
--- /dev/null
+++ b/toolkit/actors/AutoScrollParent.sys.mjs
@@ -0,0 +1,48 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class AutoScrollParent extends JSWindowActorParent {
+  receiveMessage(msg) {
+    let browser = this.manager.browsingContext.top.embedderElement;
+    if (!browser) {
+      return null;
+    }
+
+    // If another tab is activated, we shouldn't start autoscroll requested
+    // for the previous active window if and only if the browser is a remote
+    // browser.  This is required for web apps which don't prevent default of
+    // middle click after opening a new window.  If the active tab is our
+    // documents like about:*, we don't need this check since our documents
+    // should do it correctly.
+    const requestedInForegroundTab = browser.isRemoteBrowser
+      ? Services.focus.focusedElement == browser
+      : true;
+
+    let data = msg.data;
+    switch (msg.name) {
+      case "Autoscroll:Start":
+        // Don't start autoscroll if the tab has already been a background tab.
+        if (!requestedInForegroundTab) {
+          return Promise.resolve({ autoscrollEnabled: false, usingAPZ: false });
+        }
+        return Promise.resolve(browser.startScroll(data));
+      case "Autoscroll:MaybeStartInParent":
+        // Don't start autoscroll if the tab has already been a background tab.
+        if (!requestedInForegroundTab) {
+          return Promise.resolve({ autoscrollEnabled: false, usingAPZ: false });
+        }
+        let parent = this.browsingContext.parent;
+        if (parent) {
+          let actor = parent.currentWindowGlobal.getActor("AutoScroll");
+          actor.sendAsyncMessage("Autoscroll:MaybeStart", data);
+        }
+        break;
+      case "Autoscroll:Cancel":
+        browser.cancelScroll();
+        break;
+    }
+    return null;
+  }
+}
diff --git a/toolkit/actors/AutoplayChild.sys.mjs b/toolkit/actors/AutoplayChild.sys.mjs
new file mode 100644
index 0000000000..87fa966cb1
--- /dev/null
+++ b/toolkit/actors/AutoplayChild.sys.mjs
@@ -0,0 +1,10 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class AutoplayChild extends JSWindowActorChild {
+  handleEvent(event) {
+    this.sendAsyncMessage("GloballyAutoplayBlocked", {});
+  }
+}
diff --git a/toolkit/actors/AutoplayParent.sys.mjs b/toolkit/actors/AutoplayParent.sys.mjs
new file mode 100644
index 0000000000..3e9f807b3a
--- /dev/null
+++ b/toolkit/actors/AutoplayParent.sys.mjs
@@ -0,0 +1,17 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class AutoplayParent extends JSWindowActorParent {
+  receiveMessage(aMessage) {
+    let topBrowsingContext = this.manager.browsingContext.top;
+    let browser = topBrowsingContext.embedderElement;
+    let document = browser.ownerDocument;
+    let event = document.createEvent("CustomEvent");
+    event.initCustomEvent("GloballyAutoplayBlocked", true, false, {
+      url: this.documentURI,
+    });
+    browser.dispatchEvent(event);
+  }
+}
diff --git a/toolkit/actors/BackgroundThumbnailsChild.sys.mjs b/toolkit/actors/BackgroundThumbnailsChild.sys.mjs
new file mode 100644
index 0000000000..3af51b3b80
--- /dev/null
+++ b/toolkit/actors/BackgroundThumbnailsChild.sys.mjs
@@ -0,0 +1,102 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  PageThumbUtils: "resource://gre/modules/PageThumbUtils.sys.mjs",
+});
+
+// NOTE: Copied from nsSandboxFlags.h
+/**
+ * This flag prevents content from creating new auxiliary browsing contexts,
+ * e.g. using the target attribute, or the window.open() method.
+ */
+const SANDBOXED_AUXILIARY_NAVIGATION = 0x2;
+
+export class BackgroundThumbnailsChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "Browser:Thumbnail:ContentInfo": {
+        if (
+          message.data.isImage ||
+          this.contentWindow.ImageDocument.isInstance(this.document)
+        ) {
+          // To avoid sending additional messages between processes, we return
+          // the image data directly with the size info.
+          return lazy.PageThumbUtils.createImageThumbnailCanvas(
+            this.contentWindow,
+            this.document.location,
+            message.data.targetWidth,
+            message.data.backgroundColor
+          );
+        }
+
+        let [width, height] = lazy.PageThumbUtils.getContentSize(
+          this.contentWindow
+        );
+        return { width, height };
+      }
+
+      case "Browser:Thumbnail:LoadURL": {
+        let docShell = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+
+        // We want a low network priority for this service - lower than b/g tabs
+        // etc - so set it to the lowest priority available.
+        docShell
+          .QueryInterface(Ci.nsIDocumentLoader)
+          .loadGroup.QueryInterface(Ci.nsISupportsPriority).priority =
+          Ci.nsISupportsPriority.PRIORITY_LOWEST;
+
+        docShell.allowMedia = false;
+        docShell.allowPlugins = false;
+        docShell.allowContentRetargeting = false;
+        let defaultFlags =
+          Ci.nsIRequest.LOAD_ANONYMOUS |
+          Ci.nsIRequest.LOAD_BYPASS_CACHE |
+          Ci.nsIRequest.INHIBIT_CACHING |
+          Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_HISTORY;
+        docShell.defaultLoadFlags = defaultFlags;
+        this.browsingContext.sandboxFlags |= SANDBOXED_AUXILIARY_NAVIGATION;
+        docShell.useTrackingProtection = true;
+
+        // Get the document to force a content viewer to be created, otherwise
+        // the first load can fail.
+        if (!this.document) {
+          return false;
+        }
+
+        let loadURIOptions = {
+          // Bug 1498603 verify usages of systemPrincipal here
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+          loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_STOP_CONTENT,
+        };
+        try {
+          docShell.loadURI(
+            Services.io.newURI(message.data.url),
+            loadURIOptions
+          );
+        } catch (ex) {
+          return false;
+        }
+
+        return true;
+      }
+    }
+
+    return undefined;
+  }
+
+  handleEvent(event) {
+    if (event.type == "DOMDocElementInserted") {
+      // Arrange to prevent (most) popup dialogs for this window - popups done
+      // in the parent (eg, auth) aren't prevented, but alert() etc are.
+      // disableDialogs only works on the current inner window, so it has
+      // to be called every page load, but before scripts run.
+      this.contentWindow.windowUtils.disableDialogs();
+    }
+  }
+}
diff --git a/toolkit/actors/BrowserElementChild.sys.mjs b/toolkit/actors/BrowserElementChild.sys.mjs
new file mode 100644
index 0000000000..97e1327821
--- /dev/null
+++ b/toolkit/actors/BrowserElementChild.sys.mjs
@@ -0,0 +1,35 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class BrowserElementChild extends JSWindowActorChild {
+  handleEvent(event) {
+    if (
+      event.type == "DOMWindowClose" &&
+      !this.manager.browsingContext.parent
+    ) {
+      this.sendAsyncMessage("DOMWindowClose", {});
+    }
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "EnterModalState": {
+        this.contentWindow.windowUtils.enterModalState();
+        break;
+      }
+
+      case "LeaveModalState": {
+        if (
+          !message.data.forceLeave &&
+          !this.contentWindow.windowUtils.isInModalState()
+        ) {
+          break;
+        }
+        this.contentWindow.windowUtils.leaveModalState();
+        break;
+      }
+    }
+  }
+}
diff --git a/toolkit/actors/BrowserElementParent.sys.mjs b/toolkit/actors/BrowserElementParent.sys.mjs
new file mode 100644
index 0000000000..c3cb0991a3
--- /dev/null
+++ b/toolkit/actors/BrowserElementParent.sys.mjs
@@ -0,0 +1,36 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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/. */
+
+/**
+ * The BrowserElementParent is for performing actions on one or more subframes of
+ * a <xul:browser> from the browser element binding.
+ */
+export class BrowserElementParent extends JSWindowActorParent {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "DOMWindowClose": {
+        // This message is sent whenever window.close() is called within a window
+        // that had originally been opened via window.open. Double-check that this is
+        // coming from a top-level frame, and then dispatch the DOMWindowClose event
+        // on the browser so that the front-end code can do the right thing with the
+        // request to close.
+        if (!this.manager.browsingContext.parent) {
+          let browser = this.manager.browsingContext.embedderElement;
+          let win = browser.ownerGlobal;
+          // If this is a non-remote browser, the DOMWindowClose event will bubble
+          // up naturally, and doesn't need to be re-dispatched.
+          if (browser.isRemoteBrowser) {
+            browser.dispatchEvent(
+              new win.CustomEvent("DOMWindowClose", {
+                bubbles: true,
+              })
+            );
+          }
+        }
+        break;
+      }
+    }
+  }
+}
diff --git a/toolkit/actors/ClipboardReadPasteChild.sys.mjs b/toolkit/actors/ClipboardReadPasteChild.sys.mjs
new file mode 100644
index 0000000000..e8b113bd68
--- /dev/null
+++ b/toolkit/actors/ClipboardReadPasteChild.sys.mjs
@@ -0,0 +1,39 @@
+/* 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/. */
+
+/**
+ * Propagates "MozClipboardReadPaste" events from a content process to the
+ * chrome process.
+ * Receives messages from the chrome process.
+ */
+export class ClipboardReadPasteChild extends JSWindowActorChild {
+  constructor() {
+    super();
+  }
+
+  // EventListener interface.
+  handleEvent(aEvent) {
+    if (aEvent.type == "MozClipboardReadPaste" && aEvent.isTrusted) {
+      this.sendAsyncMessage("ClipboardReadPaste:ShowMenupopup", {});
+    }
+  }
+
+  // For JSWindowActorChild.
+  receiveMessage(value) {
+    switch (value.name) {
+      case "ClipboardReadPaste:PasteMenuItemClicked": {
+        this.contentWindow.navigator.clipboard.onUserReactedToPasteMenuPopup(
+          true
+        );
+        break;
+      }
+      case "ClipboardReadPaste:PasteMenuItemDismissed": {
+        this.contentWindow.navigator.clipboard.onUserReactedToPasteMenuPopup(
+          false
+        );
+        break;
+      }
+    }
+  }
+}
diff --git a/toolkit/actors/ClipboardReadPasteParent.sys.mjs b/toolkit/actors/ClipboardReadPasteParent.sys.mjs
new file mode 100644
index 0000000000..a0b37f1041
--- /dev/null
+++ b/toolkit/actors/ClipboardReadPasteParent.sys.mjs
@@ -0,0 +1,194 @@
+/* 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 kMenuPopupId = "clipboardReadPasteMenuPopup";
+
+// Exchanges messages with the child actor and handles events from the
+// pasteMenuHandler.
+export class ClipboardReadPasteParent extends JSWindowActorParent {
+  constructor() {
+    super();
+
+    this._menupopup = null;
+    this._menuitem = null;
+    this._delayTimer = null;
+    this._pasteMenuItemClicked = false;
+    this._lastBeepTime = 0;
+  }
+
+  didDestroy() {
+    if (this._menupopup) {
+      this._menupopup.hidePopup(true);
+    }
+  }
+
+  // EventListener interface.
+  handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case "command": {
+        this.onCommand();
+        break;
+      }
+      case "popuphiding": {
+        this.onPopupHiding();
+        break;
+      }
+      case "keydown": {
+        this.onKeyDown(aEvent);
+        break;
+      }
+    }
+  }
+
+  onCommand() {
+    this._pasteMenuItemClicked = true;
+    this.sendAsyncMessage("ClipboardReadPaste:PasteMenuItemClicked");
+  }
+
+  onPopupHiding() {
+    // Remove the listeners before potentially sending the async message
+    // below, because that might throw.
+    this._removeMenupopupEventListeners();
+    this._clearDelayTimer();
+    this._stopWatchingForSpammyActivation();
+
+    if (this._pasteMenuItemClicked) {
+      // A message was already sent. Reset the state to handle further
+      // click/dismiss events properly.
+      this._pasteMenuItemClicked = false;
+    } else {
+      this.sendAsyncMessage("ClipboardReadPaste:PasteMenuItemDismissed");
+    }
+  }
+
+  onKeyDown(aEvent) {
+    if (!this._menuitem.disabled) {
+      return;
+    }
+
+    let accesskey = this._menuitem.getAttribute("accesskey");
+    if (
+      aEvent.key == accesskey.toLowerCase() ||
+      aEvent.key == accesskey.toUpperCase()
+    ) {
+      if (Date.now() - this._lastBeepTime > 1000) {
+        Cc["@mozilla.org/sound;1"].getService(Ci.nsISound).beep();
+        this._lastBeepTime = Date.now();
+      }
+      this._refreshDelayTimer();
+    }
+  }
+
+  // For JSWindowActorParent.
+  receiveMessage(value) {
+    if (value.name == "ClipboardReadPaste:ShowMenupopup") {
+      if (!this._menupopup) {
+        this._menupopup = this._getMenupopup();
+        this._menuitem = this._menupopup.firstElementChild;
+      }
+
+      this._addMenupopupEventListeners();
+
+      const browser = this.browsingContext.top.embedderElement;
+      const window = browser.ownerGlobal;
+      const windowUtils = window.windowUtils;
+
+      let mouseXInCSSPixels = {};
+      let mouseYInCSSPixels = {};
+      windowUtils.getLastOverWindowPointerLocationInCSSPixels(
+        mouseXInCSSPixels,
+        mouseYInCSSPixels
+      );
+
+      this._menuitem.disabled = true;
+      this._startWatchingForSpammyActivation();
+      // `openPopup` is a no-op if the popup is already opened.
+      // That property is used when `navigator.clipboard.readText()` or
+      // `navigator.clipboard.read()`is called from two different frames, e.g.
+      // an iframe and the top level frame. In that scenario, the two frames
+      // correspond to different `navigator.clipboard` instances. When
+      // `readText()` or `read()` is called from both frames, an actor pair is
+      // instantiated for each of them. Both actor parents will call `openPopup`
+      // on the same `_menupopup` object. If the popup is already open,
+      // `openPopup` is a no-op. When the popup is clicked or dismissed both
+      // actor parents will receive the corresponding event.
+      this._menupopup.openPopup(
+        null,
+        "overlap" /* options */,
+        mouseXInCSSPixels.value,
+        mouseYInCSSPixels.value,
+        true /* isContextMenu */
+      );
+
+      this._refreshDelayTimer();
+    }
+  }
+
+  _addMenupopupEventListeners() {
+    this._menupopup.addEventListener("command", this);
+    this._menupopup.addEventListener("popuphiding", this);
+  }
+
+  _removeMenupopupEventListeners() {
+    this._menupopup.removeEventListener("command", this);
+    this._menupopup.removeEventListener("popuphiding", this);
+  }
+
+  _createMenupopup(aChromeDoc) {
+    let menuitem = aChromeDoc.createXULElement("menuitem");
+    menuitem.id = "clipboardReadPasteMenuItem";
+    menuitem.setAttribute("data-l10n-id", "text-action-paste");
+
+    let menupopup = aChromeDoc.createXULElement("menupopup");
+    menupopup.id = kMenuPopupId;
+    menupopup.appendChild(menuitem);
+    return menupopup;
+  }
+
+  _getMenupopup() {
+    let browser = this.browsingContext.top.embedderElement;
+    let window = browser.ownerGlobal;
+    let chromeDoc = window.document;
+
+    let menupopup = chromeDoc.getElementById(kMenuPopupId);
+    if (menupopup == null) {
+      menupopup = this._createMenupopup(chromeDoc);
+      const parent =
+        chromeDoc.querySelector("popupset") || chromeDoc.documentElement;
+      parent.appendChild(menupopup);
+    }
+
+    return menupopup;
+  }
+
+  _startWatchingForSpammyActivation() {
+    let doc = this._menuitem.ownerDocument;
+    Services.els.addSystemEventListener(doc, "keydown", this, true);
+  }
+
+  _stopWatchingForSpammyActivation() {
+    let doc = this._menuitem.ownerDocument;
+    Services.els.removeSystemEventListener(doc, "keydown", this, true);
+  }
+
+  _clearDelayTimer() {
+    if (this._delayTimer) {
+      let window = this._menuitem.ownerGlobal;
+      window.clearTimeout(this._delayTimer);
+      this._delayTimer = null;
+    }
+  }
+
+  _refreshDelayTimer() {
+    this._clearDelayTimer();
+
+    let window = this._menuitem.ownerGlobal;
+    let delay = Services.prefs.getIntPref("security.dialog_enable_delay");
+    this._delayTimer = window.setTimeout(() => {
+      this._menuitem.disabled = false;
+      this._stopWatchingForSpammyActivation();
+      this._delayTimer = null;
+    }, delay);
+  }
+}
diff --git a/toolkit/actors/ContentMetaChild.sys.mjs b/toolkit/actors/ContentMetaChild.sys.mjs
new file mode 100644
index 0000000000..929d92db47
--- /dev/null
+++ b/toolkit/actors/ContentMetaChild.sys.mjs
@@ -0,0 +1,199 @@
+/* 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/. */
+
+// Debounce time in milliseconds - this should be long enough to account for
+// sync script tags that could appear between desired meta tags
+const TIMEOUT_DELAY = 1000;
+
+const ACCEPTED_PROTOCOLS = ["http:", "https:"];
+
+// Possible description tags, listed in order from least favourable to most favourable
+const DESCRIPTION_RULES = [
+  "twitter:description",
+  "description",
+  "og:description",
+];
+
+// Possible image tags, listed in order from least favourable to most favourable
+const PREVIEW_IMAGE_RULES = [
+  "thumbnail",
+  "twitter:image",
+  "og:image",
+  "og:image:url",
+  "og:image:secure_url",
+];
+
+/*
+ * Checks if the incoming meta tag has a greater score than the current best
+ * score by checking the index of the meta tag in the list of rules provided.
+ *
+ * @param {Array} aRules
+ *          The list of rules for a given type of meta tag
+ * @param {String} aTag
+ *          The name or property of the incoming meta tag
+ * @param {String} aEntry
+ *          The current best entry for the given meta tag
+ *
+ * @returns {Boolean} true if the incoming meta tag is better than the current
+ *                    best meta tag of that same kind, false otherwise
+ */
+function shouldExtractMetadata(aRules, aTag, aEntry) {
+  return aRules.indexOf(aTag) > aEntry.currMaxScore;
+}
+
+/*
+ * Ensure that the preview image URL is safe and valid before storing
+ *
+ * @param {URL} aURL
+ *          A URL object that needs to be checked for valid principal and protocol
+ *
+ * @returns {Boolean} true if the preview URL is safe and can be stored, false otherwise
+ */
+function checkLoadURIStr(aURL) {
+  if (!ACCEPTED_PROTOCOLS.includes(aURL.protocol)) {
+    return false;
+  }
+  try {
+    let ssm = Services.scriptSecurityManager;
+    let principal = ssm.createNullPrincipal({});
+    ssm.checkLoadURIStrWithPrincipal(
+      principal,
+      aURL.href,
+      ssm.DISALLOW_INHERIT_PRINCIPAL
+    );
+  } catch (e) {
+    return false;
+  }
+  return true;
+}
+
+/*
+ * This listens to DOMMetaAdded events and collects relevant metadata about the
+ * meta tag received. Then, it sends the metadata gathered from the meta tags
+ * and the url of the page as it's payload to be inserted into moz_places.
+ */
+export class ContentMetaChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    // Store a mapping of the best description and preview
+    // image collected so far for a given URL.
+    this.metaTags = new Map();
+  }
+
+  didDestroy() {
+    for (let entry of this.metaTags.values()) {
+      entry.timeout.cancel();
+    }
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "DOMContentLoaded":
+        const metaTags = this.contentWindow.document.querySelectorAll("meta");
+        for (let metaTag of metaTags) {
+          this.onMetaTag(metaTag);
+        }
+        break;
+      case "DOMMetaAdded":
+        this.onMetaTag(event.originalTarget);
+        break;
+      default:
+    }
+  }
+
+  onMetaTag(metaTag) {
+    const window = metaTag.ownerGlobal;
+
+    // If there's no meta tag, ignore this. Also verify that the window
+    // matches just to be safe.
+    if (!metaTag || !metaTag.ownerDocument || window != this.contentWindow) {
+      return;
+    }
+
+    const url = metaTag.ownerDocument.documentURI;
+
+    let name = metaTag.name;
+    let prop = metaTag.getAttributeNS(null, "property");
+    if (!name && !prop) {
+      return;
+    }
+
+    let tag = name || prop;
+
+    const entry = this.metaTags.get(url) || {
+      description: { value: null, currMaxScore: -1 },
+      image: { value: null, currMaxScore: -1 },
+      timeout: null,
+    };
+
+    // Malformed meta tag - do not store it
+    const content = metaTag.getAttributeNS(null, "content");
+    if (!content) {
+      return;
+    }
+
+    if (shouldExtractMetadata(DESCRIPTION_RULES, tag, entry.description)) {
+      // Extract the description
+      entry.description.value = content;
+      entry.description.currMaxScore = DESCRIPTION_RULES.indexOf(tag);
+    } else if (shouldExtractMetadata(PREVIEW_IMAGE_RULES, tag, entry.image)) {
+      // Extract the preview image
+      let value;
+      try {
+        value = new URL(content, url);
+      } catch (e) {
+        return;
+      }
+      if (value && checkLoadURIStr(value)) {
+        entry.image.value = value.href;
+        entry.image.currMaxScore = PREVIEW_IMAGE_RULES.indexOf(tag);
+      }
+    } else {
+      // We don't care about other meta tags
+      return;
+    }
+
+    if (!this.metaTags.has(url)) {
+      this.metaTags.set(url, entry);
+    }
+
+    if (entry.timeout) {
+      entry.timeout.delay = TIMEOUT_DELAY;
+    } else {
+      // We want to debounce incoming meta tags until we're certain we have the
+      // best one for description and preview image, and only store that one
+      entry.timeout = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+      entry.timeout.initWithCallback(
+        () => {
+          entry.timeout = null;
+          this.metaTags.delete(url);
+          // We try to cancel the timers when we get destroyed, but if
+          // there's a race, catch it:
+          if (!this.manager || this.manager.isClosed) {
+            return;
+          }
+
+          // Save description and preview image to moz_places
+          this.sendAsyncMessage("Meta:SetPageInfo", {
+            url,
+            description: entry.description.value,
+            previewImageURL: entry.image.value,
+          });
+
+          // Telemetry for recording the size of page metadata
+          let metadataSize = entry.description.value
+            ? entry.description.value.length
+            : 0;
+          metadataSize += entry.image.value ? entry.image.value.length : 0;
+          Services.telemetry
+            .getHistogramById("PAGE_METADATA_SIZE")
+            .add(metadataSize);
+        },
+        TIMEOUT_DELAY,
+        Ci.nsITimer.TYPE_ONE_SHOT
+      );
+    }
+  }
+}
diff --git a/toolkit/actors/ContentMetaParent.sys.mjs b/toolkit/actors/ContentMetaParent.sys.mjs
new file mode 100644
index 0000000000..4cdc6c3c85
--- /dev/null
+++ b/toolkit/actors/ContentMetaParent.sys.mjs
@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class ContentMetaParent extends JSWindowActorParent {
+  receiveMessage(message) {
+    if (message.name == "Meta:SetPageInfo") {
+      let browser = this.manager.browsingContext.top.embedderElement;
+      if (browser) {
+        let event = new browser.ownerGlobal.CustomEvent("pageinfo", {
+          bubbles: true,
+          cancelable: false,
+          detail: {
+            url: message.data.url,
+            description: message.data.description,
+            previewImageURL: message.data.previewImageURL,
+          },
+        });
+        browser.dispatchEvent(event);
+      }
+    }
+  }
+}
diff --git a/toolkit/actors/ControllersChild.sys.mjs b/toolkit/actors/ControllersChild.sys.mjs
new file mode 100644
index 0000000000..d975c1f431
--- /dev/null
+++ b/toolkit/actors/ControllersChild.sys.mjs
@@ -0,0 +1,63 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class ControllersChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "ControllerCommands:Do":
+        if (this.docShell && this.docShell.isCommandEnabled(message.data)) {
+          this.docShell.doCommand(message.data);
+        }
+        break;
+
+      case "ControllerCommands:DoWithParams":
+        var data = message.data;
+        if (this.docShell && this.docShell.isCommandEnabled(data.cmd)) {
+          var params = Cu.createCommandParams();
+          let substituteXY = false;
+          let x = 0;
+          let y = 0;
+          if (
+            data.cmd == "cmd_lookUpDictionary" &&
+            "x" in data.params &&
+            "y" in data.params &&
+            data.params.x.type == "long" &&
+            data.params.y.type == "long"
+          ) {
+            substituteXY = true;
+            x = parseInt(data.params.x.value);
+            y = parseInt(data.params.y.value);
+
+            let rect =
+              this.contentWindow.windowUtils.convertFromParentProcessWidgetToLocal(
+                x,
+                y,
+                1,
+                1
+              );
+            x = Math.round(rect.x);
+            y = Math.round(rect.y);
+          }
+
+          for (var name in data.params) {
+            var value = data.params[name];
+            if (value.type == "long") {
+              if (substituteXY && name === "x") {
+                params.setLongValue(name, x);
+              } else if (substituteXY && name === "y") {
+                params.setLongValue(name, y);
+              } else {
+                params.setLongValue(name, parseInt(value.value));
+              }
+            } else {
+              throw Components.Exception("", Cr.NS_ERROR_NOT_IMPLEMENTED);
+            }
+          }
+          this.docShell.doCommandWithParams(data.cmd, params);
+        }
+        break;
+    }
+  }
+}
diff --git a/toolkit/actors/ControllersParent.sys.mjs b/toolkit/actors/ControllersParent.sys.mjs
new file mode 100644
index 0000000000..05ea166112
--- /dev/null
+++ b/toolkit/actors/ControllersParent.sys.mjs
@@ -0,0 +1,90 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class ControllersParent extends JSWindowActorParent {
+  constructor() {
+    super();
+
+    // A map of commands that have had their enabled/disabled state assigned. The
+    // value of each key will be true if enabled, and false if disabled.
+    this.supportedCommands = {};
+  }
+
+  get browser() {
+    return this.browsingContext.top.embedderElement;
+  }
+
+  // Update the set of enabled and disabled commands.
+  enableDisableCommands(aAction, aEnabledCommands, aDisabledCommands) {
+    // Clear the list first
+    this.supportedCommands = {};
+
+    for (let command of aEnabledCommands) {
+      this.supportedCommands[command] = true;
+    }
+
+    for (let command of aDisabledCommands) {
+      this.supportedCommands[command] = false;
+    }
+
+    let browser = this.browser;
+    if (browser) {
+      browser.ownerGlobal.updateCommands(aAction);
+    }
+  }
+
+  isCommandEnabled(aCommand) {
+    return this.supportedCommands[aCommand] || false;
+  }
+
+  supportsCommand(aCommand) {
+    return aCommand in this.supportedCommands;
+  }
+
+  doCommand(aCommand) {
+    this.sendAsyncMessage("ControllerCommands:Do", aCommand);
+  }
+
+  getCommandStateWithParams(aCommand, aCommandParams) {
+    throw Components.Exception("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
+  }
+
+  doCommandWithParams(aCommand, aCommandParams) {
+    let cmd = {
+      cmd: aCommand,
+      params: null,
+    };
+    if (aCommand == "cmd_lookUpDictionary") {
+      cmd.params = {
+        x: {
+          type: "long",
+          value: aCommandParams.getLongValue("x"),
+        },
+        y: {
+          type: "long",
+          value: aCommandParams.getLongValue("y"),
+        },
+      };
+    } else {
+      throw Components.Exception(
+        "Not implemented",
+        Cr.NS_ERROR_NOT_IMPLEMENTED
+      );
+    }
+    this.sendAsyncMessage("ControllerCommands:DoWithParams", cmd);
+  }
+
+  getSupportedCommands() {
+    throw Components.Exception("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
+  }
+
+  onEvent() {}
+}
+
+ControllersParent.prototype.QueryInterface = ChromeUtils.generateQI([
+  "nsIBrowserController",
+  "nsIController",
+  "nsICommandController",
+]);
diff --git a/toolkit/actors/DateTimePickerChild.sys.mjs b/toolkit/actors/DateTimePickerChild.sys.mjs
new file mode 100644
index 0000000000..9ef55af435
--- /dev/null
+++ b/toolkit/actors/DateTimePickerChild.sys.mjs
@@ -0,0 +1,204 @@
+/* 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, {
+  LayoutUtils: "resource://gre/modules/LayoutUtils.sys.mjs",
+});
+
+/**
+ * DateTimePickerChild is the communication channel between the input box
+ * (content) for date/time input types and its picker (chrome).
+ */
+export class DateTimePickerChild extends JSWindowActorChild {
+  /**
+   * On init, just listen for the event to open the picker, once the picker is
+   * opened, we'll listen for update and close events.
+   */
+  constructor() {
+    super();
+
+    this._inputElement = null;
+  }
+
+  /**
+   * Cleanup function called when picker is closed.
+   */
+  close() {
+    this.removeListeners(this._inputElement);
+    let dateTimeBoxElement = this._inputElement.dateTimeBoxElement;
+    if (!dateTimeBoxElement) {
+      this._inputElement = null;
+      return;
+    }
+
+    // dateTimeBoxElement is within UA Widget Shadow DOM.
+    // An event dispatch to it can't be accessed by document.
+    let win = this._inputElement.ownerGlobal;
+    dateTimeBoxElement.dispatchEvent(
+      new win.CustomEvent("MozSetDateTimePickerState", { detail: false })
+    );
+
+    this._inputElement = null;
+  }
+
+  /**
+   * Called after picker is opened to start listening for input box update
+   * events.
+   */
+  addListeners(aElement) {
+    aElement.ownerGlobal.addEventListener("pagehide", this);
+  }
+
+  /**
+   * Stop listeneing for events when picker is closed.
+   */
+  removeListeners(aElement) {
+    aElement.ownerGlobal.removeEventListener("pagehide", this);
+  }
+
+  /**
+   * Helper function that returns the CSS direction property of the element.
+   */
+  getComputedDirection(aElement) {
+    return aElement.ownerGlobal
+      .getComputedStyle(aElement)
+      .getPropertyValue("direction");
+  }
+
+  /**
+   * Helper function that returns the rect of the element, which is the position
+   * relative to the left/top of the content area.
+   */
+  getBoundingContentRect(aElement) {
+    return lazy.LayoutUtils.getElementBoundingScreenRect(aElement);
+  }
+
+  getTimePickerPref() {
+    return Services.prefs.getBoolPref("dom.forms.datetime.timepicker");
+  }
+
+  /**
+   * nsIMessageListener.
+   */
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "FormDateTime:PickerClosed": {
+        if (!this._inputElement) {
+          return;
+        }
+
+        this.close();
+        break;
+      }
+      case "FormDateTime:PickerValueChanged": {
+        if (!this._inputElement) {
+          return;
+        }
+
+        let dateTimeBoxElement = this._inputElement.dateTimeBoxElement;
+        if (!dateTimeBoxElement) {
+          return;
+        }
+
+        let win = this._inputElement.ownerGlobal;
+
+        // dateTimeBoxElement is within UA Widget Shadow DOM.
+        // An event dispatch to it can't be accessed by document.
+        dateTimeBoxElement.dispatchEvent(
+          new win.CustomEvent("MozPickerValueChanged", {
+            detail: Cu.cloneInto(aMessage.data, win),
+          })
+        );
+        break;
+      }
+      default:
+        break;
+    }
+  }
+
+  /**
+   * nsIDOMEventListener, for chrome events sent by the input element and other
+   * DOM events.
+   */
+  handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case "MozOpenDateTimePicker": {
+        // Time picker is disabled when preffed off
+        if (
+          !aEvent.originalTarget.ownerGlobal.HTMLInputElement.isInstance(
+            aEvent.originalTarget
+          ) ||
+          (aEvent.originalTarget.type == "time" && !this.getTimePickerPref())
+        ) {
+          return;
+        }
+
+        if (this._inputElement) {
+          // This happens when we're trying to open a picker when another picker
+          // is still open. We ignore this request to let the first picker
+          // close gracefully.
+          return;
+        }
+
+        this._inputElement = aEvent.originalTarget;
+
+        let dateTimeBoxElement = this._inputElement.dateTimeBoxElement;
+        if (!dateTimeBoxElement) {
+          throw new Error("How do we get this event without a UA Widget?");
+        }
+
+        // dateTimeBoxElement is within UA Widget Shadow DOM.
+        // An event dispatch to it can't be accessed by document, because
+        // the event is not composed.
+        let win = this._inputElement.ownerGlobal;
+        dateTimeBoxElement.dispatchEvent(
+          new win.CustomEvent("MozSetDateTimePickerState", { detail: true })
+        );
+
+        this.addListeners(this._inputElement);
+
+        let value = this._inputElement.getDateTimeInputBoxValue();
+        this.sendAsyncMessage("FormDateTime:OpenPicker", {
+          rect: this.getBoundingContentRect(this._inputElement),
+          dir: this.getComputedDirection(this._inputElement),
+          type: this._inputElement.type,
+          detail: {
+            // Pass partial value if it's available, otherwise pass input
+            // element's value.
+            value: Object.keys(value).length ? value : this._inputElement.value,
+            min: this._inputElement.getMinimum(),
+            max: this._inputElement.getMaximum(),
+            step: this._inputElement.getStep(),
+            stepBase: this._inputElement.getStepBase(),
+          },
+        });
+        break;
+      }
+      case "MozUpdateDateTimePicker": {
+        let value = this._inputElement.getDateTimeInputBoxValue();
+        value.type = this._inputElement.type;
+        this.sendAsyncMessage("FormDateTime:UpdatePicker", { value });
+        break;
+      }
+      case "MozCloseDateTimePicker": {
+        this.sendAsyncMessage("FormDateTime:ClosePicker", {});
+        this.close();
+        break;
+      }
+      case "pagehide": {
+        if (
+          this._inputElement &&
+          this._inputElement.ownerDocument == aEvent.target
+        ) {
+          this.sendAsyncMessage("FormDateTime:ClosePicker", {});
+          this.close();
+        }
+        break;
+      }
+      default:
+        break;
+    }
+  }
+}
diff --git a/toolkit/actors/DateTimePickerParent.sys.mjs b/toolkit/actors/DateTimePickerParent.sys.mjs
new file mode 100644
index 0000000000..ba78a39ffb
--- /dev/null
+++ b/toolkit/actors/DateTimePickerParent.sys.mjs
@@ -0,0 +1,157 @@
+/* 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 DEBUG = false;
+function debug(aStr) {
+  if (DEBUG) {
+    dump("-*- DateTimePickerParent: " + aStr + "\n");
+  }
+}
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  DateTimePickerPanel: "resource://gre/modules/DateTimePickerPanel.sys.mjs",
+});
+
+/*
+ * DateTimePickerParent receives message from content side (input box) and
+ * is reposible for opening, closing and updating the picker. Similarly,
+ * DateTimePickerParent listens for picker's events and notifies the content
+ * side (input box) about them.
+ */
+export class DateTimePickerParent extends JSWindowActorParent {
+  receiveMessage(aMessage) {
+    debug("receiveMessage: " + aMessage.name);
+    switch (aMessage.name) {
+      case "FormDateTime:OpenPicker": {
+        this.showPicker(aMessage.data);
+        break;
+      }
+      case "FormDateTime:ClosePicker": {
+        if (!this._picker) {
+          return;
+        }
+        this.close();
+        break;
+      }
+      case "FormDateTime:UpdatePicker": {
+        if (!this._picker) {
+          return;
+        }
+        this._picker.setPopupValue(aMessage.data);
+        break;
+      }
+      default:
+        break;
+    }
+  }
+
+  handleEvent(aEvent) {
+    debug("handleEvent: " + aEvent.type);
+    switch (aEvent.type) {
+      case "DateTimePickerValueCleared": {
+        this.sendAsyncMessage("FormDateTime:PickerValueChanged", null);
+        break;
+      }
+      case "DateTimePickerValueChanged": {
+        this.sendAsyncMessage("FormDateTime:PickerValueChanged", aEvent.detail);
+        break;
+      }
+      case "popuphidden": {
+        this.sendAsyncMessage("FormDateTime:PickerClosed", {});
+        this.close();
+        break;
+      }
+      default:
+        break;
+    }
+  }
+
+  // Get picker from browser and show it anchored to the input box.
+  showPicker(aData) {
+    let rect = aData.rect;
+    let type = aData.type;
+    let detail = aData.detail;
+
+    debug("Opening picker with details: " + JSON.stringify(detail));
+    let topBC = this.browsingContext.top;
+    let window = topBC.topChromeWindow;
+    if (Services.focus.activeWindow != window) {
+      debug("Not in the active window");
+      return;
+    }
+
+    {
+      let browser = topBC.embedderElement;
+      if (
+        browser &&
+        browser.ownerGlobal.gBrowser &&
+        browser.ownerGlobal.gBrowser.selectedBrowser != browser
+      ) {
+        debug("In background tab");
+        return;
+      }
+    }
+
+    let doc = window.document;
+    let panel = doc.getElementById("DateTimePickerPanel");
+    if (!panel) {
+      panel = doc.createXULElement("panel");
+      panel.id = "DateTimePickerPanel";
+      panel.setAttribute("type", "arrow");
+      panel.setAttribute("orient", "vertical");
+      panel.setAttribute("ignorekeys", "true");
+      panel.setAttribute("noautofocus", "true");
+      // This ensures that clicks on the anchored input box are never consumed.
+      panel.setAttribute("consumeoutsideclicks", "never");
+      panel.setAttribute("level", "parent");
+      panel.setAttribute("tabspecific", "true");
+      let container =
+        doc.getElementById("mainPopupSet") ||
+        doc.querySelector("popupset") ||
+        doc.documentElement.appendChild(doc.createXULElement("popupset"));
+      container.appendChild(panel);
+    }
+    this._oldFocus = doc.activeElement;
+    this._picker = new lazy.DateTimePickerPanel(panel);
+    this._picker.openPicker(type, rect, detail);
+    this.addPickerListeners();
+  }
+
+  // Close the picker and do some cleanup.
+  close() {
+    this._picker.closePicker();
+    // Restore focus to where it was before the picker opened.
+    this._oldFocus?.focus();
+    this._oldFocus = null;
+    this.removePickerListeners();
+    this._picker = null;
+  }
+
+  // Listen to picker's event.
+  addPickerListeners() {
+    if (!this._picker) {
+      return;
+    }
+    this._picker.element.addEventListener("popuphidden", this);
+    this._picker.element.addEventListener("DateTimePickerValueChanged", this);
+    this._picker.element.addEventListener("DateTimePickerValueCleared", this);
+  }
+
+  // Stop listening to picker's event.
+  removePickerListeners() {
+    if (!this._picker) {
+      return;
+    }
+    this._picker.element.removeEventListener("popuphidden", this);
+    this._picker.element.removeEventListener(
+      "DateTimePickerValueChanged",
+      this
+    );
+    this._picker.element.removeEventListener(
+      "DateTimePickerValueCleared",
+      this
+    );
+  }
+}
diff --git a/toolkit/actors/ExtFindChild.sys.mjs b/toolkit/actors/ExtFindChild.sys.mjs
new file mode 100644
index 0000000000..dc6d296d88
--- /dev/null
+++ b/toolkit/actors/ExtFindChild.sys.mjs
@@ -0,0 +1,31 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  FindContent: "resource://gre/modules/FindContent.sys.mjs",
+});
+
+export class ExtFindChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    if (!this._findContent) {
+      this._findContent = new lazy.FindContent(this.docShell);
+    }
+
+    switch (message.name) {
+      case "ext-Finder:CollectResults":
+        this.finderInited = true;
+        return this._findContent.findRanges(message.data);
+      case "ext-Finder:HighlightResults":
+        return this._findContent.highlightResults(message.data);
+      case "ext-Finder:ClearHighlighting":
+        this._findContent.highlighter.highlight(false);
+        break;
+    }
+
+    return null;
+  }
+}
diff --git a/toolkit/actors/FindBarChild.sys.mjs b/toolkit/actors/FindBarChild.sys.mjs
new file mode 100644
index 0000000000..a82615a253
--- /dev/null
+++ b/toolkit/actors/FindBarChild.sys.mjs
@@ -0,0 +1,157 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  BrowserUtils: "resource://gre/modules/BrowserUtils.sys.mjs",
+});
+
+export class FindBarChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    this._findKey = null;
+
+    XPCOMUtils.defineLazyProxy(
+      this,
+      "FindBarContent",
+      () => {
+        const { FindBarContent } = ChromeUtils.importESModule(
+          "resource://gre/modules/FindBarContent.sys.mjs"
+        );
+        return new FindBarContent(this);
+      },
+      { inQuickFind: false, inPassThrough: false }
+    );
+  }
+
+  receiveMessage(msg) {
+    if (msg.name == "Findbar:UpdateState") {
+      let { FindBarContent } = this;
+      FindBarContent.updateState(msg.data);
+    }
+  }
+
+  /**
+   * Check whether this key event will start the findbar in the parent,
+   * in which case we should pass any further key events to the parent to avoid
+   * them being lost.
+   * @param aEvent the key event to check.
+   */
+  eventMatchesFindShortcut(aEvent) {
+    if (!this._findKey) {
+      this._findKey = Services.cpmm.sharedData.get("Findbar:Shortcut");
+      if (!this._findKey) {
+        return false;
+      }
+    }
+    for (let k in this._findKey) {
+      if (this._findKey[k] != aEvent[k]) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  handleEvent(event) {
+    if (event.type == "keypress") {
+      this.onKeypress(event);
+    }
+  }
+
+  onKeypress(event) {
+    let { FindBarContent } = this;
+
+    if (!FindBarContent.inPassThrough && this.eventMatchesFindShortcut(event)) {
+      return FindBarContent.start(event);
+    }
+
+    // disable FAYT in about:blank to prevent FAYT opening unexpectedly.
+    let location = this.document.location.href;
+    if (location == "about:blank") {
+      return null;
+    }
+
+    if (
+      event.ctrlKey ||
+      event.altKey ||
+      event.metaKey ||
+      event.defaultPrevented ||
+      !lazy.BrowserUtils.mimeTypeIsTextBased(this.document.contentType) ||
+      !lazy.BrowserUtils.canFindInPage(location)
+    ) {
+      return null;
+    }
+
+    if (FindBarContent.inPassThrough || FindBarContent.inQuickFind) {
+      return FindBarContent.onKeypress(event);
+    }
+
+    if (event.charCode && this.shouldFastFind(event.target)) {
+      let key = String.fromCharCode(event.charCode);
+      if ((key == "/" || key == "'") && FindBarChild.manualFAYT) {
+        return FindBarContent.startQuickFind(event);
+      }
+      if (key != " " && FindBarChild.findAsYouType) {
+        return FindBarContent.startQuickFind(event, true);
+      }
+    }
+    return null;
+  }
+
+  /**
+   * Return true if we should FAYT for this node:
+   *
+   * @param elt
+   *        The element that is focused
+   */
+  shouldFastFind(elt) {
+    if (elt) {
+      let win = elt.ownerGlobal;
+      if (win.HTMLInputElement.isInstance(elt) && elt.mozIsTextField(false)) {
+        return false;
+      }
+
+      if (elt.isContentEditable || win.document.designMode == "on") {
+        return false;
+      }
+
+      if (
+        win.HTMLTextAreaElement.isInstance(elt) ||
+        win.HTMLSelectElement.isInstance(elt) ||
+        win.HTMLObjectElement.isInstance(elt) ||
+        win.HTMLEmbedElement.isInstance(elt)
+      ) {
+        return false;
+      }
+
+      if (
+        (win.HTMLIFrameElement.isInstance(elt) && elt.mozbrowser) ||
+        win.XULFrameElement.isInstance(elt)
+      ) {
+        // If we're targeting a mozbrowser iframe or an embedded XULFrameElement
+        // (e.g. about:addons extensions inline options page), do not activate
+        // fast find.
+        return false;
+      }
+    }
+
+    return true;
+  }
+}
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  FindBarChild,
+  "findAsYouType",
+  "accessibility.typeaheadfind"
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  FindBarChild,
+  "manualFAYT",
+  "accessibility.typeaheadfind.manual"
+);
diff --git a/toolkit/actors/FindBarParent.sys.mjs b/toolkit/actors/FindBarParent.sys.mjs
new file mode 100644
index 0000000000..eaf42b634e
--- /dev/null
+++ b/toolkit/actors/FindBarParent.sys.mjs
@@ -0,0 +1,47 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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/. */
+
+// Map of browser elements to findbars.
+let findbars = new WeakMap();
+
+export class FindBarParent extends JSWindowActorParent {
+  setFindbar(browser, findbar) {
+    if (findbar) {
+      findbars.set(browser, findbar);
+    } else {
+      findbars.delete(browser, findbar);
+    }
+  }
+
+  receiveMessage(message) {
+    let browser = this.manager.browsingContext.top.embedderElement;
+    if (!browser) {
+      return;
+    }
+
+    let respondToMessage = () => {
+      let findBar = findbars.get(browser);
+      if (!findBar) {
+        return;
+      }
+
+      switch (message.name) {
+        case "Findbar:Keypress":
+          findBar._onBrowserKeypress(message.data);
+          break;
+        case "Findbar:Mouseup":
+          findBar.onMouseUp();
+          break;
+      }
+    };
+
+    let findPromise = browser.ownerGlobal.gFindBarPromise;
+    if (findPromise) {
+      findPromise.then(respondToMessage);
+    } else {
+      respondToMessage();
+    }
+  }
+}
diff --git a/toolkit/actors/FinderChild.sys.mjs b/toolkit/actors/FinderChild.sys.mjs
new file mode 100644
index 0000000000..6a245cd606
--- /dev/null
+++ b/toolkit/actors/FinderChild.sys.mjs
@@ -0,0 +1,129 @@
+// -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+// vim: set ts=2 sw=2 sts=2 et tw=80: */
+// 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, {
+  Finder: "resource://gre/modules/Finder.sys.mjs",
+});
+
+export class FinderChild extends JSWindowActorChild {
+  get finder() {
+    if (!this._finder) {
+      this._finder = new lazy.Finder(this.docShell);
+    }
+    return this._finder;
+  }
+
+  receiveMessage(aMessage) {
+    let data = aMessage.data;
+
+    switch (aMessage.name) {
+      case "Finder:CaseSensitive":
+        this.finder.caseSensitive = data.caseSensitive;
+        break;
+
+      case "Finder:MatchDiacritics":
+        this.finder.matchDiacritics = data.matchDiacritics;
+        break;
+
+      case "Finder:EntireWord":
+        this.finder.entireWord = data.entireWord;
+        break;
+
+      case "Finder:SetSearchStringToSelection": {
+        return new Promise(resolve => {
+          resolve(this.finder.setSearchStringToSelection());
+        });
+      }
+
+      case "Finder:GetInitialSelection": {
+        return new Promise(resolve => {
+          resolve(this.finder.getActiveSelectionText());
+        });
+      }
+
+      case "Finder:Find":
+        return this.finder.find(data);
+
+      case "Finder:Highlight":
+        return this.finder
+          .highlight(
+            data.highlight,
+            data.searchString,
+            data.linksOnly,
+            data.useSubFrames
+          )
+          .then(result => {
+            if (result) {
+              result.browsingContextId = this.browsingContext.id;
+            }
+            return result;
+          });
+
+      case "Finder:UpdateHighlightAndMatchCount":
+        return this.finder.updateHighlightAndMatchCount(data).then(result => {
+          if (result) {
+            result.browsingContextId = this.browsingContext.id;
+          }
+          return result;
+        });
+
+      case "Finder:HighlightAllChange":
+        this.finder.onHighlightAllChange(data.highlightAll);
+        break;
+
+      case "Finder:EnableSelection":
+        this.finder.enableSelection();
+        break;
+
+      case "Finder:RemoveSelection":
+        this.finder.removeSelection(data.keepHighlight);
+        break;
+
+      case "Finder:FocusContent":
+        this.finder.focusContent();
+        break;
+
+      case "Finder:FindbarClose":
+        this.finder.onFindbarClose();
+        break;
+
+      case "Finder:FindbarOpen":
+        this.finder.onFindbarOpen();
+        break;
+
+      case "Finder:KeyPress":
+        var KeyboardEvent = this.finder._getWindow().KeyboardEvent;
+        this.finder.keyPress(new KeyboardEvent("keypress", data));
+        break;
+
+      case "Finder:MatchesCount":
+        return this.finder
+          .requestMatchesCount(
+            data.searchString,
+            data.linksOnly,
+            data.useSubFrames
+          )
+          .then(result => {
+            if (result) {
+              result.browsingContextId = this.browsingContext.id;
+            }
+            return result;
+          });
+
+      case "Finder:ModalHighlightChange":
+        this.finder.onModalHighlightChange(data.useModalHighlight);
+        break;
+
+      case "Finder:EnableMarkTesting":
+        this.finder.highlighter.enableTesting(data.enable);
+        break;
+    }
+
+    return null;
+  }
+}
diff --git a/toolkit/actors/InlineSpellCheckerChild.sys.mjs b/toolkit/actors/InlineSpellCheckerChild.sys.mjs
new file mode 100644
index 0000000000..c8262c749a
--- /dev/null
+++ b/toolkit/actors/InlineSpellCheckerChild.sys.mjs
@@ -0,0 +1,38 @@
+/* -*- mode: js; indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  InlineSpellCheckerContent:
+    "resource://gre/modules/InlineSpellCheckerContent.sys.mjs",
+});
+
+export class InlineSpellCheckerChild extends JSWindowActorChild {
+  receiveMessage(msg) {
+    switch (msg.name) {
+      case "InlineSpellChecker:selectDictionaries":
+        lazy.InlineSpellCheckerContent.selectDictionaries(msg.data.localeCodes);
+        break;
+
+      case "InlineSpellChecker:replaceMisspelling":
+        lazy.InlineSpellCheckerContent.replaceMisspelling(msg.data.suggestion);
+        break;
+
+      case "InlineSpellChecker:toggleEnabled":
+        lazy.InlineSpellCheckerContent.toggleEnabled();
+        break;
+
+      case "InlineSpellChecker:recheck":
+        lazy.InlineSpellCheckerContent.recheck();
+        break;
+
+      case "InlineSpellChecker:uninit":
+        lazy.InlineSpellCheckerContent.uninitContextMenu();
+        break;
+    }
+  }
+}
diff --git a/toolkit/actors/InlineSpellCheckerParent.sys.mjs b/toolkit/actors/InlineSpellCheckerParent.sys.mjs
new file mode 100644
index 0000000000..ff5f55724e
--- /dev/null
+++ b/toolkit/actors/InlineSpellCheckerParent.sys.mjs
@@ -0,0 +1,50 @@
+/* -*- mode: js; indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class InlineSpellCheckerParent extends JSWindowActorParent {
+  selectDictionaries({ localeCodes }) {
+    this.sendAsyncMessage("InlineSpellChecker:selectDictionaries", {
+      localeCodes,
+    });
+  }
+
+  replaceMisspelling({ suggestion }) {
+    this.sendAsyncMessage("InlineSpellChecker:replaceMisspelling", {
+      suggestion,
+    });
+  }
+
+  toggleEnabled() {
+    this.sendAsyncMessage("InlineSpellChecker:toggleEnabled", {});
+  }
+
+  recheckSpelling() {
+    this.sendAsyncMessage("InlineSpellChecker:recheck", {});
+  }
+
+  uninit() {
+    // This method gets called by InlineSpellChecker when the context menu
+    // goes away and the InlineSpellChecker instance is still alive.
+    // Stop referencing it and tidy the child end of us.
+    this.sendAsyncMessage("InlineSpellChecker:uninit", {});
+  }
+
+  _destructionObservers = new Set();
+  registerDestructionObserver(obj) {
+    this._destructionObservers.add(obj);
+  }
+
+  unregisterDestructionObserver(obj) {
+    this._destructionObservers.delete(obj);
+  }
+
+  didDestroy() {
+    for (let obs of this._destructionObservers) {
+      obs.actorDestroyed(this);
+    }
+    this._destructionObservers = null;
+  }
+}
diff --git a/toolkit/actors/KeyPressEventModelCheckerChild.sys.mjs b/toolkit/actors/KeyPressEventModelCheckerChild.sys.mjs
new file mode 100644
index 0000000000..8b4fe82f17
--- /dev/null
+++ b/toolkit/actors/KeyPressEventModelCheckerChild.sys.mjs
@@ -0,0 +1,107 @@
+/* -*- mode: js; indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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";
+
+export class KeyPressEventModelCheckerChild extends JSWindowActorChild {
+  // Currently, the event is dispatched only when the document becomes editable
+  // because of contenteditable.  If you need to add new editor which is in
+  // designMode, you need to change MaybeDispatchCheckKeyPressEventModelEvent()
+  // of Document.
+  handleEvent(aEvent) {
+    if (!AppConstants.DEBUG) {
+      // Stop propagation in opt build to save the propagation cost.
+      // However, the event is necessary for running test_bug1514940.html.
+      // Therefore, we need to keep propagating it at least on debug build.
+      aEvent.stopImmediatePropagation();
+    }
+
+    // Currently, even if we set Document.KEYPRESS_EVENT_MODEL_CONFLATED
+    // here, conflated model isn't used forcibly.  If you need it, you need
+    // to change WidgetKeyboardEvent, dom::KeyboardEvent and PresShell.
+    let model = Document.KEYPRESS_EVENT_MODEL_DEFAULT;
+    if (
+      this._isOldOfficeOnlineServer(aEvent.target) ||
+      this._isOldConfluence(aEvent.target.ownerGlobal)
+    ) {
+      model = Document.KEYPRESS_EVENT_MODEL_SPLIT;
+    }
+    aEvent.target.setKeyPressEventModel(model);
+  }
+
+  _isOldOfficeOnlineServer(aDocument) {
+    let editingElement = aDocument.getElementById(
+      "WACViewPanel_EditingElement"
+    );
+    // If it's not Office Online Server, don't include it into the telemetry
+    // because we just need to collect percentage of old version in all loaded
+    // Office Online Server instances.
+    if (!editingElement) {
+      return false;
+    }
+    let isOldVersion = !editingElement.classList.contains(
+      "WACViewPanel_DisableLegacyKeyCodeAndCharCode"
+    );
+    Services.telemetry.keyedScalarAdd(
+      "dom.event.office_online_load_count",
+      isOldVersion ? "old" : "new",
+      1
+    );
+    return isOldVersion;
+  }
+
+  _isOldConfluence(aWindow) {
+    if (!aWindow) {
+      return false;
+    }
+    // aWindow should be an editor window in <iframe>.  However, we don't know
+    // whether it can be without <iframe>.  Anyway, there should be tinyMCE
+    // object in the parent window or in the window.
+    let tinyMCEObject;
+    // First, try to retrieve tinyMCE object from parent window.
+    try {
+      tinyMCEObject = ChromeUtils.waiveXrays(aWindow.parent).tinyMCE;
+    } catch (e) {
+      // Ignore the exception for now.
+    }
+    // Next, if there is no tinyMCE object in the parent window, let's check
+    // the window.
+    if (!tinyMCEObject) {
+      try {
+        tinyMCEObject = ChromeUtils.waiveXrays(aWindow).tinyMCE;
+      } catch (e) {
+        // Fallthrough to return false below.
+      }
+      // If we couldn't find tinyMCE object, let's assume that it's not
+      // Confluence instance.
+      if (!tinyMCEObject) {
+        return false;
+      }
+    }
+    // If there is tinyMCE object, we can assume that we loaded Confluence
+    // instance.  So, let's check the version whether it allows conflated
+    // keypress event model.
+    try {
+      let { author, version } =
+        new tinyMCEObject.plugins.CursorTargetPlugin().getInfo();
+      // If it's not Confluence, don't include it into the telemetry because
+      // we just need to collect percentage of old version in all loaded
+      // Confluence instances.
+      if (author !== "Atlassian") {
+        return false;
+      }
+      let isOldVersion = version === "1.0";
+      Services.telemetry.keyedScalarAdd(
+        "dom.event.confluence_load_count",
+        isOldVersion ? "old" : "new",
+        1
+      );
+      return isOldVersion;
+    } catch (e) {
+      return false;
+    }
+  }
+}
diff --git a/toolkit/actors/NetErrorChild.sys.mjs b/toolkit/actors/NetErrorChild.sys.mjs
new file mode 100644
index 0000000000..671eb22baa
--- /dev/null
+++ b/toolkit/actors/NetErrorChild.sys.mjs
@@ -0,0 +1,244 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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",
+});
+
+import { RemotePageChild } from "resource://gre/actors/RemotePageChild.sys.mjs";
+
+export class NetErrorChild extends RemotePageChild {
+  actorCreated() {
+    super.actorCreated();
+
+    // If you add a new function, remember to add it to RemotePageAccessManager.sys.mjs
+    // to allow content-privileged about:neterror or about:certerror to use it.
+    const exportableFunctions = [
+      "RPMGetAppBuildID",
+      "RPMGetInnerMostURI",
+      "RPMAddToHistogram",
+      "RPMRecordTelemetryEvent",
+      "RPMCheckAlternateHostAvailable",
+      "RPMGetHttpResponseHeader",
+      "RPMIsTRROnlyFailure",
+      "RPMIsFirefox",
+      "RPMIsNativeFallbackFailure",
+      "RPMOpenPreferences",
+      "RPMGetTRRSkipReason",
+      "RPMGetTRRDomain",
+      "RPMIsSiteSpecificTRRError",
+      "RPMSetTRRDisabledLoadFlags",
+      "RPMGetCurrentTRRMode",
+    ];
+    this.exportFunctions(exportableFunctions);
+  }
+
+  getFailedCertChain(docShell) {
+    let securityInfo =
+      docShell.failedChannel && docShell.failedChannel.securityInfo;
+    if (!securityInfo) {
+      return [];
+    }
+    return securityInfo.failedCertChain.map(cert => cert.getBase64DERString());
+  }
+
+  handleEvent(aEvent) {
+    // Documents have a null ownerDocument.
+    let doc = aEvent.originalTarget.ownerDocument || aEvent.originalTarget;
+
+    switch (aEvent.type) {
+      case "click":
+        let elem = aEvent.originalTarget;
+        if (elem.id == "viewCertificate") {
+          // Call through the superclass to avoid the security check.
+          this.sendAsyncMessage("Browser:CertExceptionError", {
+            location: doc.location.href,
+            elementId: elem.id,
+            failedCertChain: this.getFailedCertChain(doc.defaultView.docShell),
+          });
+        }
+        break;
+    }
+  }
+
+  RPMGetInnerMostURI(uriString) {
+    let uri = Services.io.newURI(uriString);
+    if (uri instanceof Ci.nsINestedURI) {
+      uri = uri.QueryInterface(Ci.nsINestedURI).innermostURI;
+    }
+
+    return uri.spec;
+  }
+
+  RPMGetAppBuildID() {
+    return Services.appinfo.appBuildID;
+  }
+
+  RPMAddToHistogram(histID, bin) {
+    Services.telemetry.getHistogramById(histID).add(bin);
+  }
+
+  RPMRecordTelemetryEvent(category, event, object, value, extra) {
+    Services.telemetry.recordEvent(category, event, object, value, extra);
+  }
+
+  RPMCheckAlternateHostAvailable() {
+    const host = this.contentWindow.location.host.trim();
+
+    // Adapted from UrlbarUtils::looksLikeSingleWordHost
+    // https://searchfox.org/mozilla-central/rev/a26af613a476fafe6c3eba05a81bef63dff3c9f1/browser/components/urlbar/UrlbarUtils.sys.mjs#893
+    const REGEXP_SINGLE_WORD = /^[^\s@:/?#]+(:\d+)?$/;
+    if (!REGEXP_SINGLE_WORD.test(host)) {
+      return;
+    }
+
+    let info = Services.uriFixup.forceHttpFixup(
+      this.contentWindow.location.href
+    );
+
+    if (!info.fixupCreatedAlternateURI && !info.fixupChangedProtocol) {
+      return;
+    }
+
+    let { displayHost, displaySpec, pathQueryRef } = info.fixedURI;
+
+    if (pathQueryRef.endsWith("/")) {
+      pathQueryRef = pathQueryRef.slice(0, pathQueryRef.length - 1);
+    }
+
+    let weakDoc = Cu.getWeakReference(this.contentWindow.document);
+    let onLookupCompleteListener = {
+      onLookupComplete(request, record, status) {
+        let doc = weakDoc.get();
+        if (!doc || !Components.isSuccessCode(status)) {
+          return;
+        }
+
+        let link = doc.createElement("a");
+        link.href = displaySpec;
+        link.setAttribute("data-l10n-name", "website");
+
+        let span = doc.createElement("span");
+        span.appendChild(link);
+        doc.l10n.setAttributes(span, "neterror-dns-not-found-with-suggestion", {
+          hostAndPath: displayHost + pathQueryRef,
+        });
+
+        const shortDesc = doc.getElementById("errorShortDesc");
+        shortDesc.textContent += " ";
+        shortDesc.appendChild(span);
+      },
+    };
+
+    Services.uriFixup.checkHost(
+      info.fixedURI,
+      onLookupCompleteListener,
+      this.document.nodePrincipal.originAttributes
+    );
+  }
+
+  // Get the header from the http response of the failed channel. This function
+  // is used in the 'about:neterror' page.
+  RPMGetHttpResponseHeader(responseHeader) {
+    let channel = this.contentWindow.docShell.failedChannel;
+    if (!channel) {
+      return "";
+    }
+
+    let httpChannel = channel.QueryInterface(Ci.nsIHttpChannel);
+    if (!httpChannel) {
+      return "";
+    }
+
+    try {
+      return httpChannel.getResponseHeader(responseHeader);
+    } catch (e) {}
+
+    return "";
+  }
+
+  RPMIsTRROnlyFailure() {
+    // We will only show this in Firefox because the options may direct users to settings only available on Firefox Desktop
+    let channel = this.contentWindow?.docShell?.failedChannel?.QueryInterface(
+      Ci.nsIHttpChannelInternal
+    );
+    if (!channel) {
+      return false;
+    }
+    return channel.effectiveTRRMode == Ci.nsIRequest.TRR_ONLY_MODE;
+  }
+
+  RPMIsFirefox() {
+    return lazy.AppInfo.isFirefox;
+  }
+
+  _getTRRSkipReason() {
+    let channel = this.contentWindow?.docShell?.failedChannel?.QueryInterface(
+      Ci.nsIHttpChannelInternal
+    );
+    return channel?.trrSkipReason ?? Ci.nsITRRSkipReason.TRR_UNSET;
+  }
+
+  RPMIsNativeFallbackFailure() {
+    if (!this.contentWindow?.navigator.onLine) {
+      return false;
+    }
+
+    let skipReason = this._getTRRSkipReason();
+
+    if (
+      Services.dns.currentTrrMode === Ci.nsIDNSService.MODE_TRRFIRST &&
+      skipReason === Ci.nsITRRSkipReason.TRR_NOT_CONFIRMED
+    ) {
+      return true;
+    }
+
+    const warningReasons = new Set([
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_GOOGLE_SAFESEARCH,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_YOUTUBE_SAFESEARCH,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_ZSCALER_CANARY,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_CANARY,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_MODIFIED_ROOTS,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_PARENTAL_CONTROLS,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_THIRD_PARTY_ROOTS,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_ENTERPRISE_POLICY,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_VPN,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_PROXY,
+      Ci.nsITRRSkipReason.TRR_HEURISTIC_TRIPPED_NRPT,
+    ]);
+
+    return (
+      Services.dns.currentTrrMode === Ci.nsIDNSService.MODE_NATIVEONLY &&
+      warningReasons.has(skipReason)
+    );
+  }
+
+  RPMGetTRRSkipReason() {
+    let skipReason = this._getTRRSkipReason();
+    return Services.dns.getTRRSkipReasonName(skipReason);
+  }
+
+  RPMGetTRRDomain() {
+    return Services.dns.trrDomain;
+  }
+
+  RPMIsSiteSpecificTRRError() {
+    let skipReason = this._getTRRSkipReason();
+    switch (skipReason) {
+      case Ci.nsITRRSkipReason.TRR_NXDOMAIN:
+      case Ci.nsITRRSkipReason.TRR_RCODE_FAIL:
+      case Ci.nsITRRSkipReason.TRR_NO_ANSWERS:
+        return true;
+    }
+    return false;
+  }
+
+  RPMSetTRRDisabledLoadFlags() {
+    this.contentWindow.docShell.browsingContext.defaultLoadFlags |=
+      Ci.nsIRequest.LOAD_TRR_DISABLED_MODE;
+  }
+}
diff --git a/toolkit/actors/NetErrorParent.sys.mjs b/toolkit/actors/NetErrorParent.sys.mjs
new file mode 100644
index 0000000000..7d1f0f3f0f
--- /dev/null
+++ b/toolkit/actors/NetErrorParent.sys.mjs
@@ -0,0 +1,353 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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 { PrivateBrowsingUtils } from "resource://gre/modules/PrivateBrowsingUtils.sys.mjs";
+import { TelemetryController } from "resource://gre/modules/TelemetryController.sys.mjs";
+
+const PREF_SSL_IMPACT_ROOTS = [
+  "security.tls.version.",
+  "security.ssl3.",
+  "security.tls13.",
+];
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  BrowserUtils: "resource://gre/modules/BrowserUtils.sys.mjs",
+});
+
+ChromeUtils.defineModuleGetter(
+  lazy,
+  "HomePage",
+  "resource:///modules/HomePage.jsm"
+);
+
+class CaptivePortalObserver {
+  constructor(actor) {
+    this.actor = actor;
+    Services.obs.addObserver(this, "captive-portal-login-abort");
+    Services.obs.addObserver(this, "captive-portal-login-success");
+  }
+
+  stop() {
+    Services.obs.removeObserver(this, "captive-portal-login-abort");
+    Services.obs.removeObserver(this, "captive-portal-login-success");
+  }
+
+  observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+      case "captive-portal-login-abort":
+      case "captive-portal-login-success":
+        // Send a message to the content when a captive portal is freed
+        // so that error pages can refresh themselves.
+        this.actor.sendAsyncMessage("AboutNetErrorCaptivePortalFreed");
+        break;
+    }
+  }
+}
+
+export class NetErrorParent extends JSWindowActorParent {
+  constructor() {
+    super();
+    this.captivePortalObserver = new CaptivePortalObserver(this);
+  }
+
+  didDestroy() {
+    if (this.captivePortalObserver) {
+      this.captivePortalObserver.stop();
+    }
+  }
+
+  get browser() {
+    return this.browsingContext.top.embedderElement;
+  }
+
+  hasChangedCertPrefs() {
+    let prefSSLImpact = PREF_SSL_IMPACT_ROOTS.reduce((prefs, root) => {
+      return prefs.concat(Services.prefs.getChildList(root));
+    }, []);
+    for (let prefName of prefSSLImpact) {
+      if (Services.prefs.prefHasUserValue(prefName)) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  async ReportBlockingError(bcID, scheme, host, port, path, xfoAndCspInfo) {
+    // For reporting X-Frame-Options error and CSP: frame-ancestors errors, We
+    // are collecting 4 pieces of information.
+    // 1. The X-Frame-Options in the response header.
+    // 2. The CSP: frame-ancestors in the response header.
+    // 3. The URI of the frame who triggers this error.
+    // 4. The top-level URI which loads the frame.
+    //
+    // We will exclude the query strings from the reporting URIs.
+    //
+    // More details about the data we send can be found in
+    // https://firefox-source-docs.mozilla.org/toolkit/components/telemetry/telemetry/data/xfocsp-error-report-ping.html
+    //
+
+    let topBC = BrowsingContext.get(bcID).top;
+    let topURI = topBC.currentWindowGlobal.documentURI;
+
+    // Get the URLs without query strings.
+    let frame_uri = `${scheme}://${host}${port == -1 ? "" : ":" + port}${path}`;
+    let top_uri = `${topURI.scheme}://${topURI.hostPort}${topURI.filePath}`;
+
+    TelemetryController.submitExternalPing(
+      "xfocsp-error-report",
+      {
+        ...xfoAndCspInfo,
+        frame_hostname: host,
+        top_hostname: topURI.host,
+        frame_uri,
+        top_uri,
+      },
+      { addClientId: false, addEnvironment: false }
+    );
+  }
+
+  /**
+   * Return the default start page for the cases when the user's own homepage is
+   * infected, so we can get them somewhere safe.
+   */
+  getDefaultHomePage(win) {
+    let url;
+    if (
+      !PrivateBrowsingUtils.isWindowPrivate(win) &&
+      AppConstants.MOZ_BUILD_APP == "browser"
+    ) {
+      url = lazy.HomePage.getDefault();
+    }
+    url ||= win.BROWSER_NEW_TAB_URL || "about:blank";
+
+    // If url is a pipe-delimited set of pages, just take the first one.
+    if (url.includes("|")) {
+      url = url.split("|")[0];
+    }
+    return url;
+  }
+
+  /**
+   * Re-direct the browser to the previous page or a known-safe page if no
+   * previous page is found in history.  This function is used when the user
+   * browses to a secure page with certificate issues and is presented with
+   * about:certerror.  The "Go Back" button should take the user to the previous
+   * or a default start page so that even when their own homepage is on a server
+   * that has certificate errors, we can get them somewhere safe.
+   */
+  goBackFromErrorPage(browser) {
+    if (!browser.canGoBack) {
+      // If the unsafe page is the first or the only one in history, go to the
+      // start page.
+      browser.fixupAndLoadURIString(
+        this.getDefaultHomePage(browser.ownerGlobal),
+        {
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+        }
+      );
+    } else {
+      browser.goBack();
+    }
+  }
+
+  /**
+   * This function does a canary request to a reliable, maintained endpoint, in
+   * order to help network code detect a system-wide man-in-the-middle.
+   */
+  primeMitm(browser) {
+    // If we already have a mitm canary issuer stored, then don't bother with the
+    // extra request. This will be cleared on every update ping.
+    if (Services.prefs.getStringPref("security.pki.mitm_canary_issuer", null)) {
+      return;
+    }
+
+    let url = Services.prefs.getStringPref(
+      "security.certerrors.mitm.priming.endpoint"
+    );
+    let request = new XMLHttpRequest({ mozAnon: true });
+    request.open("HEAD", url);
+    request.channel.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
+    request.channel.loadFlags |= Ci.nsIRequest.INHIBIT_CACHING;
+
+    request.addEventListener("error", event => {
+      // Make sure the user is still on the cert error page.
+      if (!browser.documentURI.spec.startsWith("about:certerror")) {
+        return;
+      }
+
+      let secInfo = request.channel.securityInfo;
+      if (secInfo.errorCodeString != "SEC_ERROR_UNKNOWN_ISSUER") {
+        return;
+      }
+
+      // When we get to this point there's already something deeply wrong, it's very likely
+      // that there is indeed a system-wide MitM.
+      if (secInfo.serverCert && secInfo.serverCert.issuerName) {
+        // Grab the issuer of the certificate used in the exchange and store it so that our
+        // network-level MitM detection code has a comparison baseline.
+        Services.prefs.setStringPref(
+          "security.pki.mitm_canary_issuer",
+          secInfo.serverCert.issuerName
+        );
+
+        // MitM issues are sometimes caused by software not registering their root certs in the
+        // Firefox root store. We might opt for using third party roots from the system root store.
+        if (
+          Services.prefs.getBoolPref(
+            "security.certerrors.mitm.auto_enable_enterprise_roots"
+          )
+        ) {
+          if (
+            !Services.prefs.getBoolPref("security.enterprise_roots.enabled")
+          ) {
+            // Loading enterprise roots happens on a background thread, so wait for import to finish.
+            lazy.BrowserUtils.promiseObserved(
+              "psm:enterprise-certs-imported"
+            ).then(() => {
+              if (browser.documentURI.spec.startsWith("about:certerror")) {
+                browser.reload();
+              }
+            });
+
+            Services.prefs.setBoolPref(
+              "security.enterprise_roots.enabled",
+              true
+            );
+            // Record that this pref was automatically set.
+            Services.prefs.setBoolPref(
+              "security.enterprise_roots.auto-enabled",
+              true
+            );
+          }
+        } else {
+          // Need to reload the page to make sure network code picks up the canary issuer pref.
+          browser.reload();
+        }
+      }
+    });
+
+    request.send(null);
+  }
+
+  displayOfflineSupportPage(supportPageSlug) {
+    const AVAILABLE_PAGES = ["connection-not-secure", "time-errors"];
+    if (!AVAILABLE_PAGES.includes(supportPageSlug)) {
+      console.log(
+        `[Not supported] Offline support is not yet available for ${supportPageSlug} errors.`
+      );
+      return;
+    }
+
+    let offlinePagePath = `chrome://global/content/neterror/supportpages/${supportPageSlug}.html`;
+    let triggeringPrincipal =
+      Services.scriptSecurityManager.getSystemPrincipal();
+    this.browser.loadURI(Services.io.newURI(offlinePagePath), {
+      triggeringPrincipal,
+    });
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "Browser:EnableOnlineMode":
+        // Reset network state and refresh the page.
+        Services.io.offline = false;
+        this.browser.reload();
+        break;
+      case "Browser:OpenCaptivePortalPage":
+        this.browser.ownerGlobal.CaptivePortalWatcher.ensureCaptivePortalTab();
+        break;
+      case "Browser:PrimeMitm":
+        this.primeMitm(this.browser);
+        break;
+      case "Browser:ResetEnterpriseRootsPref":
+        Services.prefs.clearUserPref("security.enterprise_roots.enabled");
+        Services.prefs.clearUserPref("security.enterprise_roots.auto-enabled");
+        break;
+      case "Browser:ResetSSLPreferences":
+        let prefSSLImpact = PREF_SSL_IMPACT_ROOTS.reduce((prefs, root) => {
+          return prefs.concat(Services.prefs.getChildList(root));
+        }, []);
+        for (let prefName of prefSSLImpact) {
+          Services.prefs.clearUserPref(prefName);
+        }
+        this.browser.reload();
+        break;
+      case "Browser:SSLErrorGoBack":
+        this.goBackFromErrorPage(this.browser);
+        break;
+      case "Browser:SSLErrorReportTelemetry":
+        let reportStatus = message.data.reportStatus;
+        Services.telemetry
+          .getHistogramById("TLS_ERROR_REPORT_UI")
+          .add(reportStatus);
+        break;
+      case "GetChangedCertPrefs":
+        let hasChangedCertPrefs = this.hasChangedCertPrefs();
+        this.sendAsyncMessage("HasChangedCertPrefs", {
+          hasChangedCertPrefs,
+        });
+        break;
+      case "ReportBlockingError":
+        this.ReportBlockingError(
+          this.browsingContext.id,
+          message.data.scheme,
+          message.data.host,
+          message.data.port,
+          message.data.path,
+          message.data.xfoAndCspInfo
+        );
+        break;
+      case "DisplayOfflineSupportPage":
+        this.displayOfflineSupportPage(message.data.supportPageSlug);
+        break;
+      case "Browser:CertExceptionError":
+        switch (message.data.elementId) {
+          case "viewCertificate": {
+            let certs = message.data.failedCertChain.map(certBase64 =>
+              encodeURIComponent(certBase64)
+            );
+            let certsStringURL = certs.map(elem => `cert=${elem}`);
+            certsStringURL = certsStringURL.join("&");
+            let url = `about:certificate?${certsStringURL}`;
+
+            let window = this.browser.ownerGlobal;
+            if (AppConstants.MOZ_BUILD_APP === "browser") {
+              window.switchToTabHavingURI(url, true, {});
+            } else {
+              window.open(url, "_blank");
+            }
+            break;
+          }
+        }
+        break;
+      case "Browser:AddTRRExcludedDomain":
+        let domain = message.data.hostname;
+        let excludedDomains = Services.prefs.getStringPref(
+          "network.trr.excluded-domains"
+        );
+        excludedDomains += `, ${domain}`;
+        Services.prefs.setStringPref(
+          "network.trr.excluded-domains",
+          excludedDomains
+        );
+        break;
+      case "OpenTRRPreferences":
+        let browser = this.browsingContext.top.embedderElement;
+        if (!browser) {
+          break;
+        }
+
+        let win = browser.ownerGlobal;
+        win.openPreferences("privacy-doh");
+        break;
+    }
+  }
+}
diff --git a/toolkit/actors/PictureInPictureChild.sys.mjs b/toolkit/actors/PictureInPictureChild.sys.mjs
new file mode 100644
index 0000000000..ffaabbd5da
--- /dev/null
+++ b/toolkit/actors/PictureInPictureChild.sys.mjs
@@ -0,0 +1,3152 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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, {
+  ContentDOMReference: "resource://gre/modules/ContentDOMReference.sys.mjs",
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+  KEYBOARD_CONTROLS: "resource://gre/modules/PictureInPictureControls.sys.mjs",
+  NimbusFeatures: "resource://nimbus/ExperimentAPI.sys.mjs",
+  Rect: "resource://gre/modules/Geometry.sys.mjs",
+  TOGGLE_POLICIES: "resource://gre/modules/PictureInPictureControls.sys.mjs",
+  TOGGLE_POLICY_STRINGS:
+    "resource://gre/modules/PictureInPictureControls.sys.mjs",
+});
+
+import { WebVTT } from "resource://gre/modules/vtt.sys.mjs";
+import { setTimeout, clearTimeout } from "resource://gre/modules/Timer.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "DISPLAY_TEXT_TRACKS_PREF",
+  "media.videocontrols.picture-in-picture.display-text-tracks.enabled",
+  false
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "IMPROVED_CONTROLS_ENABLED_PREF",
+  "media.videocontrols.picture-in-picture.improved-video-controls.enabled",
+  false
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "MIN_VIDEO_LENGTH",
+  "media.videocontrols.picture-in-picture.video-toggle.min-video-secs",
+  45
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "PIP_TOGGLE_ALWAYS_SHOW",
+  "media.videocontrols.picture-in-picture.video-toggle.always-show",
+  false
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "PIP_URLBAR_BUTTON",
+  "media.videocontrols.picture-in-picture.urlbar-button.enabled",
+  false
+);
+
+const PIP_ENABLED_PREF = "media.videocontrols.picture-in-picture.enabled";
+const TOGGLE_ENABLED_PREF =
+  "media.videocontrols.picture-in-picture.video-toggle.enabled";
+const TOGGLE_FIRST_SEEN_PREF =
+  "media.videocontrols.picture-in-picture.video-toggle.first-seen-secs";
+const TOGGLE_FIRST_TIME_DURATION_DAYS = 28;
+const TOGGLE_HAS_USED_PREF =
+  "media.videocontrols.picture-in-picture.video-toggle.has-used";
+const TOGGLE_TESTING_PREF =
+  "media.videocontrols.picture-in-picture.video-toggle.testing";
+const TOGGLE_VISIBILITY_THRESHOLD_PREF =
+  "media.videocontrols.picture-in-picture.video-toggle.visibility-threshold";
+const TEXT_TRACK_FONT_SIZE =
+  "media.videocontrols.picture-in-picture.display-text-tracks.size";
+
+const MOUSEMOVE_PROCESSING_DELAY_MS = 50;
+const TOGGLE_HIDING_TIMEOUT_MS = 3000;
+// If you change this, also change VideoControlsWidget.SEEK_TIME_SECS:
+const SEEK_TIME_SECS = 5;
+const EMPTIED_TIMEOUT_MS = 1000;
+
+// The ToggleChild does not want to capture events from the PiP
+// windows themselves. This set contains all currently open PiP
+// players' content windows
+var gPlayerContents = new WeakSet();
+
+// To make it easier to write tests, we have a process-global
+// WeakSet of all <video> elements that are being tracked for
+// mouseover
+var gWeakIntersectingVideosForTesting = new WeakSet();
+
+// Overrides are expected to stay constant for the lifetime of a
+// content process, so we set this as a lazy process global.
+// See PictureInPictureToggleChild.getSiteOverrides for a
+// sense of what the return types are.
+XPCOMUtils.defineLazyGetter(lazy, "gSiteOverrides", () => {
+  return PictureInPictureToggleChild.getSiteOverrides();
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "logConsole", () => {
+  return console.createInstance({
+    prefix: "PictureInPictureChild",
+    maxLogLevel: Services.prefs.getBoolPref(
+      "media.videocontrols.picture-in-picture.log",
+      false
+    )
+      ? "Debug"
+      : "Error",
+  });
+});
+
+/**
+ * Creates and returns an instance of the PictureInPictureChildVideoWrapper class responsible
+ * for applying site-specific wrapper methods around the original video.
+ *
+ * The Picture-In-Picture add-on can use this to provide site-specific wrappers for
+ * sites that require special massaging to control.
+ * @param {Object} pipChild reference to PictureInPictureChild class calling this function
+ * @param {Element} originatingVideo
+ *   The <video> element to wrap.
+ * @returns {PictureInPictureChildVideoWrapper} instance of PictureInPictureChildVideoWrapper
+ */
+function applyWrapper(pipChild, originatingVideo) {
+  let originatingDoc = originatingVideo.ownerDocument;
+  let originatingDocumentURI = originatingDoc.documentURI;
+
+  let overrides = lazy.gSiteOverrides.find(([matcher]) => {
+    return matcher.matches(originatingDocumentURI);
+  });
+
+  // gSiteOverrides is a list of tuples where the first element is the MatchPattern
+  // for a supported site and the second is the actual overrides object for it.
+  let wrapperPath = overrides ? overrides[1].videoWrapperScriptPath : null;
+  return new PictureInPictureChildVideoWrapper(
+    wrapperPath,
+    originatingVideo,
+    pipChild
+  );
+}
+
+export class PictureInPictureLauncherChild extends JSWindowActorChild {
+  handleEvent(event) {
+    switch (event.type) {
+      case "MozTogglePictureInPicture": {
+        if (event.isTrusted) {
+          this.togglePictureInPicture({
+            video: event.target,
+            reason: event.detail?.reason,
+          });
+        }
+        break;
+      }
+    }
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "PictureInPicture:KeyToggle": {
+        this.keyToggle();
+        break;
+      }
+    }
+  }
+
+  /**
+   * Tells the parent to open a Picture-in-Picture window hosting
+   * a clone of the passed video. If we know about a pre-existing
+   * Picture-in-Picture window existing, this tells the parent to
+   * close it before opening the new one.
+   *
+   * @param {Object} pipObject An object containing the video and reason
+   * for toggling the PiP video
+   *
+   * @return {Promise}
+   * @resolves {undefined} Once the new Picture-in-Picture window
+   * has been requested.
+   */
+  async togglePictureInPicture(pipObject) {
+    let { video, reason } = pipObject;
+    if (video.isCloningElementVisually) {
+      // The only way we could have entered here for the same video is if
+      // we are toggling via the context menu or via the urlbar button,
+      // since we hide the inline Picture-in-Picture toggle when a video
+      // is being displayed in Picture-in-Picture. Turn off PiP in this case
+      const stopPipEvent = new this.contentWindow.CustomEvent(
+        "MozStopPictureInPicture",
+        {
+          bubbles: true,
+          detail: { reason },
+        }
+      );
+      video.dispatchEvent(stopPipEvent);
+      return;
+    }
+
+    if (!PictureInPictureChild.videoWrapper) {
+      PictureInPictureChild.videoWrapper = applyWrapper(
+        PictureInPictureChild,
+        video
+      );
+    }
+
+    let timestamp = undefined;
+    let scrubberPosition = undefined;
+
+    if (lazy.IMPROVED_CONTROLS_ENABLED_PREF) {
+      timestamp = PictureInPictureChild.videoWrapper.formatTimestamp(
+        PictureInPictureChild.videoWrapper.getCurrentTime(video),
+        PictureInPictureChild.videoWrapper.getDuration(video)
+      );
+
+      // Scrubber is hidden if undefined, so only set it to something else
+      // if the timestamp is not undefined.
+      scrubberPosition =
+        timestamp === undefined
+          ? undefined
+          : PictureInPictureChild.videoWrapper.getCurrentTime(video) /
+            PictureInPictureChild.videoWrapper.getDuration(video);
+    }
+
+    // All other requests to toggle PiP should open a new PiP
+    // window
+    const videoRef = lazy.ContentDOMReference.get(video);
+    this.sendAsyncMessage("PictureInPicture:Request", {
+      isMuted: PictureInPictureChild.videoIsMuted(video),
+      playing: PictureInPictureChild.videoIsPlaying(video),
+      videoHeight: video.videoHeight,
+      videoWidth: video.videoWidth,
+      videoRef,
+      ccEnabled: lazy.DISPLAY_TEXT_TRACKS_PREF,
+      webVTTSubtitles: !!video.textTracks?.length,
+      scrubberPosition,
+      timestamp,
+    });
+
+    let args = {
+      firstTimeToggle: (!Services.prefs.getBoolPref(
+        TOGGLE_HAS_USED_PREF
+      )).toString(),
+    };
+
+    Services.telemetry.recordEvent(
+      "pictureinpicture",
+      "opened_method",
+      reason,
+      null,
+      args
+    );
+  }
+
+  /**
+   * The keyboard was used to attempt to open Picture-in-Picture. If a video is focused,
+   * select that video. Otherwise find the first playing video, or if none, the largest
+   * dimension video. We suspect this heuristic will handle most cases, though we
+   * might refine this later on. Note that we assume that this method will only be
+   * called for the focused document.
+   */
+  keyToggle() {
+    let doc = this.document;
+    if (doc) {
+      let video = doc.activeElement;
+      if (!HTMLVideoElement.isInstance(video)) {
+        let listOfVideos = [...doc.querySelectorAll("video")].filter(
+          video => !isNaN(video.duration)
+        );
+        // Get the first non-paused video, otherwise the longest video. This
+        // fallback is designed to skip over "preview"-style videos on sidebars.
+        video =
+          listOfVideos.filter(v => !v.paused)[0] ||
+          listOfVideos.sort((a, b) => b.duration - a.duration)[0];
+      }
+      if (video) {
+        this.togglePictureInPicture({ video, reason: "shortcut" });
+      }
+    }
+  }
+}
+
+/**
+ * The PictureInPictureToggleChild is responsible for displaying the overlaid
+ * Picture-in-Picture toggle over top of <video> elements that the mouse is
+ * hovering.
+ */
+export class PictureInPictureToggleChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    // We need to maintain some state about various things related to the
+    // Picture-in-Picture toggles - however, for now, the same
+    // PictureInPictureToggleChild might be re-used for different documents.
+    // We keep the state stashed inside of this WeakMap, keyed on the document
+    // itself.
+    this.weakDocStates = new WeakMap();
+    this.toggleEnabled =
+      Services.prefs.getBoolPref(TOGGLE_ENABLED_PREF) &&
+      Services.prefs.getBoolPref(PIP_ENABLED_PREF);
+    this.toggleTesting = Services.prefs.getBoolPref(TOGGLE_TESTING_PREF, false);
+
+    // Bug 1570744 - JSWindowActorChild's cannot be used as nsIObserver's
+    // directly, so we create a new function here instead to act as our
+    // nsIObserver, which forwards the notification to the observe method.
+    this.observerFunction = (subject, topic, data) => {
+      this.observe(subject, topic, data);
+    };
+    Services.prefs.addObserver(TOGGLE_ENABLED_PREF, this.observerFunction);
+    Services.prefs.addObserver(PIP_ENABLED_PREF, this.observerFunction);
+    Services.prefs.addObserver(TOGGLE_FIRST_SEEN_PREF, this.observerFunction);
+    Services.cpmm.sharedData.addEventListener("change", this);
+
+    this.eligiblePipVideos = new WeakSet();
+    this.trackingVideos = new WeakSet();
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "PictureInPicture:UrlbarToggle": {
+        this.urlbarToggle();
+        break;
+      }
+    }
+    return null;
+  }
+
+  didDestroy() {
+    this.stopTrackingMouseOverVideos();
+    Services.prefs.removeObserver(TOGGLE_ENABLED_PREF, this.observerFunction);
+    Services.prefs.removeObserver(PIP_ENABLED_PREF, this.observerFunction);
+    Services.prefs.removeObserver(
+      TOGGLE_FIRST_SEEN_PREF,
+      this.observerFunction
+    );
+    Services.cpmm.sharedData.removeEventListener("change", this);
+
+    // remove the observer on the <video> element
+    let state = this.docState;
+    if (state?.intersectionObserver) {
+      state.intersectionObserver.disconnect();
+    }
+
+    // ensure the sandbox created by the video is destroyed
+    this.videoWrapper?.destroy();
+    this.videoWrapper = null;
+
+    for (let video of ChromeUtils.nondeterministicGetWeakSetKeys(
+      this.eligiblePipVideos
+    )) {
+      video.removeEventListener("emptied", this);
+      video.removeEventListener("loadedmetadata", this);
+      video.removeEventListener("durationchange", this);
+    }
+
+    for (let video of ChromeUtils.nondeterministicGetWeakSetKeys(
+      this.trackingVideos
+    )) {
+      video.removeEventListener("emptied", this);
+      video.removeEventListener("loadedmetadata", this);
+      video.removeEventListener("durationchange", this);
+    }
+
+    // ensure we don't access the state
+    this.isDestroyed = true;
+  }
+
+  observe(subject, topic, data) {
+    if (topic != "nsPref:changed") {
+      return;
+    }
+
+    this.toggleEnabled =
+      Services.prefs.getBoolPref(TOGGLE_ENABLED_PREF) &&
+      Services.prefs.getBoolPref(PIP_ENABLED_PREF);
+
+    if (this.toggleEnabled) {
+      // We have enabled the Picture-in-Picture toggle, so we need to make
+      // sure we register all of the videos that might already be on the page.
+      this.contentWindow.requestIdleCallback(() => {
+        let videos = this.document.querySelectorAll("video");
+        for (let video of videos) {
+          this.registerVideo(video);
+        }
+      });
+    }
+
+    switch (data) {
+      case TOGGLE_FIRST_SEEN_PREF:
+        const firstSeenSeconds = Services.prefs.getIntPref(
+          TOGGLE_FIRST_SEEN_PREF
+        );
+        if (!firstSeenSeconds || firstSeenSeconds < 0) {
+          return;
+        }
+        this.changeToIconIfDurationEnd(firstSeenSeconds);
+        break;
+    }
+  }
+
+  /**
+   * Returns the state for the current document referred to via
+   * this.document. If no such state exists, creates it, stores it
+   * and returns it.
+   */
+  get docState() {
+    if (this.isDestroyed || !this.document) {
+      return false;
+    }
+
+    let state = this.weakDocStates.get(this.document);
+
+    let visibilityThresholdPref = Services.prefs.getFloatPref(
+      TOGGLE_VISIBILITY_THRESHOLD_PREF,
+      "1.0"
+    );
+
+    if (!state) {
+      state = {
+        // A reference to the IntersectionObserver that's monitoring for videos
+        // to become visible.
+        intersectionObserver: null,
+        // A WeakSet of videos that are supposedly visible, according to the
+        // IntersectionObserver.
+        weakVisibleVideos: new WeakSet(),
+        // The number of videos that are supposedly visible, according to the
+        // IntersectionObserver
+        visibleVideosCount: 0,
+        // The DeferredTask that we'll arm every time a mousemove event occurs
+        // on a page where we have one or more visible videos.
+        mousemoveDeferredTask: null,
+        // A weak reference to the last video we displayed the toggle over.
+        weakOverVideo: null,
+        // True if the user is in the midst of clicking the toggle.
+        isClickingToggle: false,
+        // Set to the original target element on pointerdown if the user is clicking
+        // the toggle - this way, we can determine if a "click" event will need to be
+        // suppressed ("click" events don't fire if a "mouseup" occurs on a different
+        // element from the "pointerdown" / "mousedown" event).
+        clickedElement: null,
+        // This is a DeferredTask to hide the toggle after a period of mouse
+        // inactivity.
+        hideToggleDeferredTask: null,
+        // If we reach a point where we're tracking videos for mouse movements,
+        // then this will be true. If there are no videos worth tracking, then
+        // this is false.
+        isTrackingVideos: false,
+        togglePolicy: lazy.TOGGLE_POLICIES.DEFAULT,
+        toggleVisibilityThreshold: visibilityThresholdPref,
+        // The documentURI that has been checked with toggle policies and
+        // visibility thresholds for this document. Note that the documentURI
+        // might change for a document via the history API, so we remember
+        // the last checked documentURI to determine if we need to check again.
+        checkedPolicyDocumentURI: null,
+      };
+      this.weakDocStates.set(this.document, state);
+    }
+
+    return state;
+  }
+
+  /**
+   * Returns the video that the user was last hovering with the mouse if it
+   * still exists.
+   *
+   * @return {Element} the <video> element that the user was last hovering,
+   * or null if there was no such <video>, or the <video> no longer exists.
+   */
+  getWeakOverVideo() {
+    let { weakOverVideo } = this.docState;
+    if (weakOverVideo) {
+      // Bug 800957 - Accessing weakrefs at the wrong time can cause us to
+      // throw NS_ERROR_XPC_BAD_CONVERT_NATIVE
+      try {
+        return weakOverVideo.get();
+      } catch (e) {
+        return null;
+      }
+    }
+    return null;
+  }
+
+  handleEvent(event) {
+    if (!event.isTrusted) {
+      // We don't care about synthesized events that might be coming from
+      // content JS.
+      return;
+    }
+
+    // Don't capture events from Picture-in-Picture content windows
+    if (gPlayerContents.has(this.contentWindow)) {
+      return;
+    }
+
+    switch (event.type) {
+      case "touchstart": {
+        // Even if this is a touch event, there may be subsequent click events.
+        // Suppress those events after selecting the toggle to prevent playback changes
+        // when opening the Picture-in-Picture window.
+        if (this.docState.isClickingToggle) {
+          event.stopImmediatePropagation();
+          event.preventDefault();
+        }
+        break;
+      }
+      case "change": {
+        const { changedKeys } = event;
+        if (changedKeys.includes("PictureInPicture:SiteOverrides")) {
+          // For now we only update our cache if the site overrides change.
+          // the user will need to refresh the page for changes to apply.
+          try {
+            lazy.gSiteOverrides =
+              PictureInPictureToggleChild.getSiteOverrides();
+          } catch (e) {
+            // Ignore resulting TypeError if gSiteOverrides is still unloaded
+            if (!(e instanceof TypeError)) {
+              throw e;
+            }
+          }
+        }
+        break;
+      }
+      case "UAWidgetSetupOrChange": {
+        if (
+          this.toggleEnabled &&
+          this.contentWindow.HTMLVideoElement.isInstance(event.target) &&
+          event.target.ownerDocument == this.document
+        ) {
+          this.registerVideo(event.target);
+        }
+        break;
+      }
+      case "contextmenu": {
+        if (this.toggleEnabled) {
+          this.checkContextMenu(event);
+        }
+        break;
+      }
+      case "mouseout": {
+        this.onMouseOut(event);
+        break;
+      }
+      case "click":
+        if (event.detail == 0) {
+          let shadowRoot = event.originalTarget.containingShadowRoot;
+          let toggle = this.getToggleElement(shadowRoot);
+          if (event.originalTarget == toggle) {
+            this.startPictureInPicture(event, shadowRoot.host, toggle);
+            return;
+          }
+        }
+      // fall through
+      case "mousedown":
+      case "pointerup":
+      case "mouseup": {
+        this.onMouseButtonEvent(event);
+        break;
+      }
+      case "pointerdown": {
+        this.onPointerDown(event);
+        break;
+      }
+      case "mousemove": {
+        this.onMouseMove(event);
+        break;
+      }
+      case "pageshow": {
+        this.onPageShow(event);
+        break;
+      }
+      case "pagehide": {
+        this.onPageHide(event);
+        break;
+      }
+      case "durationchange":
+      // Intentional fall-through
+      case "emptied":
+      // Intentional fall-through
+      case "loadedmetadata": {
+        this.updatePipVideoEligibility(event.target);
+        break;
+      }
+    }
+  }
+
+  /**
+   * Adds a <video> to the IntersectionObserver so that we know when it becomes
+   * visible.
+   *
+   * @param {Element} video The <video> element to register.
+   */
+  registerVideo(video) {
+    let state = this.docState;
+    if (!state.intersectionObserver) {
+      let fn = this.onIntersection.bind(this);
+      state.intersectionObserver = new this.contentWindow.IntersectionObserver(
+        fn,
+        {
+          threshold: [0.0, 0.5],
+        }
+      );
+    }
+
+    state.intersectionObserver.observe(video);
+
+    if (!lazy.PIP_URLBAR_BUTTON) {
+      return;
+    }
+
+    video.addEventListener("emptied", this);
+    video.addEventListener("loadedmetadata", this);
+    video.addEventListener("durationchange", this);
+
+    this.trackingVideos.add(video);
+
+    this.updatePipVideoEligibility(video);
+  }
+
+  updatePipVideoEligibility(video) {
+    let isEligible = this.isVideoPiPEligible(video);
+    if (isEligible) {
+      if (!this.eligiblePipVideos.has(video)) {
+        this.eligiblePipVideos.add(video);
+
+        let mutationObserver = new this.contentWindow.MutationObserver(
+          mutationList => {
+            this.handleEligiblePipVideoMutation(mutationList);
+          }
+        );
+        mutationObserver.observe(video.parentElement, { childList: true });
+      }
+    } else if (this.eligiblePipVideos.has(video)) {
+      this.eligiblePipVideos.delete(video);
+    }
+
+    let videos = ChromeUtils.nondeterministicGetWeakSetKeys(
+      this.eligiblePipVideos
+    );
+
+    this.sendAsyncMessage("PictureInPicture:UpdateEligiblePipVideoCount", {
+      pipCount: videos.length,
+      pipDisabledCount: videos.reduce(
+        (accumulator, currentVal) =>
+          accumulator +
+          (currentVal.getAttribute("disablePictureInPicture") === "true"
+            ? 1
+            : 0),
+        0
+      ),
+    });
+  }
+
+  handleEligiblePipVideoMutation(mutationList) {
+    for (let mutationRecord of mutationList) {
+      let video = mutationRecord.removedNodes[0];
+      this.eligiblePipVideos.delete(video);
+    }
+
+    let videos = ChromeUtils.nondeterministicGetWeakSetKeys(
+      this.eligiblePipVideos
+    );
+
+    this.sendAsyncMessage("PictureInPicture:UpdateEligiblePipVideoCount", {
+      pipCount: videos.length,
+      pipDisabledCount: videos.reduce(
+        (accumulator, currentVal) =>
+          accumulator +
+          (currentVal.getAttribute("disablePictureInPicture") === "true"
+            ? 1
+            : 0),
+        0
+      ),
+    });
+  }
+
+  urlbarToggle() {
+    let video = ChromeUtils.nondeterministicGetWeakSetKeys(
+      this.eligiblePipVideos
+    )[0];
+    if (video) {
+      let pipEvent = new this.contentWindow.CustomEvent(
+        "MozTogglePictureInPicture",
+        {
+          bubbles: true,
+          detail: { reason: "urlBar" },
+        }
+      );
+      video.dispatchEvent(pipEvent);
+    }
+  }
+
+  isVideoPiPEligible(video) {
+    if (lazy.PIP_TOGGLE_ALWAYS_SHOW) {
+      return true;
+    }
+
+    if (isNaN(video.duration) || video.duration < lazy.MIN_VIDEO_LENGTH) {
+      return false;
+    }
+
+    const MIN_VIDEO_DIMENSION = 140; // pixels
+    if (
+      video.clientWidth < MIN_VIDEO_DIMENSION ||
+      video.clientHeight < MIN_VIDEO_DIMENSION
+    ) {
+      return false;
+    }
+
+    return true;
+  }
+
+  /**
+   * Changes from the first-time toggle to the icon toggle if the Nimbus variable `displayDuration`'s
+   * end date is reached when hovering over a video. The end date is calculated according to the timestamp
+   * indicating when the PiP toggle was first seen.
+   * @param {Number} firstSeenStartSeconds the timestamp in seconds indicating when the PiP toggle was first seen
+   */
+  changeToIconIfDurationEnd(firstSeenStartSeconds) {
+    const { displayDuration } =
+      lazy.NimbusFeatures.pictureinpicture.getAllVariables({
+        defaultValues: {
+          displayDuration: TOGGLE_FIRST_TIME_DURATION_DAYS,
+        },
+      });
+    if (!displayDuration || displayDuration < 0) {
+      return;
+    }
+
+    let daysInSeconds = displayDuration * 24 * 60 * 60;
+    let firstSeenEndSeconds = daysInSeconds + firstSeenStartSeconds;
+    let currentDateSeconds = Math.round(Date.now() / 1000);
+
+    lazy.logConsole.debug(
+      "Toggle duration experiment - first time toggle seen on:",
+      new Date(firstSeenStartSeconds * 1000).toLocaleDateString()
+    );
+    lazy.logConsole.debug(
+      "Toggle duration experiment - first time toggle will change on:",
+      new Date(firstSeenEndSeconds * 1000).toLocaleDateString()
+    );
+    lazy.logConsole.debug(
+      "Toggle duration experiment - current date:",
+      new Date(currentDateSeconds * 1000).toLocaleDateString()
+    );
+
+    if (currentDateSeconds >= firstSeenEndSeconds) {
+      this.sendAsyncMessage("PictureInPicture:SetHasUsed", {
+        hasUsed: true,
+      });
+    }
+  }
+
+  /**
+   * Called by the IntersectionObserver callback once a video becomes visible.
+   * This adds some fine-grained checking to ensure that a sufficient amount of
+   * the video is visible before we consider showing the toggles on it. For now,
+   * that means that the entirety of the video must be in the viewport.
+   *
+   * @param {IntersectionEntry} intersectionEntry An IntersectionEntry passed to
+   * the IntersectionObserver callback.
+   * @return bool Whether or not we should start tracking mousemove events for
+   * this registered video.
+   */
+  worthTracking(intersectionEntry) {
+    return intersectionEntry.isIntersecting;
+  }
+
+  /**
+   * Called by the IntersectionObserver once a video crosses one of the
+   * thresholds dictated by the IntersectionObserver configuration.
+   *
+   * @param {Array<IntersectionEntry>} A collection of one or more
+   * IntersectionEntry's for <video> elements that might have entered or exited
+   * the viewport.
+   */
+  onIntersection(entries) {
+    // The IntersectionObserver will also fire when a previously intersecting
+    // element is removed from the DOM. We know, however, that the node is
+    // still alive and referrable from the WeakSet because the
+    // IntersectionObserverEntry holds a strong reference to the video.
+    let state = this.docState;
+    if (!state) {
+      return;
+    }
+    let oldVisibleVideosCount = state.visibleVideosCount;
+    for (let entry of entries) {
+      let video = entry.target;
+      if (this.worthTracking(entry)) {
+        if (!state.weakVisibleVideos.has(video)) {
+          state.weakVisibleVideos.add(video);
+          state.visibleVideosCount++;
+          if (this.toggleTesting) {
+            gWeakIntersectingVideosForTesting.add(video);
+          }
+        }
+      } else if (state.weakVisibleVideos.has(video)) {
+        state.weakVisibleVideos.delete(video);
+        state.visibleVideosCount--;
+        if (this.toggleTesting) {
+          gWeakIntersectingVideosForTesting.delete(video);
+        }
+      }
+    }
+
+    // For testing, especially in debug or asan builds, we might not
+    // run this idle callback within an acceptable time. While we're
+    // testing, we'll bypass the idle callback performance optimization
+    // and run our callbacks as soon as possible during the next idle
+    // period.
+    if (!oldVisibleVideosCount && state.visibleVideosCount) {
+      if (this.toggleTesting || !this.contentWindow) {
+        this.beginTrackingMouseOverVideos();
+      } else {
+        this.contentWindow.requestIdleCallback(() => {
+          this.beginTrackingMouseOverVideos();
+        });
+      }
+    } else if (oldVisibleVideosCount && !state.visibleVideosCount) {
+      if (this.toggleTesting || !this.contentWindow) {
+        this.stopTrackingMouseOverVideos();
+      } else {
+        this.contentWindow.requestIdleCallback(() => {
+          this.stopTrackingMouseOverVideos();
+        });
+      }
+    }
+  }
+
+  addMouseButtonListeners() {
+    // We want to try to cancel the mouse events from continuing
+    // on into content if the user has clicked on the toggle, so
+    // we don't use the mozSystemGroup here, and add the listener
+    // to the parent target of the window, which in this case,
+    // is the windowRoot. Since this event listener is attached to
+    // part of the outer window, we need to also remove it in a
+    // pagehide event listener in the event that the page unloads
+    // before stopTrackingMouseOverVideos fires.
+    this.contentWindow.windowRoot.addEventListener("pointerdown", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("mousedown", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("mouseup", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("pointerup", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("click", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("mouseout", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.addEventListener("touchstart", this, {
+      capture: true,
+    });
+  }
+
+  removeMouseButtonListeners() {
+    // This can be null when closing the tab, but the event
+    // listeners should be removed in that case already.
+    if (!this.contentWindow || !this.contentWindow.windowRoot) {
+      return;
+    }
+
+    this.contentWindow.windowRoot.removeEventListener("pointerdown", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("mousedown", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("mouseup", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("pointerup", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("click", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("mouseout", this, {
+      capture: true,
+    });
+    this.contentWindow.windowRoot.removeEventListener("touchstart", this, {
+      capture: true,
+    });
+  }
+
+  /**
+   * One of the challenges of displaying this toggle is that many sites put
+   * things over top of <video> elements, like custom controls, or images, or
+   * all manner of things that might intercept mouseevents that would normally
+   * fire directly on the <video>. In order to properly detect when the mouse
+   * is over top of one of the <video> elements in this situation, we currently
+   * add a mousemove event handler to the entire document, and stash the most
+   * recent mousemove that fires. At periodic intervals, that stashed mousemove
+   * event is checked to see if it's hovering over one of our registered
+   * <video> elements.
+   *
+   * This sort of thing will not be necessary once bug 1539652 is fixed.
+   */
+  beginTrackingMouseOverVideos() {
+    let state = this.docState;
+    if (!state.mousemoveDeferredTask) {
+      state.mousemoveDeferredTask = new lazy.DeferredTask(() => {
+        this.checkLastMouseMove();
+      }, MOUSEMOVE_PROCESSING_DELAY_MS);
+    }
+    this.document.addEventListener("mousemove", this, {
+      mozSystemGroup: true,
+      capture: true,
+    });
+    this.contentWindow.addEventListener("pageshow", this, {
+      mozSystemGroup: true,
+    });
+    this.contentWindow.addEventListener("pagehide", this, {
+      mozSystemGroup: true,
+    });
+    this.addMouseButtonListeners();
+    state.isTrackingVideos = true;
+  }
+
+  /**
+   * If we no longer have any interesting videos in the viewport, we deregister
+   * the mousemove and click listeners, and also remove any toggles that might
+   * be on the page still.
+   */
+  stopTrackingMouseOverVideos() {
+    let state = this.docState;
+    // We initialize `mousemoveDeferredTask` in `beginTrackingMouseOverVideos`.
+    // If it doesn't exist, that can't have happened. Nothing else ever sets
+    // this value (though we arm/disarm in various places). So we don't need
+    // to do anything else here and can return early.
+    if (!state.mousemoveDeferredTask) {
+      return;
+    }
+    state.mousemoveDeferredTask.disarm();
+    this.document.removeEventListener("mousemove", this, {
+      mozSystemGroup: true,
+      capture: true,
+    });
+    if (this.contentWindow) {
+      this.contentWindow.removeEventListener("pageshow", this, {
+        mozSystemGroup: true,
+      });
+      this.contentWindow.removeEventListener("pagehide", this, {
+        mozSystemGroup: true,
+      });
+    }
+    this.removeMouseButtonListeners();
+    let oldOverVideo = this.getWeakOverVideo();
+    if (oldOverVideo) {
+      this.onMouseLeaveVideo(oldOverVideo);
+    }
+    state.isTrackingVideos = false;
+  }
+
+  /**
+   * This pageshow event handler will get called if and when we complete a tab
+   * tear out or in. If we happened to be tracking videos before the tear
+   * occurred, we re-add the mouse event listeners so that they're attached to
+   * the right WindowRoot.
+   *
+   * @param {Event} event The pageshow event fired when completing a tab tear
+   * out or in.
+   */
+  onPageShow(event) {
+    let state = this.docState;
+    if (state.isTrackingVideos) {
+      this.addMouseButtonListeners();
+    }
+  }
+
+  /**
+   * This pagehide event handler will get called if and when we start a tab
+   * tear out or in. If we happened to be tracking videos before the tear
+   * occurred, we remove the mouse event listeners. We'll re-add them when the
+   * pageshow event fires.
+   *
+   * @param {Event} event The pagehide event fired when starting a tab tear
+   * out or in.
+   */
+  onPageHide(event) {
+    let state = this.docState;
+    if (state.isTrackingVideos) {
+      this.removeMouseButtonListeners();
+    }
+  }
+
+  /**
+   * If we're tracking <video> elements, this pointerdown event handler is run anytime
+   * a pointerdown occurs on the document. This function is responsible for checking
+   * if the user clicked on the Picture-in-Picture toggle. It does this by first
+   * checking if the video is visible beneath the point that was clicked. Then
+   * it tests whether or not the pointerdown occurred within the rectangle of the
+   * toggle. If so, the event's propagation is stopped, and Picture-in-Picture is
+   * triggered.
+   *
+   * @param {Event} event The mousemove event.
+   */
+  onPointerDown(event) {
+    // The toggle ignores non-primary mouse clicks.
+    if (event.button != 0) {
+      return;
+    }
+
+    let video = this.getWeakOverVideo();
+    if (!video) {
+      return;
+    }
+
+    let shadowRoot = video.openOrClosedShadowRoot;
+    if (!shadowRoot) {
+      return;
+    }
+
+    let state = this.docState;
+
+    let overVideo = (() => {
+      let { clientX, clientY } = event;
+      let winUtils = this.contentWindow.windowUtils;
+      // We use winUtils.nodesFromRect instead of document.elementsFromPoint,
+      // since document.elementsFromPoint always flushes layout. The 1's in that
+      // function call are for the size of the rect that we want, which is 1x1.
+      //
+      // We pass the aOnlyVisible boolean argument to check that the video isn't
+      // occluded by anything visible at the point of mousedown. If it is, we'll
+      // ignore the mousedown.
+      let elements = winUtils.nodesFromRect(
+        clientX,
+        clientY,
+        1,
+        1,
+        1,
+        1,
+        true,
+        false,
+        /* aOnlyVisible = */ true,
+        state.toggleVisibilityThreshold
+      );
+
+      for (let element of elements) {
+        if (element == video || element.containingShadowRoot == shadowRoot) {
+          return true;
+        }
+      }
+
+      return false;
+    })();
+
+    if (!overVideo) {
+      return;
+    }
+
+    let toggle = this.getToggleElement(shadowRoot);
+    if (this.isMouseOverToggle(toggle, event)) {
+      state.isClickingToggle = true;
+      state.clickedElement = Cu.getWeakReference(event.originalTarget);
+      event.stopImmediatePropagation();
+
+      this.startPictureInPicture(event, video, toggle);
+    }
+  }
+
+  startPictureInPicture(event, video, toggle) {
+    Services.telemetry.keyedScalarAdd(
+      "pictureinpicture.opened_method",
+      "toggle",
+      1
+    );
+
+    let pipEvent = new this.contentWindow.CustomEvent(
+      "MozTogglePictureInPicture",
+      {
+        bubbles: true,
+        detail: { reason: "toggle" },
+      }
+    );
+    video.dispatchEvent(pipEvent);
+
+    // Since we've initiated Picture-in-Picture, we can go ahead and
+    // hide the toggle now.
+    this.onMouseLeaveVideo(video);
+  }
+
+  /**
+   * Called for mousedown, pointerup, mouseup and click events. If we
+   * detected that the user is clicking on the Picture-in-Picture toggle,
+   * these events are cancelled in the capture-phase before they reach
+   * content. The state for suppressing these events is cleared on the
+   * click event (unless the mouseup occurs on a different element from
+   * the mousedown, in which case, the state is cleared on mouseup).
+   *
+   * @param {Event} event A mousedown, pointerup, mouseup or click event.
+   */
+  onMouseButtonEvent(event) {
+    // The toggle ignores non-primary mouse clicks.
+    if (event.button != 0) {
+      return;
+    }
+
+    let state = this.docState;
+    if (state.isClickingToggle) {
+      event.stopImmediatePropagation();
+
+      // If this is a mouseup event, check to see if we have a record of what
+      // the original target was on pointerdown. If so, and if it doesn't match
+      // the mouseup original target, that means we won't get a click event, and
+      // we can clear the "clicking the toggle" state right away.
+      //
+      // Otherwise, we wait for the click event to do that.
+      let isMouseUpOnOtherElement =
+        event.type == "mouseup" &&
+        (!state.clickedElement ||
+          state.clickedElement.get() != event.originalTarget);
+
+      if (
+        isMouseUpOnOtherElement ||
+        event.type == "click" ||
+        // pointerup event still triggers after a touchstart event. We just need to detect
+        // the pointer type and determine if we got to this part of the code through a touch event.
+        event.pointerType == "touch"
+      ) {
+        // The click is complete, so now we reset the state so that
+        // we stop suppressing these events.
+        state.isClickingToggle = false;
+        state.clickedElement = null;
+      }
+    }
+  }
+
+  /**
+   * Called on mouseout events to determine whether or not the mouse has
+   * exited the window.
+   *
+   * @param {Event} event The mouseout event.
+   */
+  onMouseOut(event) {
+    if (!event.relatedTarget) {
+      // For mouseout events, if there's no relatedTarget (which normally
+      // maps to the element that the mouse entered into) then this means that
+      // we left the window.
+      let video = this.getWeakOverVideo();
+      if (!video) {
+        return;
+      }
+
+      this.onMouseLeaveVideo(video);
+    }
+  }
+
+  /**
+   * Called for each mousemove event when we're tracking those events to
+   * determine if the cursor is hovering over a <video>.
+   *
+   * @param {Event} event The mousemove event.
+   */
+  onMouseMove(event) {
+    let state = this.docState;
+
+    if (state.hideToggleDeferredTask) {
+      state.hideToggleDeferredTask.disarm();
+      state.hideToggleDeferredTask.arm();
+    }
+
+    state.lastMouseMoveEvent = event;
+    state.mousemoveDeferredTask.arm();
+  }
+
+  /**
+   * Called by the DeferredTask after MOUSEMOVE_PROCESSING_DELAY_MS
+   * milliseconds. Checked to see if that mousemove happens to be overtop of
+   * any interesting <video> elements that we want to display the toggle
+   * on. If so, puts the toggle on that video.
+   */
+  checkLastMouseMove() {
+    let state = this.docState;
+    let event = state.lastMouseMoveEvent;
+    let { clientX, clientY } = event;
+    lazy.logConsole.debug("Visible videos count:", state.visibleVideosCount);
+    lazy.logConsole.debug("Tracking videos:", state.isTrackingVideos);
+    let winUtils = this.contentWindow.windowUtils;
+    // We use winUtils.nodesFromRect instead of document.elementsFromPoint,
+    // since document.elementsFromPoint always flushes layout. The 1's in that
+    // function call are for the size of the rect that we want, which is 1x1.
+    let elements = winUtils.nodesFromRect(
+      clientX,
+      clientY,
+      1,
+      1,
+      1,
+      1,
+      true,
+      false,
+      /* aOnlyVisible = */ true
+    );
+
+    for (let element of elements) {
+      lazy.logConsole.debug("Element id under cursor:", element.id);
+      lazy.logConsole.debug(
+        "Node name of an element under cursor:",
+        element.nodeName
+      );
+      lazy.logConsole.debug(
+        "Supported <video> element:",
+        state.weakVisibleVideos.has(element)
+      );
+      lazy.logConsole.debug(
+        "PiP window is open:",
+        element.isCloningElementVisually
+      );
+
+      // Check for hovering over the video controls or so too, not only
+      // directly over the video.
+      for (let el = element; el; el = el.containingShadowRoot?.host) {
+        if (state.weakVisibleVideos.has(el) && !el.isCloningElementVisually) {
+          lazy.logConsole.debug("Found supported element");
+          this.onMouseOverVideo(el, event);
+          return;
+        }
+      }
+    }
+
+    let oldOverVideo = this.getWeakOverVideo();
+    if (oldOverVideo) {
+      this.onMouseLeaveVideo(oldOverVideo);
+    }
+  }
+
+  /**
+   * Called once it has been determined that the mouse is overtop of a video
+   * that is in the viewport.
+   *
+   * @param {Element} video The video the mouse is over.
+   */
+  onMouseOverVideo(video, event) {
+    let oldOverVideo = this.getWeakOverVideo();
+    let shadowRoot = video.openOrClosedShadowRoot;
+
+    if (shadowRoot.firstChild && video != oldOverVideo) {
+      if (video.getTransformToViewport().a == -1) {
+        shadowRoot.firstChild.setAttribute("flipped", true);
+      } else {
+        shadowRoot.firstChild.removeAttribute("flipped");
+      }
+    }
+
+    // It seems from automated testing that if it's still very early on in the
+    // lifecycle of a <video> element, it might not yet have a shadowRoot,
+    // in which case, we can bail out here early.
+    if (!shadowRoot) {
+      if (oldOverVideo) {
+        // We also clear the hover state on the old video we were hovering,
+        // if there was one.
+        this.onMouseLeaveVideo(oldOverVideo);
+      }
+
+      return;
+    }
+
+    let state = this.docState;
+    let toggle = this.getToggleElement(shadowRoot);
+    let controlsOverlay = shadowRoot.querySelector(".controlsOverlay");
+
+    if (state.checkedPolicyDocumentURI != this.document.documentURI) {
+      state.togglePolicy = lazy.TOGGLE_POLICIES.DEFAULT;
+      // We cache the matchers process-wide. We'll skip this while running tests to make that
+      // easier.
+      let siteOverrides = this.toggleTesting
+        ? PictureInPictureToggleChild.getSiteOverrides()
+        : lazy.gSiteOverrides;
+
+      let visibilityThresholdPref = Services.prefs.getFloatPref(
+        TOGGLE_VISIBILITY_THRESHOLD_PREF,
+        "1.0"
+      );
+
+      if (!this.videoWrapper) {
+        this.videoWrapper = applyWrapper(this, video);
+      }
+
+      // Do we have any toggle overrides? If so, try to apply them.
+      for (let [override, { policy, visibilityThreshold }] of siteOverrides) {
+        if (
+          (policy || visibilityThreshold) &&
+          override.matches(this.document.documentURI)
+        ) {
+          state.togglePolicy = this.videoWrapper?.shouldHideToggle(video)
+            ? lazy.TOGGLE_POLICIES.HIDDEN
+            : policy || lazy.TOGGLE_POLICIES.DEFAULT;
+          state.toggleVisibilityThreshold =
+            visibilityThreshold || visibilityThresholdPref;
+          break;
+        }
+      }
+
+      state.checkedPolicyDocumentURI = this.document.documentURI;
+    }
+
+    // The built-in <video> controls are along the bottom, which would overlap the
+    // toggle if the override is set to BOTTOM, so we ignore overrides that set
+    // a policy of BOTTOM for <video> elements with controls.
+    if (
+      state.togglePolicy != lazy.TOGGLE_POLICIES.DEFAULT &&
+      !(state.togglePolicy == lazy.TOGGLE_POLICIES.BOTTOM && video.controls)
+    ) {
+      toggle.setAttribute(
+        "policy",
+        lazy.TOGGLE_POLICY_STRINGS[state.togglePolicy]
+      );
+    } else {
+      toggle.removeAttribute("policy");
+    }
+
+    // nimbusExperimentVariables will be defaultValues when the experiment is disabled
+    const nimbusExperimentVariables =
+      lazy.NimbusFeatures.pictureinpicture.getAllVariables({
+        defaultValues: {
+          oldToggle: true,
+          title: null,
+          message: false,
+          showIconOnly: false,
+          displayDuration: TOGGLE_FIRST_TIME_DURATION_DAYS,
+        },
+      });
+
+    /**
+     * If a Nimbus variable exists for the first-time PiP toggle design,
+     * override the old design via a classname "experiment".
+     */
+    if (!nimbusExperimentVariables.oldToggle) {
+      let controlsContainer = shadowRoot.querySelector(".controlsContainer");
+      let pipWrapper = shadowRoot.querySelector(".pip-wrapper");
+
+      controlsContainer.classList.add("experiment");
+      pipWrapper.classList.add("experiment");
+    } else {
+      let controlsContainer = shadowRoot.querySelector(".controlsContainer");
+      let pipWrapper = shadowRoot.querySelector(".pip-wrapper");
+
+      controlsContainer.classList.remove("experiment");
+      pipWrapper.classList.remove("experiment");
+    }
+
+    if (nimbusExperimentVariables.title) {
+      let pipExplainer = shadowRoot.querySelector(".pip-explainer");
+      let pipLabel = shadowRoot.querySelector(".pip-label");
+
+      if (pipExplainer && nimbusExperimentVariables.message) {
+        pipExplainer.innerText = nimbusExperimentVariables.message;
+      }
+      pipLabel.innerText = nimbusExperimentVariables.title;
+    } else if (nimbusExperimentVariables.showIconOnly) {
+      // We only want to show the PiP icon in this experiment scenario
+      let pipExpanded = shadowRoot.querySelector(".pip-expanded");
+      pipExpanded.style.display = "none";
+      let pipSmall = shadowRoot.querySelector(".pip-small");
+      pipSmall.style.opacity = "1";
+
+      let pipIcon = shadowRoot.querySelectorAll(".pip-icon")[1];
+      pipIcon.style.display = "block";
+    }
+
+    controlsOverlay.removeAttribute("hidetoggle");
+
+    // The hideToggleDeferredTask we create here is for automatically hiding
+    // the toggle after a period of no mousemove activity for
+    // TOGGLE_HIDING_TIMEOUT_MS. If the mouse moves, then the DeferredTask
+    // timer is reset.
+    //
+    // We disable the toggle hiding timeout during testing to reduce
+    // non-determinism from timers when testing the toggle.
+    if (!state.hideToggleDeferredTask && !this.toggleTesting) {
+      state.hideToggleDeferredTask = new lazy.DeferredTask(() => {
+        controlsOverlay.setAttribute("hidetoggle", true);
+      }, TOGGLE_HIDING_TIMEOUT_MS);
+    }
+
+    if (oldOverVideo) {
+      if (oldOverVideo == video) {
+        // If we're still hovering the old video, we might have entered or
+        // exited the toggle region.
+        this.checkHoverToggle(toggle, event);
+        return;
+      }
+
+      // We had an old video that we were hovering, and we're not hovering
+      // it anymore. Let's leave it.
+      this.onMouseLeaveVideo(oldOverVideo);
+    }
+
+    state.weakOverVideo = Cu.getWeakReference(video);
+    controlsOverlay.classList.add("hovering");
+
+    if (
+      state.togglePolicy != lazy.TOGGLE_POLICIES.HIDDEN &&
+      !toggle.hasAttribute("hidden")
+    ) {
+      Services.telemetry.scalarAdd("pictureinpicture.saw_toggle", 1);
+      const hasUsedPiP = Services.prefs.getBoolPref(TOGGLE_HAS_USED_PREF);
+      let args = {
+        firstTime: (!hasUsedPiP).toString(),
+      };
+      Services.telemetry.recordEvent(
+        "pictureinpicture",
+        "saw_toggle",
+        "toggle",
+        null,
+        args
+      );
+      // only record if this is the first time seeing the toggle
+      if (!hasUsedPiP) {
+        lazy.NimbusFeatures.pictureinpicture.recordExposureEvent();
+
+        const firstSeenSeconds = Services.prefs.getIntPref(
+          TOGGLE_FIRST_SEEN_PREF,
+          0
+        );
+
+        if (!firstSeenSeconds || firstSeenSeconds < 0) {
+          let firstTimePiPStartDate = Math.round(Date.now() / 1000);
+          this.sendAsyncMessage("PictureInPicture:SetFirstSeen", {
+            dateSeconds: firstTimePiPStartDate,
+          });
+        } else if (nimbusExperimentVariables.displayDuration) {
+          this.changeToIconIfDurationEnd(firstSeenSeconds);
+        }
+      }
+    }
+
+    // Now that we're hovering the video, we'll check to see if we're
+    // hovering the toggle too.
+    this.checkHoverToggle(toggle, event);
+  }
+
+  /**
+   * Checks if a mouse event is happening over a toggle element. If it is,
+   * sets the hovering class on it. Otherwise, it clears the hovering
+   * class.
+   *
+   * @param {Element} toggle The Picture-in-Picture toggle to check.
+   * @param {MouseEvent} event A MouseEvent to test.
+   */
+  checkHoverToggle(toggle, event) {
+    toggle.classList.toggle("hovering", this.isMouseOverToggle(toggle, event));
+  }
+
+  /**
+   * Called once it has been determined that the mouse is no longer overlapping
+   * a video that we'd previously called onMouseOverVideo with.
+   *
+   * @param {Element} video The video that the mouse left.
+   */
+  onMouseLeaveVideo(video) {
+    let state = this.docState;
+    let shadowRoot = video.openOrClosedShadowRoot;
+
+    if (shadowRoot) {
+      let controlsOverlay = shadowRoot.querySelector(".controlsOverlay");
+      let toggle = this.getToggleElement(shadowRoot);
+      controlsOverlay.classList.remove("hovering");
+      toggle.classList.remove("hovering");
+    }
+
+    state.weakOverVideo = null;
+
+    if (!this.toggleTesting) {
+      state.hideToggleDeferredTask.disarm();
+      state.mousemoveDeferredTask.disarm();
+    }
+
+    state.hideToggleDeferredTask = null;
+  }
+
+  /**
+   * Given a reference to a Picture-in-Picture toggle element, determines
+   * if a MouseEvent event is occurring within its bounds.
+   *
+   * @param {Element} toggle The Picture-in-Picture toggle.
+   * @param {MouseEvent} event A MouseEvent to test.
+   *
+   * @return {Boolean}
+   */
+  isMouseOverToggle(toggle, event) {
+    let toggleRect =
+      toggle.ownerGlobal.windowUtils.getBoundsWithoutFlushing(toggle);
+
+    // The way the toggle is currently implemented with
+    // absolute positioning, the root toggle element bounds don't actually
+    // contain all of the toggle child element bounds. Until we find a way to
+    // sort that out, we workaround the issue by having each clickable child
+    // elements of the toggle have a clicklable class, and then compute the
+    // smallest rect that contains all of their bounding rects and use that
+    // as the hitbox.
+    toggleRect = lazy.Rect.fromRect(toggleRect);
+    let clickableChildren = toggle.querySelectorAll(".clickable");
+    for (let child of clickableChildren) {
+      let childRect = lazy.Rect.fromRect(
+        child.ownerGlobal.windowUtils.getBoundsWithoutFlushing(child)
+      );
+      toggleRect.expandToContain(childRect);
+    }
+
+    // If the toggle has no dimensions, we're definitely not over it.
+    if (!toggleRect.width || !toggleRect.height) {
+      return false;
+    }
+
+    let { clientX, clientY } = event;
+
+    return (
+      clientX >= toggleRect.left &&
+      clientX <= toggleRect.right &&
+      clientY >= toggleRect.top &&
+      clientY <= toggleRect.bottom
+    );
+  }
+
+  /**
+   * Checks a contextmenu event to see if the mouse is currently over the
+   * Picture-in-Picture toggle. If so, sends a message to the parent process
+   * to open up the Picture-in-Picture toggle context menu.
+   *
+   * @param {MouseEvent} event A contextmenu event.
+   */
+  checkContextMenu(event) {
+    let video = this.getWeakOverVideo();
+    if (!video) {
+      return;
+    }
+
+    let shadowRoot = video.openOrClosedShadowRoot;
+    if (!shadowRoot) {
+      return;
+    }
+
+    let toggle = this.getToggleElement(shadowRoot);
+    if (this.isMouseOverToggle(toggle, event)) {
+      let devicePixelRatio = toggle.ownerGlobal.devicePixelRatio;
+      this.sendAsyncMessage("PictureInPicture:OpenToggleContextMenu", {
+        screenXDevPx: event.screenX * devicePixelRatio,
+        screenYDevPx: event.screenY * devicePixelRatio,
+        mozInputSource: event.mozInputSource,
+      });
+      event.stopImmediatePropagation();
+      event.preventDefault();
+    }
+  }
+
+  /**
+   * Returns the appropriate root element for the Picture-in-Picture toggle,
+   * depending on whether or not we're using the experimental toggle preference.
+   *
+   * @param {Element} shadowRoot The shadowRoot of the video element.
+   * @returns {Element} The toggle element.
+   */
+  getToggleElement(shadowRoot) {
+    return shadowRoot.getElementById("pictureInPictureToggle");
+  }
+
+  /**
+   * This is a test-only function that returns true if a video is being tracked
+   * for mouseover events after having intersected the viewport.
+   */
+  static isTracking(video) {
+    return gWeakIntersectingVideosForTesting.has(video);
+  }
+
+  /**
+   * Gets any Picture-in-Picture site-specific overrides stored in the
+   * sharedData struct, and returns them as an Array of two-element Arrays,
+   * where the first element is a MatchPattern and the second element is an
+   * object of the form { policy, disabledKeyboardControls } (where each property
+   * may be missing or undefined).
+   *
+   * @returns {Array<Array<2>>} Array of 2-element Arrays where the first element
+   * is a MatchPattern and the second element is an object with optional policy
+   * and/or disabledKeyboardControls properties.
+   */
+  static getSiteOverrides() {
+    let result = [];
+    let patterns = Services.cpmm.sharedData.get(
+      "PictureInPicture:SiteOverrides"
+    );
+    for (let pattern in patterns) {
+      let matcher = new MatchPattern(pattern);
+      result.push([matcher, patterns[pattern]]);
+    }
+    return result;
+  }
+}
+
+export class PictureInPictureChild extends JSWindowActorChild {
+  #subtitlesEnabled = false;
+  // A weak reference to this PiP window's video element
+  weakVideo = null;
+
+  // A weak reference to this PiP window's content window
+  weakPlayerContent = null;
+
+  // A reference to current WebVTT track currently displayed on the content window
+  _currentWebVTTTrack = null;
+
+  observerFunction = null;
+
+  observe(subject, topic, data) {
+    if (topic != "nsPref:changed") {
+      return;
+    }
+
+    switch (data) {
+      case "media.videocontrols.picture-in-picture.display-text-tracks.enabled": {
+        const originatingVideo = this.getWeakVideo();
+        let isTextTrackPrefEnabled = Services.prefs.getBoolPref(
+          "media.videocontrols.picture-in-picture.display-text-tracks.enabled"
+        );
+
+        // Enable or disable text track support
+        if (isTextTrackPrefEnabled) {
+          this.setupTextTracks(originatingVideo);
+        } else {
+          this.removeTextTracks(originatingVideo);
+        }
+        break;
+      }
+    }
+  }
+
+  /**
+   * Creates a link element with a reference to the css stylesheet needed
+   * for text tracks responsive styling.
+   * @returns {Element} the link element containing text tracks stylesheet.
+   */
+  createTextTracksStyleSheet() {
+    let headStyleElement = this.document.createElement("link");
+    headStyleElement.setAttribute("rel", "stylesheet");
+    headStyleElement.setAttribute(
+      "href",
+      "chrome://global/skin/pictureinpicture/texttracks.css"
+    );
+    headStyleElement.setAttribute("type", "text/css");
+    return headStyleElement;
+  }
+
+  /**
+   * Sets up Picture-in-Picture to support displaying text tracks from WebVTT
+   * or if WebVTT isn't supported we will register the caption change mutation observer if
+   * the site wrapper exists.
+   *
+   * If the originating video supports WebVTT, try to read the
+   * active track and cues. Display any active cues on the pip window
+   * right away if applicable.
+   *
+   * @param originatingVideo {Element|null}
+   *  The <video> being displayed in Picture-in-Picture mode, or null if that <video> no longer exists.
+   */
+  setupTextTracks(originatingVideo) {
+    const isWebVTTSupported = !!originatingVideo.textTracks?.length;
+
+    if (!isWebVTTSupported) {
+      this.setUpCaptionChangeListener(originatingVideo);
+      return;
+    }
+
+    // Verify active track for originating video
+    this.setActiveTextTrack(originatingVideo.textTracks);
+
+    if (!this._currentWebVTTTrack) {
+      // If WebVTT track is invalid, try using a video wrapper
+      this.setUpCaptionChangeListener(originatingVideo);
+      return;
+    }
+
+    // Listen for changes in tracks and active cues
+    originatingVideo.textTracks.addEventListener("change", this);
+    this._currentWebVTTTrack.addEventListener("cuechange", this.onCueChange);
+
+    const cues = this._currentWebVTTTrack.activeCues;
+    this.updateWebVTTTextTracksDisplay(cues);
+  }
+
+  /**
+   * Toggle the visibility of the subtitles in the PiP window
+   */
+  toggleTextTracks() {
+    let textTracks = this.document.getElementById("texttracks");
+    textTracks.style.display =
+      textTracks.style.display === "none" ? "" : "none";
+  }
+
+  /**
+   * Removes existing text tracks on the Picture in Picture window.
+   *
+   * If the originating video supports WebVTT, clear references to active
+   * tracks and cues. No longer listen for any track or cue changes.
+   *
+   * @param originatingVideo {Element|null}
+   *  The <video> being displayed in Picture-in-Picture mode, or null if that <video> no longer exists.
+   */
+  removeTextTracks(originatingVideo) {
+    const isWebVTTSupported = !!originatingVideo.textTracks;
+
+    if (!isWebVTTSupported) {
+      return;
+    }
+
+    // No longer listen for changes to tracks and active cues
+    originatingVideo.textTracks.removeEventListener("change", this);
+    this._currentWebVTTTrack?.removeEventListener(
+      "cuechange",
+      this.onCueChange
+    );
+    this._currentWebVTTTrack = null;
+    this.updateWebVTTTextTracksDisplay(null);
+  }
+
+  /**
+   * Moves the text tracks container position above the pip window's video controls
+   * if their positions visually overlap. Since pip controls are within the parent
+   * process, we determine if pip video controls and text tracks visually overlap by
+   * comparing their relative positions with DOMRect.
+   *
+   * If overlap is found, set attribute "overlap-video-controls" to move text tracks
+   * and define a new relative bottom position according to pip window size and the
+   * position of video controls.
+   *  @param {Object} data args needed to determine if text tracks must be moved
+   */
+  moveTextTracks(data) {
+    const {
+      isFullscreen,
+      isVideoControlsShowing,
+      playerBottomControlsDOMRect,
+      isScrubberShowing,
+    } = data;
+    let textTracks = this.document.getElementById("texttracks");
+    const originatingWindow = this.getWeakVideo().ownerGlobal;
+    const isReducedMotionEnabled = originatingWindow.matchMedia(
+      "(prefers-reduced-motion: reduce)"
+    ).matches;
+    const textTracksFontScale = this.document
+      .querySelector(":root")
+      .style.getPropertyValue("--font-scale");
+
+    if (isFullscreen || isReducedMotionEnabled) {
+      textTracks.removeAttribute("overlap-video-controls");
+      return;
+    }
+
+    if (isVideoControlsShowing) {
+      let playerVideoRect = textTracks.parentElement.getBoundingClientRect();
+      let isOverlap =
+        playerVideoRect.bottom - textTracksFontScale * playerVideoRect.height >
+        playerBottomControlsDOMRect.top;
+
+      if (isOverlap) {
+        const root = this.document.querySelector(":root");
+        if (isScrubberShowing) {
+          root.style.setProperty("--player-controls-scrubber-height", "30px");
+        } else {
+          root.style.setProperty("--player-controls-scrubber-height", "0px");
+        }
+        textTracks.setAttribute("overlap-video-controls", true);
+      } else {
+        textTracks.removeAttribute("overlap-video-controls");
+      }
+    } else {
+      textTracks.removeAttribute("overlap-video-controls");
+    }
+  }
+
+  /**
+   * Updates the text content for the container that holds and displays text tracks
+   * on the pip window.
+   * @param textTrackCues {TextTrackCueList|null}
+   *  Collection of TextTrackCue objects containing text displayed, or null if there is no cue to display.
+   */
+  updateWebVTTTextTracksDisplay(textTrackCues) {
+    let pipWindowTracksContainer = this.document.getElementById("texttracks");
+    let playerVideo = this.document.getElementById("playervideo");
+    let playerVideoWindow = playerVideo.ownerGlobal;
+
+    // To prevent overlap with previous cues, clear all text from the pip window
+    pipWindowTracksContainer.replaceChildren();
+
+    if (!textTrackCues) {
+      return;
+    }
+
+    if (!this.isSubtitlesEnabled) {
+      this.isSubtitlesEnabled = true;
+      this.sendAsyncMessage("PictureInPicture:EnableSubtitlesButton");
+    }
+
+    let allCuesArray = [...textTrackCues];
+    // Re-order cues
+    this.getOrderedWebVTTCues(allCuesArray);
+    // Parse through WebVTT cue using vtt.js to ensure
+    // semantic markup like <b> and <i> tags are rendered.
+    allCuesArray.forEach(cue => {
+      let text = cue.text;
+      // Trim extra newlines and whitespaces
+      const re = /(\s*\n{2,}\s*)/g;
+      text = text.trim();
+      text = text.replace(re, "\n");
+      let cueTextNode = WebVTT.convertCueToDOMTree(playerVideoWindow, text);
+      let cueDiv = this.document.createElement("div");
+      cueDiv.appendChild(cueTextNode);
+      pipWindowTracksContainer.appendChild(cueDiv);
+    });
+  }
+
+  /**
+   * Re-orders list of multiple active cues to ensure cues are rendered in the correct order.
+   * How cues are ordered depends on the VTTCue.line value of the cue.
+   *
+   * If line is string "auto", we want to reverse the order of cues.
+   * Cues are read from top to bottom in a vtt file, but are inserted into a video from bottom to top.
+   * Ensure this order is followed.
+   *
+   * If line is an integer or percentage, we want to order cues according to numeric value.
+   * Assumptions:
+   *  1) all active cues are numeric
+   *  2) all active cues are in range 0..100
+   *  3) all actives cue are horizontal (no VTTCue.vertical)
+   *  4) all active cues with VTTCue.line integer have VTTCue.snapToLines = true
+   *  5) all active cues with VTTCue.line percentage have VTTCue.snapToLines = false
+   *
+   * vtt.jsm currently sets snapToLines to false if line is a percentage value, but
+   * cues are still ordered by line. In most cases, snapToLines is set to true by default,
+   * unless intentionally overridden.
+   * @param allCuesArray {Array<VTTCue>} array of active cues
+   */
+  getOrderedWebVTTCues(allCuesArray) {
+    if (!allCuesArray || allCuesArray.length <= 1) {
+      return;
+    }
+
+    let allCuesHaveNumericLines = allCuesArray.find(cue => cue.line !== "auto");
+
+    if (allCuesHaveNumericLines) {
+      allCuesArray.sort((cue1, cue2) => cue1.line - cue2.line);
+    } else if (allCuesArray.length >= 2) {
+      allCuesArray.reverse();
+    }
+  }
+
+  /**
+   * Returns a reference to the PiP's <video> element being displayed in Picture-in-Picture
+   * mode.
+   *
+   * @return {Element} The <video> being displayed in Picture-in-Picture mode, or null
+   * if that <video> no longer exists.
+   */
+  getWeakVideo() {
+    if (this.weakVideo) {
+      // Bug 800957 - Accessing weakrefs at the wrong time can cause us to
+      // throw NS_ERROR_XPC_BAD_CONVERT_NATIVE
+      try {
+        return this.weakVideo.get();
+      } catch (e) {
+        return null;
+      }
+    }
+    return null;
+  }
+
+  /**
+   * Returns a reference to the inner window of the about:blank document that is
+   * cloning the originating <video> in the always-on-top player <xul:browser>.
+   *
+   * @return {Window} The inner window of the about:blank player <xul:browser>, or
+   * null if that window has been closed.
+   */
+  getWeakPlayerContent() {
+    if (this.weakPlayerContent) {
+      // Bug 800957 - Accessing weakrefs at the wrong time can cause us to
+      // throw NS_ERROR_XPC_BAD_CONVERT_NATIVE
+      try {
+        return this.weakPlayerContent.get();
+      } catch (e) {
+        return null;
+      }
+    }
+    return null;
+  }
+
+  /**
+   * Returns true if the passed video happens to be the one that this
+   * content process is running in a Picture-in-Picture window.
+   *
+   * @param {Element} video The <video> element to check.
+   *
+   * @return {Boolean}
+   */
+  inPictureInPicture(video) {
+    return this.getWeakVideo() === video;
+  }
+
+  static videoIsPlaying(video) {
+    return !!(!video.paused && !video.ended && video.readyState > 2);
+  }
+
+  static videoIsMuted(video) {
+    return this.videoWrapper.isMuted(video);
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "MozStopPictureInPicture": {
+        if (event.isTrusted && event.target === this.getWeakVideo()) {
+          const reason = event.detail?.reason || "videoElRemove";
+          this.closePictureInPicture({ reason });
+        }
+        break;
+      }
+      case "pagehide": {
+        // The originating video's content document has unloaded,
+        // so close Picture-in-Picture.
+        this.closePictureInPicture({ reason: "pagehide" });
+        break;
+      }
+      case "MozDOMFullscreen:Request": {
+        this.closePictureInPicture({ reason: "fullscreen" });
+        break;
+      }
+      case "play": {
+        this.sendAsyncMessage("PictureInPicture:Playing");
+        break;
+      }
+      case "pause": {
+        this.sendAsyncMessage("PictureInPicture:Paused");
+        break;
+      }
+      case "volumechange": {
+        let video = this.getWeakVideo();
+
+        // Just double-checking that we received the event for the right
+        // video element.
+        if (video !== event.target) {
+          lazy.logConsole.error(
+            "PictureInPictureChild received volumechange for " +
+              "the wrong video!"
+          );
+          return;
+        }
+
+        if (this.constructor.videoIsMuted(video)) {
+          this.sendAsyncMessage("PictureInPicture:Muting");
+        } else {
+          this.sendAsyncMessage("PictureInPicture:Unmuting");
+        }
+        break;
+      }
+      case "resize": {
+        let video = event.target;
+        if (this.inPictureInPicture(video)) {
+          this.sendAsyncMessage("PictureInPicture:Resize", {
+            videoHeight: video.videoHeight,
+            videoWidth: video.videoWidth,
+          });
+        }
+        this.setupTextTracks(video);
+        break;
+      }
+      case "emptied": {
+        this.isSubtitlesEnabled = false;
+        if (this.emptiedTimeout) {
+          clearTimeout(this.emptiedTimeout);
+          this.emptiedTimeout = null;
+        }
+        let video = this.getWeakVideo();
+        // We may want to keep the pip window open if the video
+        // is still in DOM. But if video src is no longer defined,
+        // close Picture-in-Picture.
+        this.emptiedTimeout = setTimeout(() => {
+          if (!video || !video.src) {
+            this.closePictureInPicture({ reason: "videoElEmptied" });
+          }
+        }, EMPTIED_TIMEOUT_MS);
+        break;
+      }
+      case "change": {
+        // Clear currently stored track data (webvtt support) before reading
+        // a new track.
+        if (this._currentWebVTTTrack) {
+          this._currentWebVTTTrack.removeEventListener(
+            "cuechange",
+            this.onCueChange
+          );
+          this._currentWebVTTTrack = null;
+        }
+
+        const tracks = event.target;
+        this.setActiveTextTrack(tracks);
+        const isCurrentTrackAvailable = this._currentWebVTTTrack;
+
+        // If tracks are disabled or invalid while change occurs,
+        // remove text tracks from the pip window and stop here.
+        if (!isCurrentTrackAvailable || !tracks.length) {
+          this.updateWebVTTTextTracksDisplay(null);
+          return;
+        }
+
+        this._currentWebVTTTrack.addEventListener(
+          "cuechange",
+          this.onCueChange
+        );
+        const cues = this._currentWebVTTTrack.activeCues;
+        this.updateWebVTTTextTracksDisplay(cues);
+        break;
+      }
+      case "timeupdate":
+      case "durationchange": {
+        let video = this.getWeakVideo();
+        let currentTime = this.videoWrapper.getCurrentTime(video);
+        let duration = this.videoWrapper.getDuration(video);
+        let scrubberPosition = currentTime === 0 ? 0 : currentTime / duration;
+        let timestamp = this.videoWrapper.formatTimestamp(
+          currentTime,
+          duration
+        );
+        // There's no point in sending this message unless we have a
+        // reasonable timestamp.
+        if (timestamp !== undefined && lazy.IMPROVED_CONTROLS_ENABLED_PREF) {
+          this.sendAsyncMessage(
+            "PictureInPicture:SetTimestampAndScrubberPosition",
+            {
+              scrubberPosition,
+              timestamp,
+            }
+          );
+        }
+        break;
+      }
+    }
+  }
+
+  /**
+   * Tells the parent to close a pre-existing Picture-in-Picture
+   * window.
+   *
+   * @return {Promise}
+   *
+   * @resolves {undefined} Once the pre-existing Picture-in-Picture
+   * window has unloaded.
+   */
+  async closePictureInPicture({ reason }) {
+    let video = this.getWeakVideo();
+    if (video) {
+      this.untrackOriginatingVideo(video);
+    }
+    this.sendAsyncMessage("PictureInPicture:Close", {
+      reason,
+    });
+
+    let playerContent = this.getWeakPlayerContent();
+    if (playerContent) {
+      if (!playerContent.closed) {
+        await new Promise(resolve => {
+          playerContent.addEventListener("unload", resolve, {
+            once: true,
+          });
+        });
+      }
+      // Nothing should be holding a reference to the Picture-in-Picture
+      // player window content at this point, but just in case, we'll
+      // clear the weak reference directly so nothing else can get a hold
+      // of it from this angle.
+      this.weakPlayerContent = null;
+    }
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "PictureInPicture:SetupPlayer": {
+        const { videoRef } = message.data;
+        this.setupPlayer(videoRef);
+        break;
+      }
+      case "PictureInPicture:Play": {
+        this.play();
+        break;
+      }
+      case "PictureInPicture:Pause": {
+        if (message.data && message.data.reason == "pip-closed") {
+          let video = this.getWeakVideo();
+
+          // Currently in Firefox srcObjects are MediaStreams. However, by spec a srcObject
+          // can be either a MediaStream, MediaSource or Blob. In case of future changes
+          // we do not want to pause MediaStream srcObjects and we want to maintain current
+          // behavior for non-MediaStream srcObjects.
+          if (video && MediaStream.isInstance(video.srcObject)) {
+            break;
+          }
+        }
+        this.pause();
+        break;
+      }
+      case "PictureInPicture:Mute": {
+        this.mute();
+        break;
+      }
+      case "PictureInPicture:Unmute": {
+        this.unmute();
+        break;
+      }
+      case "PictureInPicture:SeekForward":
+      case "PictureInPicture:SeekBackward": {
+        let selectedTime;
+        let video = this.getWeakVideo();
+        let currentTime = this.videoWrapper.getCurrentTime(video);
+        if (message.name == "PictureInPicture:SeekBackward") {
+          selectedTime = currentTime - SEEK_TIME_SECS;
+          selectedTime = selectedTime >= 0 ? selectedTime : 0;
+        } else {
+          const maxtime = this.videoWrapper.getDuration(video);
+          selectedTime = currentTime + SEEK_TIME_SECS;
+          selectedTime = selectedTime <= maxtime ? selectedTime : maxtime;
+        }
+        this.videoWrapper.setCurrentTime(video, selectedTime);
+        break;
+      }
+      case "PictureInPicture:KeyDown": {
+        this.keyDown(message.data);
+        break;
+      }
+      case "PictureInPicture:EnterFullscreen":
+      case "PictureInPicture:ExitFullscreen": {
+        let textTracks = this.document.getElementById("texttracks");
+        if (textTracks) {
+          this.moveTextTracks(message.data);
+        }
+        break;
+      }
+      case "PictureInPicture:ShowVideoControls":
+      case "PictureInPicture:HideVideoControls": {
+        let textTracks = this.document.getElementById("texttracks");
+        if (textTracks) {
+          this.moveTextTracks(message.data);
+        }
+        break;
+      }
+      case "PictureInPicture:ToggleTextTracks": {
+        this.toggleTextTracks();
+        break;
+      }
+      case "PictureInPicture:ChangeFontSizeTextTracks": {
+        this.setTextTrackFontSize();
+        break;
+      }
+      case "PictureInPicture:SetVideoTime": {
+        const { scrubberPosition, wasPlaying } = message.data;
+        this.setVideoTime(scrubberPosition, wasPlaying);
+        break;
+      }
+    }
+  }
+
+  /**
+   * Set the current time of the video based of the position of the scrubber
+   * @param {Number} scrubberPosition A number between 0 and 1 representing the position of the scrubber
+   */
+  setVideoTime(scrubberPosition, wasPlaying) {
+    const video = this.getWeakVideo();
+    let duration = this.videoWrapper.getDuration(video);
+    let currentTime = scrubberPosition * duration;
+    this.videoWrapper.setCurrentTime(video, currentTime, wasPlaying);
+  }
+
+  /**
+   * @returns {boolean} true if a textTrack with mode "hidden" should be treated as "showing"
+   */
+  shouldShowHiddenTextTracks() {
+    const video = this.getWeakVideo();
+    if (!video) {
+      return false;
+    }
+    const { documentURI } = video.ownerDocument;
+    if (!documentURI) {
+      return false;
+    }
+    for (let [override, { showHiddenTextTracks }] of lazy.gSiteOverrides) {
+      if (override.matches(documentURI) && showHiddenTextTracks) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Updates this._currentWebVTTTrack if an active track is found
+   * for the originating video.
+   * @param {TextTrackList} textTrackList list of text tracks
+   */
+  setActiveTextTrack(textTrackList) {
+    this._currentWebVTTTrack = null;
+
+    for (let i = 0; i < textTrackList.length; i++) {
+      let track = textTrackList[i];
+      let isCCText = track.kind === "subtitles" || track.kind === "captions";
+      let shouldShowTrack =
+        track.mode === "showing" ||
+        (track.mode === "hidden" && this.shouldShowHiddenTextTracks());
+      if (isCCText && shouldShowTrack && track.cues) {
+        this._currentWebVTTTrack = track;
+        break;
+      }
+    }
+  }
+
+  /**
+   * Set the font size on the PiP window using the current font size value from
+   * the "media.videocontrols.picture-in-picture.display-text-tracks.size" pref
+   */
+  setTextTrackFontSize() {
+    const fontSize = Services.prefs.getStringPref(
+      TEXT_TRACK_FONT_SIZE,
+      "medium"
+    );
+    const root = this.document.querySelector(":root");
+    if (fontSize === "small") {
+      root.style.setProperty("--font-scale", "0.03");
+    } else if (fontSize === "large") {
+      root.style.setProperty("--font-scale", "0.09");
+    } else {
+      root.style.setProperty("--font-scale", "0.06");
+    }
+  }
+
+  /**
+   * Keeps an eye on the originating video's document. If it ever
+   * goes away, this will cause the Picture-in-Picture window for any
+   * of its content to go away as well.
+   */
+  trackOriginatingVideo(originatingVideo) {
+    this.observerFunction = (subject, topic, data) => {
+      this.observe(subject, topic, data);
+    };
+    Services.prefs.addObserver(
+      "media.videocontrols.picture-in-picture.display-text-tracks.enabled",
+      this.observerFunction
+    );
+
+    let originatingWindow = originatingVideo.ownerGlobal;
+    if (originatingWindow) {
+      originatingWindow.addEventListener("pagehide", this);
+      originatingVideo.addEventListener("play", this);
+      originatingVideo.addEventListener("pause", this);
+      originatingVideo.addEventListener("volumechange", this);
+      originatingVideo.addEventListener("resize", this);
+      originatingVideo.addEventListener("emptied", this);
+      originatingVideo.addEventListener("timeupdate", this);
+
+      if (lazy.DISPLAY_TEXT_TRACKS_PREF) {
+        this.setupTextTracks(originatingVideo);
+      }
+
+      let chromeEventHandler = originatingWindow.docShell.chromeEventHandler;
+      chromeEventHandler.addEventListener(
+        "MozDOMFullscreen:Request",
+        this,
+        true
+      );
+      chromeEventHandler.addEventListener(
+        "MozStopPictureInPicture",
+        this,
+        true
+      );
+    }
+  }
+
+  setUpCaptionChangeListener(originatingVideo) {
+    if (this.videoWrapper) {
+      this.videoWrapper.setCaptionContainerObserver(originatingVideo, this);
+    }
+  }
+
+  /**
+   * Stops tracking the originating video's document. This should
+   * happen once the Picture-in-Picture window goes away (or is about
+   * to go away), and we no longer care about hearing when the originating
+   * window's document unloads.
+   */
+  untrackOriginatingVideo(originatingVideo) {
+    Services.prefs.removeObserver(
+      "media.videocontrols.picture-in-picture.display-text-tracks.enabled",
+      this.observerFunction
+    );
+
+    let originatingWindow = originatingVideo.ownerGlobal;
+    if (originatingWindow) {
+      originatingWindow.removeEventListener("pagehide", this);
+      originatingVideo.removeEventListener("play", this);
+      originatingVideo.removeEventListener("pause", this);
+      originatingVideo.removeEventListener("volumechange", this);
+      originatingVideo.removeEventListener("resize", this);
+      originatingVideo.removeEventListener("emptied", this);
+      originatingVideo.removeEventListener("timeupdate", this);
+
+      if (lazy.DISPLAY_TEXT_TRACKS_PREF) {
+        this.removeTextTracks(originatingVideo);
+      }
+
+      let chromeEventHandler = originatingWindow.docShell.chromeEventHandler;
+      chromeEventHandler.removeEventListener(
+        "MozDOMFullscreen:Request",
+        this,
+        true
+      );
+      chromeEventHandler.removeEventListener(
+        "MozStopPictureInPicture",
+        this,
+        true
+      );
+    }
+  }
+
+  /**
+   * Runs in an instance of PictureInPictureChild for the
+   * player window's content, and not the originating video
+   * content. Sets up the player so that it clones the originating
+   * video. If anything goes wrong during set up, a message is
+   * sent to the parent to close the Picture-in-Picture window.
+   *
+   * @param videoRef {ContentDOMReference}
+   *    A reference to the video element that a Picture-in-Picture window
+   *    is being created for
+   * @return {Promise}
+   * @resolves {undefined} Once the player window has been set up
+   * properly, or a pre-existing Picture-in-Picture window has gone
+   * away due to an unexpected error.
+   */
+  async setupPlayer(videoRef) {
+    const video = await lazy.ContentDOMReference.resolve(videoRef);
+
+    this.weakVideo = Cu.getWeakReference(video);
+    let originatingVideo = this.getWeakVideo();
+    if (!originatingVideo) {
+      // If the video element has gone away before we've had a chance to set up
+      // Picture-in-Picture for it, tell the parent to close the Picture-in-Picture
+      // window.
+      await this.closePictureInPicture({ reason: "setupFailure" });
+      return;
+    }
+
+    this.videoWrapper = applyWrapper(this, originatingVideo);
+
+    let loadPromise = new Promise(resolve => {
+      this.contentWindow.addEventListener("load", resolve, {
+        once: true,
+        mozSystemGroup: true,
+        capture: true,
+      });
+    });
+    this.contentWindow.location.reload();
+    await loadPromise;
+
+    // We're committed to adding the video to this window now. Ensure we track
+    // the content window before we do so, so that the toggle actor can
+    // distinguish this new video we're creating from web-controlled ones.
+    this.weakPlayerContent = Cu.getWeakReference(this.contentWindow);
+    gPlayerContents.add(this.contentWindow);
+
+    let doc = this.document;
+    let playerVideo = doc.createElement("video");
+    playerVideo.id = "playervideo";
+    let textTracks = doc.createElement("div");
+
+    doc.body.style.overflow = "hidden";
+    doc.body.style.margin = "0";
+
+    // Force the player video to assume maximum height and width of the
+    // containing window
+    playerVideo.style.height = "100vh";
+    playerVideo.style.width = "100vw";
+    playerVideo.style.backgroundColor = "#000";
+
+    // Load text tracks container in the content process so that
+    // we can load text tracks without having to constantly
+    // access the parent process.
+    textTracks.id = "texttracks";
+    // When starting pip, player controls are expected to appear.
+    textTracks.setAttribute("overlap-video-controls", true);
+    doc.body.appendChild(playerVideo);
+    doc.body.appendChild(textTracks);
+    // Load text tracks stylesheet
+    let textTracksStyleSheet = this.createTextTracksStyleSheet();
+    doc.head.appendChild(textTracksStyleSheet);
+
+    this.setTextTrackFontSize();
+
+    originatingVideo.cloneElementVisually(playerVideo);
+
+    let shadowRoot = originatingVideo.openOrClosedShadowRoot;
+    if (originatingVideo.getTransformToViewport().a == -1) {
+      shadowRoot.firstChild.setAttribute("flipped", true);
+      playerVideo.style.transform = "scaleX(-1)";
+    }
+
+    this.onCueChange = this.onCueChange.bind(this);
+    this.trackOriginatingVideo(originatingVideo);
+
+    // A request to open PIP implies that the user intends to be interacting
+    // with the page, even if they open PIP by some means outside of the page
+    // itself (e.g., the keyboard shortcut or the page action button). So we
+    // manually record that the document has been activated via user gesture
+    // to make sure the video can be played regardless of autoplay permissions.
+    originatingVideo.ownerDocument.notifyUserGestureActivation();
+
+    this.contentWindow.addEventListener(
+      "unload",
+      () => {
+        let video = this.getWeakVideo();
+        if (video) {
+          this.untrackOriginatingVideo(video);
+          video.stopCloningElementVisually();
+        }
+        this.weakVideo = null;
+      },
+      { once: true }
+    );
+  }
+
+  play() {
+    let video = this.getWeakVideo();
+    if (video && this.videoWrapper) {
+      this.videoWrapper.play(video);
+    }
+  }
+
+  pause() {
+    let video = this.getWeakVideo();
+    if (video && this.videoWrapper) {
+      this.videoWrapper.pause(video);
+    }
+  }
+
+  mute() {
+    let video = this.getWeakVideo();
+    if (video && this.videoWrapper) {
+      this.videoWrapper.setMuted(video, true);
+    }
+  }
+
+  unmute() {
+    let video = this.getWeakVideo();
+    if (video && this.videoWrapper) {
+      this.videoWrapper.setMuted(video, false);
+    }
+  }
+
+  onCueChange(e) {
+    if (!lazy.DISPLAY_TEXT_TRACKS_PREF) {
+      this.updateWebVTTTextTracksDisplay(null);
+    } else {
+      const cues = this._currentWebVTTTrack.activeCues;
+      this.updateWebVTTTextTracksDisplay(cues);
+    }
+  }
+
+  /**
+   * This checks if a given keybinding has been disabled for the specific site
+   * currently being viewed.
+   */
+  isKeyDisabled(key) {
+    const video = this.getWeakVideo();
+    if (!video) {
+      return false;
+    }
+    const { documentURI } = video.ownerDocument;
+    if (!documentURI) {
+      return true;
+    }
+    for (let [override, { disabledKeyboardControls }] of lazy.gSiteOverrides) {
+      if (
+        disabledKeyboardControls !== undefined &&
+        override.matches(documentURI)
+      ) {
+        if (disabledKeyboardControls === lazy.KEYBOARD_CONTROLS.ALL) {
+          return true;
+        }
+        return !!(disabledKeyboardControls & key);
+      }
+    }
+    return false;
+  }
+
+  /**
+   * This reuses the keyHandler logic in the VideoControlsWidget
+   * https://searchfox.org/mozilla-central/rev/cfd1cc461f1efe0d66c2fdc17c024a203d5a2fd8/toolkit/content/widgets/videocontrols.js#1687-1810.
+   * There are future plans to eventually combine the two implementations.
+   */
+  /* eslint-disable complexity */
+  keyDown({ altKey, shiftKey, metaKey, ctrlKey, keyCode }) {
+    let video = this.getWeakVideo();
+    if (!video) {
+      return;
+    }
+
+    var keystroke = "";
+    if (altKey) {
+      keystroke += "alt-";
+    }
+    if (shiftKey) {
+      keystroke += "shift-";
+    }
+    if (this.contentWindow.navigator.platform.startsWith("Mac")) {
+      if (metaKey) {
+        keystroke += "accel-";
+      }
+      if (ctrlKey) {
+        keystroke += "control-";
+      }
+    } else {
+      if (metaKey) {
+        keystroke += "meta-";
+      }
+      if (ctrlKey) {
+        keystroke += "accel-";
+      }
+    }
+
+    switch (keyCode) {
+      case this.contentWindow.KeyEvent.DOM_VK_UP:
+        keystroke += "upArrow";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_DOWN:
+        keystroke += "downArrow";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_LEFT:
+        keystroke += "leftArrow";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_RIGHT:
+        keystroke += "rightArrow";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_HOME:
+        keystroke += "home";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_END:
+        keystroke += "end";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_SPACE:
+        keystroke += "space";
+        break;
+      case this.contentWindow.KeyEvent.DOM_VK_W:
+        keystroke += "w";
+        break;
+    }
+
+    const isVideoStreaming = this.videoWrapper.isLive(video);
+    var oldval, newval;
+
+    try {
+      switch (keystroke) {
+        case "space" /* Toggle Play / Pause */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.PLAY_PAUSE)) {
+            return;
+          }
+
+          if (
+            this.videoWrapper.getPaused(video) ||
+            this.videoWrapper.getEnded(video)
+          ) {
+            this.videoWrapper.play(video);
+          } else {
+            this.videoWrapper.pause(video);
+          }
+
+          break;
+        case "accel-w" /* Close video */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.CLOSE)) {
+            return;
+          }
+          this.pause();
+          this.closePictureInPicture({ reason: "closePlayerShortcut" });
+          break;
+        case "downArrow" /* Volume decrease */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.VOLUME)) {
+            return;
+          }
+          oldval = this.videoWrapper.getVolume(video);
+          this.videoWrapper.setVolume(video, oldval < 0.1 ? 0 : oldval - 0.1);
+          this.videoWrapper.setMuted(video, false);
+          break;
+        case "upArrow" /* Volume increase */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.VOLUME)) {
+            return;
+          }
+          oldval = this.videoWrapper.getVolume(video);
+          this.videoWrapper.setVolume(video, oldval > 0.9 ? 1 : oldval + 0.1);
+          this.videoWrapper.setMuted(video, false);
+          break;
+        case "accel-downArrow" /* Mute */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.MUTE_UNMUTE)) {
+            return;
+          }
+          this.videoWrapper.setMuted(video, true);
+          break;
+        case "accel-upArrow" /* Unmute */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.MUTE_UNMUTE)) {
+            return;
+          }
+          this.videoWrapper.setMuted(video, false);
+          break;
+        case "leftArrow": /* Seek back 5 seconds */
+        case "accel-leftArrow" /* Seek back 10% */:
+          if (
+            this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.SEEK) ||
+            (isVideoStreaming &&
+              this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.LIVE_SEEK))
+          ) {
+            return;
+          }
+
+          oldval = this.videoWrapper.getCurrentTime(video);
+          if (keystroke == "leftArrow") {
+            newval = oldval - SEEK_TIME_SECS;
+          } else {
+            newval = oldval - this.videoWrapper.getDuration(video) / 10;
+          }
+          this.videoWrapper.setCurrentTime(video, newval >= 0 ? newval : 0);
+          break;
+        case "rightArrow": /* Seek forward 5 seconds */
+        case "accel-rightArrow" /* Seek forward 10% */:
+          if (
+            this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.SEEK) ||
+            (isVideoStreaming &&
+              this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.LIVE_SEEK))
+          ) {
+            return;
+          }
+
+          oldval = this.videoWrapper.getCurrentTime(video);
+          var maxtime = this.videoWrapper.getDuration(video);
+          if (keystroke == "rightArrow") {
+            newval = oldval + SEEK_TIME_SECS;
+          } else {
+            newval = oldval + maxtime / 10;
+          }
+          let selectedTime = newval <= maxtime ? newval : maxtime;
+          this.videoWrapper.setCurrentTime(video, selectedTime);
+          break;
+        case "home" /* Seek to beginning */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.SEEK)) {
+            return;
+          }
+          if (!isVideoStreaming) {
+            this.videoWrapper.setCurrentTime(video, 0);
+          }
+          break;
+        case "end" /* Seek to end */:
+          if (this.isKeyDisabled(lazy.KEYBOARD_CONTROLS.SEEK)) {
+            return;
+          }
+
+          let duration = this.videoWrapper.getDuration(video);
+          if (
+            !isVideoStreaming &&
+            this.videoWrapper.getCurrentTime(video) != duration
+          ) {
+            this.videoWrapper.setCurrentTime(video, duration);
+          }
+          break;
+        default:
+      }
+    } catch (e) {
+      /* ignore any exception from setting video.currentTime */
+    }
+  }
+
+  get isSubtitlesEnabled() {
+    return this.#subtitlesEnabled;
+  }
+
+  set isSubtitlesEnabled(val) {
+    if (val) {
+      Services.telemetry.recordEvent(
+        "pictureinpicture",
+        "subtitles_shown",
+        "subtitles",
+        null,
+        {
+          webVTTSubtitles: (!!this.getWeakVideo().textTracks
+            ?.length).toString(),
+        }
+      );
+    } else {
+      this.sendAsyncMessage("PictureInPicture:DisableSubtitlesButton");
+    }
+    this.#subtitlesEnabled = val;
+  }
+}
+
+/**
+ * The PictureInPictureChildVideoWrapper class handles providing a path to a script that
+ * defines a "site wrapper" for the original <video> (or other controls API provided
+ * by the site) to command it.
+ *
+ * This "site wrapper" provided to PictureInPictureChildVideoWrapper is a script file that
+ * defines a class called `PictureInPictureVideoWrapper` and exports it. These scripts can
+ * be found under "browser/extensions/pictureinpicture/video-wrappers" as part of the
+ * Picture-In-Picture addon.
+ *
+ * Site wrappers need to adhere to a specific interface to work properly with
+ * PictureInPictureChildVideoWrapper:
+ *
+ * - The "site wrapper" script must export a class called "PictureInPictureVideoWrapper"
+ * - Method names on a site wrapper class should match its caller's name
+ *   (i.e: PictureInPictureChildVideoWrapper.play will only call `play` on a site-wrapper, if available)
+ */
+class PictureInPictureChildVideoWrapper {
+  #sandbox;
+  #siteWrapper;
+  #PictureInPictureChild;
+
+  /**
+   * Create a wrapper for the original <video>
+   *
+   * @param {String|null} videoWrapperScriptPath
+   *        Path to a wrapper script from the Picture-in-Picture addon. If a wrapper isn't
+   *        provided to the class, then we fallback on a default implementation for
+   *        commanding the original <video>.
+   * @param {HTMLVideoElement} video
+   *        The original <video> we want to create a wrapper class for.
+   * @param {Object} pipChild
+   *        Reference to PictureInPictureChild class calling this function.
+   */
+  constructor(videoWrapperScriptPath, video, pipChild) {
+    this.#sandbox = videoWrapperScriptPath
+      ? this.#createSandbox(videoWrapperScriptPath, video)
+      : null;
+    this.#PictureInPictureChild = pipChild;
+  }
+
+  /**
+   * Handles calling methods defined on the site wrapper class to perform video
+   * controls operations on the source video. If the method doesn't exist,
+   * or if an error is thrown while calling it, use a fallback implementation.
+   *
+   * @param {String} methodInfo.name
+   *        The method name to call.
+   * @param {Array} methodInfo.args
+   *        Arguments to pass to the site wrapper method being called.
+   * @param {Function} methodInfo.fallback
+   *        A fallback function that's invoked when a method doesn't exist on the site
+   *        wrapper class or an error is thrown while calling a method
+   * @param {Function} methodInfo.validateReturnVal
+   *        Validates whether or not the return value of the wrapper method is correct.
+   *        If this isn't provided or if it evaluates false for a return value, then
+   *        return null.
+   *
+   * @returns The expected output of the wrapper function.
+   */
+  #callWrapperMethod({ name, args = [], fallback = () => {}, validateRetVal }) {
+    try {
+      const wrappedMethod = this.#siteWrapper?.[name];
+      if (typeof wrappedMethod === "function") {
+        let retVal = wrappedMethod.call(this.#siteWrapper, ...args);
+
+        if (!validateRetVal) {
+          lazy.logConsole.error(
+            `No return value validator was provided for method ${name}(). Returning null.`
+          );
+          return null;
+        }
+
+        if (!validateRetVal(retVal)) {
+          lazy.logConsole.error(
+            `Calling method ${name}() returned an unexpected value: ${retVal}. Returning null.`
+          );
+          return null;
+        }
+
+        return retVal;
+      }
+    } catch (e) {
+      lazy.logConsole.error(
+        `There was an error while calling ${name}(): `,
+        e.message
+      );
+    }
+
+    return fallback();
+  }
+
+  /**
+   * Creates a sandbox with Xray vision to execute content code in an unprivileged
+   * context. This way, privileged code (PictureInPictureChild) can call into the
+   * sandbox to perform video controls operations on the originating video
+   * (content code) and still be protected from direct access by it.
+   *
+   * @param {String} videoWrapperScriptPath
+   *        Path to a wrapper script from the Picture-in-Picture addon.
+   * @param {HTMLVideoElement} video
+   *        The source video element whose window to create a sandbox for.
+   */
+  #createSandbox(videoWrapperScriptPath, video) {
+    const addonPolicy = WebExtensionPolicy.getByID(
+      "pictureinpicture@mozilla.org"
+    );
+    let wrapperScriptUrl = addonPolicy.getURL(videoWrapperScriptPath);
+    let originatingWin = video.ownerGlobal;
+    let originatingDoc = video.ownerDocument;
+
+    let sandbox = Cu.Sandbox([originatingDoc.nodePrincipal], {
+      sandboxName: "Picture-in-Picture video wrapper sandbox",
+      sandboxPrototype: originatingWin,
+      sameZoneAs: originatingWin,
+      wantXrays: false,
+    });
+
+    try {
+      Services.scriptloader.loadSubScript(wrapperScriptUrl, sandbox);
+    } catch (e) {
+      Cu.nukeSandbox(sandbox);
+      lazy.logConsole.error(
+        "Error loading wrapper script for Picture-in-Picture",
+        e
+      );
+      return null;
+    }
+
+    // The prototype of the wrapper class instantiated from the sandbox with Xray
+    // vision is `Object` and not actually `PictureInPictureVideoWrapper`. But we
+    // need to be able to access methods defined on this class to perform site-specific
+    // video control operations otherwise we fallback to a default implementation.
+    // Because of this, we need to "waive Xray vision" by adding `.wrappedObject` to the
+    // end.
+    this.#siteWrapper = new sandbox.PictureInPictureVideoWrapper(
+      video
+    ).wrappedJSObject;
+
+    return sandbox;
+  }
+
+  #isBoolean(val) {
+    return typeof val === "boolean";
+  }
+
+  #isNumber(val) {
+    return typeof val === "number";
+  }
+
+  /**
+   * Destroys the sandbox for the site wrapper class
+   */
+  destroy() {
+    if (this.#sandbox) {
+      Cu.nukeSandbox(this.#sandbox);
+    }
+  }
+
+  /**
+   * Function to display the captions on the PiP window
+   * @param text The captions to be shown on the PiP window
+   */
+  updatePiPTextTracks(text) {
+    if (!this.#PictureInPictureChild.isSubtitlesEnabled && text) {
+      this.#PictureInPictureChild.isSubtitlesEnabled = true;
+      this.#PictureInPictureChild.sendAsyncMessage(
+        "PictureInPicture:EnableSubtitlesButton"
+      );
+    }
+    let pipWindowTracksContainer =
+      this.#PictureInPictureChild.document.getElementById("texttracks");
+    pipWindowTracksContainer.textContent = text;
+  }
+
+  /* Video methods to be used for video controls from the PiP window. */
+
+  /**
+   * OVERRIDABLE - calls the play() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to handle video
+   * behaviour when a video is played.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   */
+  play(video) {
+    return this.#callWrapperMethod({
+      name: "play",
+      args: [video],
+      fallback: () => video.play(),
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the pause() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to handle video
+   * behaviour when a video is paused.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   */
+  pause(video) {
+    return this.#callWrapperMethod({
+      name: "pause",
+      args: [video],
+      fallback: () => video.pause(),
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the getPaused() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to determine if
+   * a video is paused or not.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Boolean} Boolean value true if paused, or false if video is still playing
+   */
+  getPaused(video) {
+    return this.#callWrapperMethod({
+      name: "getPaused",
+      args: [video],
+      fallback: () => video.paused,
+      validateRetVal: retVal => this.#isBoolean(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the getEnded() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to determine if
+   * video playback or streaming has stopped.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Boolean} Boolean value true if the video has ended, or false if still playing
+   */
+  getEnded(video) {
+    return this.#callWrapperMethod({
+      name: "getEnded",
+      args: [video],
+      fallback: () => video.ended,
+      validateRetVal: retVal => this.#isBoolean(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the getDuration() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to get the current
+   * duration of a video in seconds.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Number} Duration of the video in seconds
+   */
+  getDuration(video) {
+    return this.#callWrapperMethod({
+      name: "getDuration",
+      args: [video],
+      fallback: () => video.duration,
+      validateRetVal: retVal => this.#isNumber(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the getCurrentTime() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to get the current
+   * time of a video in seconds.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Number} Current time of the video in seconds
+   */
+  getCurrentTime(video) {
+    return this.#callWrapperMethod({
+      name: "getCurrentTime",
+      args: [video],
+      fallback: () => video.currentTime,
+      validateRetVal: retVal => this.#isNumber(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the setCurrentTime() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to set the current
+   * time of a video.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @param {Number} position
+   *  The current playback time of the video
+   * @param {Boolean} wasPlaying
+   *  True if the video was playing before seeking else false
+   */
+  setCurrentTime(video, position, wasPlaying) {
+    return this.#callWrapperMethod({
+      name: "setCurrentTime",
+      args: [video, position, wasPlaying],
+      fallback: () => {
+        video.currentTime = position;
+      },
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * Return hours, minutes, and seconds from seconds
+   * @param {Number} aSeconds
+   *  The time in seconds
+   * @returns {String} Timestamp string
+   **/
+  timeFromSeconds(aSeconds) {
+    aSeconds = isNaN(aSeconds) ? 0 : Math.round(aSeconds);
+    let seconds = Math.floor(aSeconds % 60),
+      minutes = Math.floor((aSeconds / 60) % 60),
+      hours = Math.floor(aSeconds / 3600);
+    seconds = seconds < 10 ? "0" + seconds : seconds;
+    minutes = hours > 0 && minutes < 10 ? "0" + minutes : minutes;
+    return aSeconds < 3600
+      ? `${minutes}:${seconds}`
+      : `${hours}:${minutes}:${seconds}`;
+  }
+
+  /**
+   * Format a timestamp from current time and total duration,
+   * output as a string in the form '0:00 / 0:00'
+   * @param {Number} aCurrentTime
+   *  The current time in seconds
+   * @param {Number} aDuration
+   *  The total duration in seconds
+   * @returns {String} Formatted timestamp
+   **/
+  formatTimestamp(aCurrentTime, aDuration) {
+    // We can't format numbers that can't be represented as decimal digits.
+    if (!Number.isFinite(aCurrentTime) || !Number.isFinite(aDuration)) {
+      return undefined;
+    }
+
+    return `${this.timeFromSeconds(aCurrentTime)} / ${this.timeFromSeconds(
+      aDuration
+    )}`;
+  }
+
+  /**
+   * OVERRIDABLE - calls the getVolume() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to get the volume
+   * value of a video.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Number} Volume of the video between 0 (muted) and 1 (loudest)
+   */
+  getVolume(video) {
+    return this.#callWrapperMethod({
+      name: "getVolume",
+      args: [video],
+      fallback: () => video.volume,
+      validateRetVal: retVal => this.#isNumber(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the setVolume() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to set the volume
+   * value of a video.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @param {Number} volume
+   *  Value between 0 (muted) and 1 (loudest)
+   */
+  setVolume(video, volume) {
+    return this.#callWrapperMethod({
+      name: "setVolume",
+      args: [video, volume],
+      fallback: () => {
+        video.volume = volume;
+      },
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the isMuted() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to get the mute
+   * state a video.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @param {Boolean} shouldMute
+   *  Boolean value true to mute the video, or false to unmute the video
+   */
+  isMuted(video) {
+    return this.#callWrapperMethod({
+      name: "isMuted",
+      args: [video],
+      fallback: () => video.muted,
+      validateRetVal: retVal => this.#isBoolean(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the setMuted() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to mute or unmute
+   * a video.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @param {Boolean} shouldMute
+   *  Boolean value true to mute the video, or false to unmute the video
+   */
+  setMuted(video, shouldMute) {
+    return this.#callWrapperMethod({
+      name: "setMuted",
+      args: [video, shouldMute],
+      fallback: () => {
+        video.muted = shouldMute;
+      },
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the setCaptionContainerObserver() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to listen for any cue changes in a
+   * video's caption container and execute a callback function responsible for updating the pip window's text tracks container whenever
+   * a cue change is triggered {@see updatePiPTextTracks()}.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @param {Function} callback
+   *  The callback function to be executed when cue changes are detected
+   */
+  setCaptionContainerObserver(video, callback) {
+    return this.#callWrapperMethod({
+      name: "setCaptionContainerObserver",
+      args: [
+        video,
+        text => {
+          this.updatePiPTextTracks(text);
+        },
+      ],
+      fallback: () => {},
+      validateRetVal: retVal => retVal == null,
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the shouldHideToggle() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to determine if the pip toggle
+   * for a video should be hidden by the site wrapper.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   * @returns {Boolean} Boolean value true if the pip toggle should be hidden by the site wrapper, or false if it should not
+   */
+  shouldHideToggle(video) {
+    return this.#callWrapperMethod({
+      name: "shouldHideToggle",
+      args: [video],
+      fallback: () => false,
+      validateRetVal: retVal => this.#isBoolean(retVal),
+    });
+  }
+
+  /**
+   * OVERRIDABLE - calls the isLive() method defined in the site wrapper script. Runs a fallback implementation
+   * if the method does not exist or if an error is thrown while calling it. This method is meant to get if the
+   * video is a live stream.
+   * @param {HTMLVideoElement} video
+   *  The originating video source element
+   */
+  isLive(video) {
+    return this.#callWrapperMethod({
+      name: "isLive",
+      args: [video],
+      fallback: () => video.duration === Infinity,
+      validateRetVal: retVal => this.#isBoolean(retVal),
+    });
+  }
+}
diff --git a/toolkit/actors/PopupBlockingChild.sys.mjs b/toolkit/actors/PopupBlockingChild.sys.mjs
new file mode 100644
index 0000000000..053f9683c5
--- /dev/null
+++ b/toolkit/actors/PopupBlockingChild.sys.mjs
@@ -0,0 +1,147 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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 no-unused-vars: ["error", {args: "none"}] */
+
+// The maximum number of popup information we'll send to the parent.
+const MAX_SENT_POPUPS = 15;
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+export class PopupBlockingChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    this.weakDocStates = new WeakMap();
+  }
+
+  /**
+   * Returns the state for the current document referred to via
+   * this.document. If no such state exists, creates it, stores it
+   * and returns it.
+   */
+  get docState() {
+    let state = this.weakDocStates.get(this.document);
+    if (!state) {
+      state = {
+        popupData: [],
+      };
+      this.weakDocStates.set(this.document, state);
+    }
+
+    return state;
+  }
+
+  receiveMessage(msg) {
+    switch (msg.name) {
+      case "UnblockPopup": {
+        let i = msg.data.index;
+        let state = this.docState;
+        let popupData = state.popupData[i];
+        if (popupData) {
+          let dwi = popupData.requestingWindow;
+
+          // If we have a requesting window and the requesting document is
+          // still the current document, open the popup.
+          if (dwi && dwi.document == popupData.requestingDocument) {
+            dwi.open(
+              popupData.popupWindowURISpec,
+              popupData.popupWindowName,
+              popupData.popupWindowFeatures
+            );
+          }
+        }
+        break;
+      }
+
+      case "GetBlockedPopupList": {
+        let state = this.docState;
+        let length = Math.min(state.popupData.length, MAX_SENT_POPUPS);
+
+        let result = [];
+
+        for (let i = 0; i < length; ++i) {
+          let popup = state.popupData[i];
+
+          let popupWindowURISpec = popup.popupWindowURISpec;
+
+          if (this.contentWindow.location.href == popupWindowURISpec) {
+            popupWindowURISpec = "<self>";
+          } else {
+            // Limit 500 chars to be sent because the URI will be cropped
+            // by the UI anyway, and data: URIs can be significantly larger.
+            popupWindowURISpec = popupWindowURISpec.substring(0, 500);
+          }
+
+          result.push({
+            popupWindowURISpec,
+          });
+        }
+
+        return result;
+      }
+    }
+
+    return null;
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "DOMPopupBlocked":
+        this.onPopupBlocked(event);
+        break;
+      case "pageshow": {
+        this.onPageShow(event);
+        break;
+      }
+    }
+  }
+
+  onPopupBlocked(event) {
+    if (event.target != this.document) {
+      return;
+    }
+
+    let state = this.docState;
+
+    // Avoid spamming the parent process with too many blocked popups.
+    if (state.popupData.length >= PopupBlockingChild.maxReportedPopups) {
+      return;
+    }
+
+    let popup = {
+      popupWindowURISpec: event.popupWindowURI
+        ? event.popupWindowURI.spec
+        : "about:blank",
+      popupWindowFeatures: event.popupWindowFeatures,
+      popupWindowName: event.popupWindowName,
+      requestingWindow: event.requestingWindow,
+      requestingDocument: event.requestingWindow.document,
+    };
+
+    state.popupData.push(popup);
+    this.updateBlockedPopups(true);
+  }
+
+  onPageShow(event) {
+    if (event.target != this.document) {
+      return;
+    }
+
+    this.updateBlockedPopups(false);
+  }
+
+  updateBlockedPopups(shouldNotify) {
+    this.sendAsyncMessage("UpdateBlockedPopups", {
+      shouldNotify,
+      count: this.docState.popupData.length,
+    });
+  }
+}
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  PopupBlockingChild,
+  "maxReportedPopups",
+  "privacy.popups.maxReported"
+);
diff --git a/toolkit/actors/PopupBlockingParent.sys.mjs b/toolkit/actors/PopupBlockingParent.sys.mjs
new file mode 100644
index 0000000000..adb49a1ba2
--- /dev/null
+++ b/toolkit/actors/PopupBlockingParent.sys.mjs
@@ -0,0 +1,268 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This class manages all popup blocking operations on a <xul:browser>, including
+ * notifying the UI about updates to the blocked popups, and allowing popups to
+ * be unblocked.
+ */
+export class PopupBlocker {
+  constructor(browser) {
+    this._browser = browser;
+    this._allBlockedPopupCounts = new WeakMap();
+    this._shouldShowNotification = false;
+  }
+
+  /**
+   * Returns whether or not there are new blocked popups for the associated
+   * <xul:browser> that the user might need to be notified about.
+   */
+  get shouldShowNotification() {
+    return this._shouldShowNotification;
+  }
+
+  /**
+   * Should be called by the UI when the user has been notified about blocked
+   * popups for the associated <xul:browser>.
+   */
+  didShowNotification() {
+    this._shouldShowNotification = false;
+  }
+
+  /**
+   * Synchronously returns the most recent count of blocked popups for
+   * the associated <xul:browser>.
+   *
+   * @return {Number}
+   *   The total number of blocked popups for this <xul:browser>.
+   */
+  getBlockedPopupCount() {
+    let totalBlockedPopups = 0;
+
+    let contextsToVisit = [this._browser.browsingContext];
+    while (contextsToVisit.length) {
+      let currentBC = contextsToVisit.pop();
+      let windowGlobal = currentBC.currentWindowGlobal;
+
+      if (!windowGlobal) {
+        continue;
+      }
+
+      let popupCountForGlobal =
+        this._allBlockedPopupCounts.get(windowGlobal) || 0;
+      totalBlockedPopups += popupCountForGlobal;
+      contextsToVisit.push(...currentBC.children);
+    }
+
+    return totalBlockedPopups;
+  }
+
+  /**
+   * Asynchronously retrieve information about the popups that have
+   * been blocked for the associated <xul:browser>. This information
+   * can be used to unblock those popups.
+   *
+   * @return {Promise}
+   * @resolves {Array}
+   *   When the blocked popup information has been gathered,
+   *   resolves with an Array of Objects with the following properties:
+   *
+   *   browsingContext {BrowsingContext}
+   *     The BrowsingContext that the popup was blocked for.
+   *
+   *   innerWindowId {Number}
+   *     The inner window ID for the blocked popup. This is used to differentiate
+   *     popups that were blocked from one page load to the next.
+   *
+   *   popupWindowURISpec {String}
+   *     A string representing part or all of the URI that tried to be opened in a
+   *     popup.
+   */
+  async getBlockedPopups() {
+    let contextsToVisit = [this._browser.browsingContext];
+    let result = [];
+    while (contextsToVisit.length) {
+      let currentBC = contextsToVisit.pop();
+      let windowGlobal = currentBC.currentWindowGlobal;
+
+      if (!windowGlobal) {
+        continue;
+      }
+
+      let popupCountForGlobal =
+        this._allBlockedPopupCounts.get(windowGlobal) || 0;
+      if (popupCountForGlobal) {
+        let actor = windowGlobal.getActor("PopupBlocking");
+        let popups = await actor.sendQuery("GetBlockedPopupList");
+
+        for (let popup of popups) {
+          if (!popup.popupWindowURISpec) {
+            continue;
+          }
+
+          result.push({
+            browsingContext: currentBC,
+            innerWindowId: windowGlobal.innerWindowId,
+            popupWindowURISpec: popup.popupWindowURISpec,
+          });
+        }
+      }
+
+      contextsToVisit.push(...currentBC.children);
+    }
+
+    return result;
+  }
+
+  /**
+   * Unblocks a popup that had been blocked. The information passed should
+   * come from the list of blocked popups returned via getBlockedPopups().
+   *
+   * Unblocking a popup causes that popup to open.
+   *
+   * @param browsingContext {BrowsingContext}
+   *   The BrowsingContext that the popup was blocked for.
+   *
+   * @param innerWindowId {Number}
+   *   The inner window ID for the blocked popup. This is used to differentiate popups
+   *   that were blocked from one page load to the next.
+   *
+   * @param popupIndex {Number}
+   *   The index of the entry in the Array returned by getBlockedPopups().
+   */
+  unblockPopup(browsingContext, innerWindowId, popupIndex) {
+    let popupFrame = browsingContext.top.embedderElement;
+    let popupBrowser = popupFrame.outerBrowser
+      ? popupFrame.outerBrowser
+      : popupFrame;
+
+    if (this._browser != popupBrowser) {
+      throw new Error(
+        "Attempting to unblock popup in a BrowsingContext no longer hosted in this browser."
+      );
+    }
+
+    let windowGlobal = browsingContext.currentWindowGlobal;
+
+    if (!windowGlobal || windowGlobal.innerWindowId != innerWindowId) {
+      // The inner window has moved on since the user clicked on
+      // the blocked popups dropdown, so we'll just exit silently.
+      return;
+    }
+
+    let actor = browsingContext.currentWindowGlobal.getActor("PopupBlocking");
+    actor.sendAsyncMessage("UnblockPopup", { index: popupIndex });
+  }
+
+  /**
+   * Goes through the most recent list of blocked popups for the associated
+   * <xul:browser> and unblocks all of them. Unblocking a popup causes the popup
+   * to open.
+   */
+  async unblockAllPopups() {
+    let popups = await this.getBlockedPopups();
+    for (let i = 0; i < popups.length; ++i) {
+      let popup = popups[i];
+      this.unblockPopup(popup.browsingContext, popup.innerWindowId, i);
+    }
+  }
+
+  /**
+   * Fires a DOMUpdateBlockedPopups chrome-only event so that the UI can
+   * update itself to represent the current state of popup blocking for
+   * the associated <xul:browser>.
+   */
+  updateBlockedPopupsUI() {
+    let event = this._browser.ownerDocument.createEvent("Events");
+    event.initEvent("DOMUpdateBlockedPopups", true, true);
+    this._browser.dispatchEvent(event);
+  }
+
+  /** Private methods **/
+
+  /**
+   * Updates the current popup count for a particular BrowsingContext based
+   * on messages from the underlying process.
+   *
+   * This should only be called by a PopupBlockingParent instance.
+   *
+   * @param browsingContext {BrowsingContext}
+   *   The BrowsingContext to update the internal blocked popup count for.
+   *
+   * @param blockedPopupData {Object}
+   *   An Object representing information about how many popups are blocked
+   *   for the BrowsingContext. The Object has the following properties:
+   *
+   *   count {Number}
+   *     The total number of blocked popups for the BrowsingContext.
+   *
+   *   shouldNotify {Boolean}
+   *     Whether or not the list of blocked popups has changed in such a way that
+   *     the UI should be updated about it.
+   */
+  _updateBlockedPopupEntries(browsingContext, blockedPopupData) {
+    let windowGlobal = browsingContext.currentWindowGlobal;
+    let { count, shouldNotify } = blockedPopupData;
+
+    if (!this.shouldShowNotification && shouldNotify) {
+      this._shouldShowNotification = true;
+    }
+
+    if (windowGlobal) {
+      this._allBlockedPopupCounts.set(windowGlobal, count);
+    }
+
+    this.updateBlockedPopupsUI();
+  }
+}
+
+/**
+ * To keep things properly encapsulated, these should only be instantiated via
+ * the PopupBlocker class for a particular <xul:browser>.
+ *
+ * Instantiated for a WindowGlobalParent for a BrowsingContext in one of two cases:
+ *
+ *   1. One or more popups have been blocked for the underlying frame represented
+ *      by the WindowGlobalParent.
+ *
+ *   2. Something in the parent process is querying a frame for information about
+ *      any popups that may have been blocked inside of it.
+ */
+export class PopupBlockingParent extends JSWindowActorParent {
+  didDestroy() {
+    this.updatePopupCountForBrowser({ count: 0, shouldNotify: false });
+  }
+
+  receiveMessage(message) {
+    if (message.name == "UpdateBlockedPopups") {
+      this.updatePopupCountForBrowser({
+        count: message.data.count,
+        shouldNotify: message.data.shouldNotify,
+      });
+    }
+  }
+
+  /**
+   * Updates the PopupBlocker for the <xul:browser> associated with this
+   * PopupBlockingParent with the most recent count of blocked popups.
+   *
+   * @param data {Object}
+   *   An Object with the following properties:
+   *
+   *     count {Number}:
+   *       The number of blocked popups for the underlying document.
+   *
+   *     shouldNotify {Boolean}:
+   *       Whether or not the list of blocked popups has changed in such a way that
+   *       the UI should be updated about it.
+   */
+  updatePopupCountForBrowser(data) {
+    let browser = this.browsingContext.top.embedderElement;
+    if (!browser) {
+      return;
+    }
+
+    browser.popupBlocker._updateBlockedPopupEntries(this.browsingContext, data);
+  }
+}
diff --git a/toolkit/actors/PrintingChild.sys.mjs b/toolkit/actors/PrintingChild.sys.mjs
new file mode 100644
index 0000000000..4ebfaf9faf
--- /dev/null
+++ b/toolkit/actors/PrintingChild.sys.mjs
@@ -0,0 +1,260 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+  ReaderMode: "resource://gre/modules/ReaderMode.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+let gPendingPreviewsMap = new Map();
+
+export class PrintingChild extends JSWindowActorChild {
+  actorCreated() {
+    // When the print preview page is loaded, the actor will change, so update
+    // the state/progress listener to the new actor.
+    let listener = gPendingPreviewsMap.get(this.browsingContext.id);
+    if (listener) {
+      listener.actor = this;
+    }
+    this.contentWindow.addEventListener("scroll", this);
+  }
+
+  didDestroy() {
+    this._scrollTask?.disarm();
+    this.contentWindow?.removeEventListener("scroll", this);
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "PrintingError": {
+        let win = event.target.defaultView;
+        let wbp = win.getInterface(Ci.nsIWebBrowserPrint);
+        let nsresult = event.detail;
+        this.sendAsyncMessage("Printing:Error", {
+          isPrinting: wbp.doingPrint,
+          nsresult,
+        });
+        break;
+      }
+
+      case "scroll":
+        if (!this._scrollTask) {
+          this._scrollTask = new lazy.DeferredTask(
+            () => this.updateCurrentPage(),
+            16,
+            16
+          );
+        }
+        this._scrollTask.arm();
+        break;
+    }
+  }
+
+  receiveMessage(message) {
+    let data = message.data;
+    switch (message.name) {
+      case "Printing:Preview:Navigate": {
+        this.navigate(data.navType, data.pageNum);
+        break;
+      }
+
+      case "Printing:Preview:ParseDocument": {
+        return this.parseDocument(
+          data.URL,
+          Services.wm.getOuterWindowWithId(data.windowID)
+        );
+      }
+    }
+
+    return undefined;
+  }
+
+  async parseDocument(URL, contentWindow) {
+    // The document in 'contentWindow' will be simplified and the resulting nodes
+    // will be inserted into this.contentWindow.
+    let thisWindow = this.contentWindow;
+
+    // By using ReaderMode primitives, we parse given document and place the
+    // resulting JS object into the DOM of current browser.
+    let article;
+    try {
+      article = await lazy.ReaderMode.parseDocument(contentWindow.document);
+    } catch (ex) {
+      console.error(ex);
+    }
+
+    await new Promise(resolve => {
+      // We make use of a web progress listener in order to know when the content we inject
+      // into the DOM has finished rendering. If our layout engine is still painting, we
+      // will wait for MozAfterPaint event to be fired.
+      let actor = thisWindow.windowGlobalChild.getActor("Printing");
+      let webProgressListener = {
+        onStateChange(webProgress, req, flags, status) {
+          if (flags & Ci.nsIWebProgressListener.STATE_STOP) {
+            webProgress.removeProgressListener(webProgressListener);
+            let domUtils = contentWindow.windowUtils;
+            // Here we tell the parent that we have parsed the document successfully
+            // using ReaderMode primitives and we are able to enter on preview mode.
+            if (domUtils.isMozAfterPaintPending) {
+              let onPaint = function () {
+                contentWindow.removeEventListener("MozAfterPaint", onPaint);
+                actor.sendAsyncMessage("Printing:Preview:ReaderModeReady");
+                resolve();
+              };
+              contentWindow.addEventListener("MozAfterPaint", onPaint);
+              // This timer is needed for when display list invalidation doesn't invalidate.
+              lazy.setTimeout(() => {
+                contentWindow.removeEventListener("MozAfterPaint", onPaint);
+                actor.sendAsyncMessage("Printing:Preview:ReaderModeReady");
+                resolve();
+              }, 100);
+            } else {
+              actor.sendAsyncMessage("Printing:Preview:ReaderModeReady");
+              resolve();
+            }
+          }
+        },
+
+        QueryInterface: ChromeUtils.generateQI([
+          "nsIWebProgressListener",
+          "nsISupportsWeakReference",
+          "nsIObserver",
+        ]),
+      };
+
+      // Here we QI the docShell into a nsIWebProgress passing our web progress listener in.
+      let webProgress = thisWindow.docShell
+        .QueryInterface(Ci.nsIInterfaceRequestor)
+        .getInterface(Ci.nsIWebProgress);
+      webProgress.addProgressListener(
+        webProgressListener,
+        Ci.nsIWebProgress.NOTIFY_STATE_REQUEST
+      );
+
+      let document = thisWindow.document;
+      document.head.innerHTML = "";
+
+      // Set base URI of document. Print preview code will read this value to
+      // populate the URL field in print settings so that it doesn't show
+      // "about:blank" as its URI.
+      let headBaseElement = document.createElement("base");
+      headBaseElement.setAttribute("href", URL);
+      document.head.appendChild(headBaseElement);
+
+      // Create link element referencing aboutReader.css and append it to head
+      let headStyleElement = document.createElement("link");
+      headStyleElement.setAttribute("rel", "stylesheet");
+      headStyleElement.setAttribute(
+        "href",
+        "chrome://global/skin/aboutReader.css"
+      );
+      headStyleElement.setAttribute("type", "text/css");
+      document.head.appendChild(headStyleElement);
+
+      // Create link element referencing simplifyMode.css and append it to head
+      headStyleElement = document.createElement("link");
+      headStyleElement.setAttribute("rel", "stylesheet");
+      headStyleElement.setAttribute(
+        "href",
+        "chrome://global/content/simplifyMode.css"
+      );
+      headStyleElement.setAttribute("type", "text/css");
+      document.head.appendChild(headStyleElement);
+
+      document.body.innerHTML = "";
+
+      // Create container div (main element) and append it to body
+      let containerElement = document.createElement("div");
+      containerElement.setAttribute("class", "container");
+      document.body.appendChild(containerElement);
+
+      // Reader Mode might return null if there's a failure when parsing the document.
+      // We'll render the error message for the Simplify Page document when that happens.
+      if (article) {
+        // Set title of document
+        document.title = article.title;
+
+        // Create header div and append it to container
+        let headerElement = document.createElement("div");
+        headerElement.setAttribute("class", "reader-header");
+        headerElement.setAttribute("class", "header");
+        containerElement.appendChild(headerElement);
+
+        // Jam the article's title and byline into header div
+        let titleElement = document.createElement("h1");
+        titleElement.setAttribute("class", "reader-title");
+        titleElement.textContent = article.title;
+        headerElement.appendChild(titleElement);
+
+        let bylineElement = document.createElement("div");
+        bylineElement.setAttribute("class", "reader-credits credits");
+        bylineElement.textContent = article.byline;
+        headerElement.appendChild(bylineElement);
+
+        // Display header element
+        headerElement.style.display = "block";
+
+        // Create content div and append it to container
+        let contentElement = document.createElement("div");
+        contentElement.setAttribute("class", "content");
+        containerElement.appendChild(contentElement);
+
+        // Jam the article's content into content div
+        let readerContent = document.createElement("div");
+        readerContent.setAttribute("class", "moz-reader-content");
+        contentElement.appendChild(readerContent);
+
+        let articleUri = Services.io.newURI(article.url);
+        let parserUtils = Cc["@mozilla.org/parserutils;1"].getService(
+          Ci.nsIParserUtils
+        );
+        let contentFragment = parserUtils.parseFragment(
+          article.content,
+          Ci.nsIParserUtils.SanitizerDropForms |
+            Ci.nsIParserUtils.SanitizerAllowStyle,
+          false,
+          articleUri,
+          readerContent
+        );
+
+        readerContent.appendChild(contentFragment);
+
+        // Display reader content element
+        readerContent.style.display = "block";
+      } else {
+        const l10n = new Localization(["toolkit/about/aboutReader.ftl"], true);
+        const errorMessage = l10n.formatValueSync("about-reader-load-error");
+
+        document.title = errorMessage;
+
+        // Create reader message div and append it to body
+        let readerMessageElement = document.createElement("div");
+        readerMessageElement.setAttribute("class", "reader-message");
+        readerMessageElement.textContent = errorMessage;
+        containerElement.appendChild(readerMessageElement);
+
+        // Display reader message element
+        readerMessageElement.style.display = "block";
+      }
+    });
+  }
+
+  updateCurrentPage() {
+    let cv = this.docShell.contentViewer;
+    cv.QueryInterface(Ci.nsIWebBrowserPrint);
+    this.sendAsyncMessage("Printing:Preview:CurrentPage", {
+      currentPage: cv.printPreviewCurrentPageNumber,
+    });
+  }
+
+  navigate(navType, pageNum) {
+    let cv = this.docShell.contentViewer;
+    cv.QueryInterface(Ci.nsIWebBrowserPrint);
+    cv.printPreviewScrollToPage(navType, pageNum);
+  }
+}
diff --git a/toolkit/actors/PrintingParent.sys.mjs b/toolkit/actors/PrintingParent.sys.mjs
new file mode 100644
index 0000000000..31b8c6bbc7
--- /dev/null
+++ b/toolkit/actors/PrintingParent.sys.mjs
@@ -0,0 +1,22 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class PrintingParent extends JSWindowActorParent {
+  receiveMessage(message) {
+    let browser = this.browsingContext.top.embedderElement;
+
+    if (message.name == "Printing:Error") {
+      browser.ownerGlobal.PrintUtils._displayPrintingError(
+        message.data.nsresult,
+        message.data.isPrinting,
+        browser
+      );
+    } else if (message.name == "Printing:Preview:CurrentPage") {
+      browser.setAttribute("current-page", message.data.currentPage);
+    }
+
+    return undefined;
+  }
+}
diff --git a/toolkit/actors/PrintingSelectionChild.sys.mjs b/toolkit/actors/PrintingSelectionChild.sys.mjs
new file mode 100644
index 0000000000..c8dfe39449
--- /dev/null
+++ b/toolkit/actors/PrintingSelectionChild.sys.mjs
@@ -0,0 +1,20 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class PrintingSelectionChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "PrintingSelection:HasSelection":
+        return this.hasSelection();
+    }
+
+    return undefined;
+  }
+
+  hasSelection() {
+    let selection = this.browsingContext.associatedWindow?.getSelection();
+    return selection && selection.type == "Range";
+  }
+}
diff --git a/toolkit/actors/PurgeSessionHistoryChild.sys.mjs b/toolkit/actors/PurgeSessionHistoryChild.sys.mjs
new file mode 100644
index 0000000000..32de7929a0
--- /dev/null
+++ b/toolkit/actors/PurgeSessionHistoryChild.sys.mjs
@@ -0,0 +1,35 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class PurgeSessionHistoryChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    if (message.name != "Browser:PurgeSessionHistory") {
+      return;
+    }
+    let sessionHistory = this.docShell.QueryInterface(
+      Ci.nsIWebNavigation
+    ).sessionHistory;
+    if (!sessionHistory) {
+      return;
+    }
+
+    // place the entry at current index at the end of the history list, so it won't get removed
+    if (sessionHistory.index < sessionHistory.count - 1) {
+      let legacy = sessionHistory.legacySHistory;
+      let indexEntry = legacy.getEntryAtIndex(sessionHistory.index);
+      indexEntry.QueryInterface(Ci.nsISHEntry);
+      legacy.addEntry(indexEntry, true);
+    }
+
+    let purge = sessionHistory.count;
+    if (this.document.location.href != "about:blank") {
+      --purge; // Don't remove the page the user's staring at from shistory
+    }
+
+    if (purge > 0) {
+      sessionHistory.legacySHistory.purgeHistory(purge);
+    }
+  }
+}
diff --git a/toolkit/actors/RemotePageChild.sys.mjs b/toolkit/actors/RemotePageChild.sys.mjs
new file mode 100644
index 0000000000..48cd4d0df9
--- /dev/null
+++ b/toolkit/actors/RemotePageChild.sys.mjs
@@ -0,0 +1,219 @@
+/* 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/. */
+
+/**
+ * RemotePageChild is a base class for an unprivileged internal page, typically
+ * an about: page. A specific implementation should subclass the RemotePageChild
+ * actor with a more specific actor for that page. Typically, the child is not
+ * needed, but the parent actor will respond to messages and provide results
+ * directly to the page.
+ */
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncPrefs: "resource://gre/modules/AsyncPrefs.sys.mjs",
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+  RemotePageAccessManager:
+    "resource://gre/modules/RemotePageAccessManager.sys.mjs",
+});
+
+export class RemotePageChild extends JSWindowActorChild {
+  actorCreated() {
+    this.listeners = new Map();
+    this.exportBaseFunctions();
+  }
+
+  exportBaseFunctions() {
+    const exportableFunctions = [
+      "RPMSendAsyncMessage",
+      "RPMSendQuery",
+      "RPMAddMessageListener",
+      "RPMRemoveMessageListener",
+      "RPMGetIntPref",
+      "RPMGetStringPref",
+      "RPMGetBoolPref",
+      "RPMSetBoolPref",
+      "RPMGetFormatURLPref",
+      "RPMIsWindowPrivate",
+    ];
+
+    this.exportFunctions(exportableFunctions);
+  }
+
+  /**
+   * Exports a list of functions to be accessible by the privileged page.
+   * Subclasses may call this function to add functions that are specific
+   * to a page. When the page calls a function, a function with the same
+   * name is called within the child actor.
+   *
+   * Only functions that appear in the whitelist in the
+   * RemotePageAccessManager for that page will be exported.
+   *
+   * @param array of function names.
+   */
+  exportFunctions(functions) {
+    let document = this.document;
+    let principal = document.nodePrincipal;
+
+    // If there is no content principal, don't export any functions.
+    if (!principal) {
+      return;
+    }
+
+    let window = this.contentWindow;
+
+    for (let fnname of functions) {
+      let allowAccess = lazy.RemotePageAccessManager.checkAllowAccessToFeature(
+        principal,
+        fnname,
+        document
+      );
+
+      if (allowAccess) {
+        // Wrap each function in an access checking function.
+        function accessCheckedFn(...args) {
+          this.checkAllowAccess(fnname, args[0]);
+          return this[fnname](...args);
+        }
+
+        Cu.exportFunction(accessCheckedFn.bind(this), window, {
+          defineAs: fnname,
+        });
+      }
+    }
+  }
+
+  handleEvent() {
+    // Do nothing. The DOMDocElementInserted event is just used to create
+    // the actor.
+  }
+
+  receiveMessage(messagedata) {
+    let message = {
+      name: messagedata.name,
+      data: messagedata.data,
+    };
+
+    let listeners = this.listeners.get(message.name);
+    if (!listeners) {
+      return;
+    }
+
+    let clonedMessage = Cu.cloneInto(message, this.contentWindow);
+    for (let listener of listeners.values()) {
+      try {
+        listener(clonedMessage);
+      } catch (e) {
+        console.error(e);
+      }
+    }
+  }
+
+  wrapPromise(promise) {
+    return new this.contentWindow.Promise((resolve, reject) =>
+      promise.then(resolve, reject)
+    );
+  }
+
+  /**
+   * Returns true if a feature cannot be accessed by the current page.
+   * Throws an exception if the feature may not be accessed.
+
+   * @param aDocument child process document to call from
+   * @param aFeature to feature to check access to
+   * @param aValue value that must be included with that feature's whitelist
+   * @returns true if access is allowed or throws an exception otherwise
+   */
+  checkAllowAccess(aFeature, aValue) {
+    let doc = this.document;
+    if (!lazy.RemotePageAccessManager.checkAllowAccess(doc, aFeature, aValue)) {
+      throw new Error(
+        "RemotePageAccessManager does not allow access to " + aFeature
+      );
+    }
+
+    return true;
+  }
+
+  addPage(aUrl, aFunctionMap) {
+    lazy.RemotePageAccessManager.addPage(aUrl, aFunctionMap);
+  }
+
+  // Implementation of functions that are exported into the page.
+
+  RPMSendAsyncMessage(aName, aData = null) {
+    this.sendAsyncMessage(aName, aData);
+  }
+
+  RPMSendQuery(aName, aData = null) {
+    return this.wrapPromise(
+      new Promise(resolve => {
+        this.sendQuery(aName, aData).then(result => {
+          resolve(Cu.cloneInto(result, this.contentWindow));
+        });
+      })
+    );
+  }
+
+  /**
+   * Adds a listener for messages. Many callbacks can be registered for the
+   * same message if necessary. An attempt to register the same callback for the
+   * same message twice will be ignored. When called the callback is passed an
+   * object with these properties:
+   *   name:   The message name
+   *   data:   Any data sent with the message
+   */
+  RPMAddMessageListener(aName, aCallback) {
+    if (!this.listeners.has(aName)) {
+      this.listeners.set(aName, new Set([aCallback]));
+    } else {
+      this.listeners.get(aName).add(aCallback);
+    }
+  }
+
+  /**
+   * Removes a listener for messages.
+   */
+  RPMRemoveMessageListener(aName, aCallback) {
+    if (!this.listeners.has(aName)) {
+      return;
+    }
+
+    this.listeners.get(aName).delete(aCallback);
+  }
+
+  RPMGetIntPref(aPref, defaultValue) {
+    // Only call with a default value if it's defined, to be able to throw
+    // errors for non-existent prefs.
+    if (defaultValue !== undefined) {
+      return Services.prefs.getIntPref(aPref, defaultValue);
+    }
+    return Services.prefs.getIntPref(aPref);
+  }
+
+  RPMGetStringPref(aPref) {
+    return Services.prefs.getStringPref(aPref);
+  }
+
+  RPMGetBoolPref(aPref, defaultValue) {
+    // Only call with a default value if it's defined, to be able to throw
+    // errors for non-existent prefs.
+    if (defaultValue !== undefined) {
+      return Services.prefs.getBoolPref(aPref, defaultValue);
+    }
+    return Services.prefs.getBoolPref(aPref);
+  }
+
+  RPMSetBoolPref(aPref, aVal) {
+    return this.wrapPromise(lazy.AsyncPrefs.set(aPref, aVal));
+  }
+
+  RPMGetFormatURLPref(aFormatURL) {
+    return Services.urlFormatter.formatURLPref(aFormatURL);
+  }
+
+  RPMIsWindowPrivate() {
+    return lazy.PrivateBrowsingUtils.isContentWindowPrivate(this.contentWindow);
+  }
+}
diff --git a/toolkit/actors/SelectChild.sys.mjs b/toolkit/actors/SelectChild.sys.mjs
new file mode 100644
index 0000000000..add2024093
--- /dev/null
+++ b/toolkit/actors/SelectChild.sys.mjs
@@ -0,0 +1,470 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+  LayoutUtils: "resource://gre/modules/LayoutUtils.sys.mjs",
+});
+
+const kStateActive = 0x00000001; // ElementState::ACTIVE
+const kStateHover = 0x00000004; // ElementState::HOVER
+
+// Duplicated in SelectParent.jsm
+// Please keep these lists in sync.
+const SUPPORTED_OPTION_OPTGROUP_PROPERTIES = [
+  "direction",
+  "color",
+  "background-color",
+  "text-shadow",
+  "text-transform",
+  "font-family",
+  "font-weight",
+  "font-size",
+  "font-style",
+];
+
+const SUPPORTED_SELECT_PROPERTIES = [
+  ...SUPPORTED_OPTION_OPTGROUP_PROPERTIES,
+  "scrollbar-width",
+  "scrollbar-color",
+];
+
+// A process global state for whether or not content thinks
+// that a <select> dropdown is open or not. This is managed
+// entirely within this module, and is read-only accessible
+// via SelectContentHelper.open.
+var gOpen = false;
+
+export var SelectContentHelper = function (aElement, aOptions, aActor) {
+  this.element = aElement;
+  this.initialSelection = aElement[aElement.selectedIndex] || null;
+  this.actor = aActor;
+  this.closedWithClickOn = false;
+  this.isOpenedViaTouch = aOptions.isOpenedViaTouch;
+  this._closeAfterBlur = true;
+  this._pseudoStylesSetup = false;
+  this._lockedDescendants = null;
+  this.init();
+  this.showDropDown();
+  this._updateTimer = new lazy.DeferredTask(this._update.bind(this), 0);
+};
+
+Object.defineProperty(SelectContentHelper, "open", {
+  get() {
+    return gOpen;
+  },
+});
+
+SelectContentHelper.prototype = {
+  init() {
+    let win = this.element.ownerGlobal;
+    win.addEventListener("pagehide", this, { mozSystemGroup: true });
+    this.element.addEventListener("blur", this, { mozSystemGroup: true });
+    this.element.addEventListener("transitionend", this, {
+      mozSystemGroup: true,
+    });
+    let MutationObserver = this.element.ownerGlobal.MutationObserver;
+    this.mut = new MutationObserver(mutations => {
+      // Something changed the <select> while it was open, so
+      // we'll poke a DeferredTask to update the parent sometime
+      // in the very near future.
+      this._updateTimer.arm();
+    });
+    this.mut.observe(this.element, {
+      childList: true,
+      subtree: true,
+      attributes: true,
+    });
+
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "disablePopupAutohide",
+      "ui.popup.disable_autohide",
+      false
+    );
+  },
+
+  uninit() {
+    this.element.openInParentProcess = false;
+    let win = this.element.ownerGlobal;
+    win.removeEventListener("pagehide", this, { mozSystemGroup: true });
+    this.element.removeEventListener("blur", this, { mozSystemGroup: true });
+    this.element.removeEventListener("transitionend", this, {
+      mozSystemGroup: true,
+    });
+    this.element = null;
+    this.actor = null;
+    this.mut.disconnect();
+    this._updateTimer.disarm();
+    this._updateTimer = null;
+    gOpen = false;
+  },
+
+  showDropDown() {
+    this.element.openInParentProcess = true;
+    this._setupPseudoClassStyles();
+    let rect = this._getBoundingContentRect();
+    let computedStyles = getComputedStyles(this.element);
+    let options = this._buildOptionList();
+    let defaultStyles = this.element.ownerGlobal.getDefaultComputedStyle(
+      this.element
+    );
+    this.actor.sendAsyncMessage("Forms:ShowDropDown", {
+      isOpenedViaTouch: this.isOpenedViaTouch,
+      options,
+      rect,
+      custom: !this.element.nodePrincipal.isSystemPrincipal,
+      selectedIndex: this.element.selectedIndex,
+      isDarkBackground: ChromeUtils.isDarkBackground(this.element),
+      style: supportedStyles(computedStyles, SUPPORTED_SELECT_PROPERTIES),
+      defaultStyle: supportedStyles(defaultStyles, SUPPORTED_SELECT_PROPERTIES),
+    });
+    this._clearPseudoClassStyles();
+    gOpen = true;
+  },
+
+  _setupPseudoClassStyles() {
+    if (this._pseudoStylesSetup) {
+      throw new Error("pseudo styles must not be set up yet");
+    }
+    // Do all of the things that change style at once, before we read
+    // any styles.
+    this._pseudoStylesSetup = true;
+    InspectorUtils.addPseudoClassLock(this.element, ":focus");
+    let lockedDescendants = (this._lockedDescendants =
+      this.element.querySelectorAll(":checked"));
+    for (let child of lockedDescendants) {
+      // Selected options have the :checked pseudo-class, which
+      // we want to disable before calculating the computed
+      // styles since the user agent styles alter the styling
+      // based on :checked.
+      InspectorUtils.addPseudoClassLock(child, ":checked", false);
+    }
+  },
+
+  _clearPseudoClassStyles() {
+    if (!this._pseudoStylesSetup) {
+      throw new Error("pseudo styles must be set up already");
+    }
+    // Undo all of the things that change style at once, after we're
+    // done reading styles.
+    InspectorUtils.clearPseudoClassLocks(this.element);
+    let lockedDescendants = this._lockedDescendants;
+    for (let child of lockedDescendants) {
+      InspectorUtils.clearPseudoClassLocks(child);
+    }
+    this._lockedDescendants = null;
+    this._pseudoStylesSetup = false;
+  },
+
+  _getBoundingContentRect() {
+    return lazy.LayoutUtils.getElementBoundingScreenRect(this.element);
+  },
+
+  _buildOptionList() {
+    if (!this._pseudoStylesSetup) {
+      throw new Error("pseudo styles must be set up");
+    }
+    let uniqueStyles = [];
+    let options = buildOptionListForChildren(this.element, uniqueStyles);
+    return { options, uniqueStyles };
+  },
+
+  _update() {
+    // The <select> was updated while the dropdown was open.
+    // Let's send up a new list of options.
+    // Technically we might not need to set this pseudo-class
+    // during _update() since the element should organically
+    // have :focus, though it is here for belt-and-suspenders.
+    this._setupPseudoClassStyles();
+    let computedStyles = getComputedStyles(this.element);
+    let defaultStyles = this.element.ownerGlobal.getDefaultComputedStyle(
+      this.element
+    );
+    this.actor.sendAsyncMessage("Forms:UpdateDropDown", {
+      options: this._buildOptionList(),
+      custom: !this.element.nodePrincipal.isSystemPrincipal,
+      selectedIndex: this.element.selectedIndex,
+      isDarkBackground: ChromeUtils.isDarkBackground(this.element),
+      style: supportedStyles(computedStyles, SUPPORTED_SELECT_PROPERTIES),
+      defaultStyle: supportedStyles(defaultStyles, SUPPORTED_SELECT_PROPERTIES),
+    });
+    this._clearPseudoClassStyles();
+  },
+
+  dispatchMouseEvent(win, target, eventName) {
+    let mouseEvent = new win.MouseEvent(eventName, {
+      view: win,
+      bubbles: true,
+      cancelable: true,
+      composed: true,
+    });
+    target.dispatchEvent(mouseEvent);
+  },
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "Forms:SelectDropDownItem":
+        this.element.selectedIndex = message.data.value;
+        this.closedWithClickOn = !message.data.closedWithEnter;
+        break;
+
+      case "Forms:DismissedDropDown": {
+        if (!this.element) {
+          return;
+        }
+
+        let win = this.element.ownerGlobal;
+
+        // Running arbitrary script below (dispatching events for example) can
+        // close us, but we should still send events consistently.
+        let element = this.element;
+
+        let selectedOption = element.item(element.selectedIndex);
+
+        // For ordering of events, we're using non-e10s as our guide here,
+        // since the spec isn't exactly clear. In non-e10s:
+        // - If the user clicks on an element in the dropdown, we fire
+        //   mousedown, mouseup, input, change, and click events.
+        // - If the user uses the keyboard to select an element in the
+        //   dropdown, we only fire input and change events.
+        // - If the user pressed ESC key or clicks outside the dropdown,
+        //   we fire nothing as the selected option is unchanged.
+        if (this.closedWithClickOn) {
+          this.dispatchMouseEvent(win, selectedOption, "mousedown");
+          this.dispatchMouseEvent(win, selectedOption, "mouseup");
+        }
+
+        // Clear active document no matter user selects via keyboard or mouse
+        InspectorUtils.removeContentState(
+          element,
+          kStateActive,
+          /* aClearActiveDocument */ true
+        );
+
+        // Fire input and change events when selected option changes
+        if (this.initialSelection !== selectedOption) {
+          let inputEvent = new win.Event("input", {
+            bubbles: true,
+            composed: true,
+          });
+
+          let changeEvent = new win.Event("change", {
+            bubbles: true,
+          });
+
+          let handlingUserInput = win.windowUtils.setHandlingUserInput(true);
+          try {
+            element.dispatchEvent(inputEvent);
+            element.dispatchEvent(changeEvent);
+          } finally {
+            handlingUserInput.destruct();
+          }
+        }
+
+        // Fire click event
+        if (this.closedWithClickOn) {
+          this.dispatchMouseEvent(win, selectedOption, "click");
+        }
+
+        this.uninit();
+        break;
+      }
+
+      case "Forms:MouseOver":
+        InspectorUtils.setContentState(this.element, kStateHover);
+        break;
+
+      case "Forms:MouseOut":
+        InspectorUtils.removeContentState(this.element, kStateHover);
+        break;
+
+      case "Forms:MouseUp":
+        let win = this.element.ownerGlobal;
+        if (message.data.onAnchor) {
+          this.dispatchMouseEvent(win, this.element, "mouseup");
+        }
+        InspectorUtils.removeContentState(this.element, kStateActive);
+        if (message.data.onAnchor) {
+          this.dispatchMouseEvent(win, this.element, "click");
+        }
+        break;
+
+      case "Forms:SearchFocused":
+        this._closeAfterBlur = false;
+        break;
+
+      case "Forms:BlurDropDown-Pong":
+        if (!this._closeAfterBlur || !gOpen) {
+          return;
+        }
+        this.actor.sendAsyncMessage("Forms:HideDropDown", {});
+        this.uninit();
+        break;
+    }
+  },
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "pagehide":
+        if (this.element.ownerDocument === event.target) {
+          this.actor.sendAsyncMessage("Forms:HideDropDown", {});
+          this.uninit();
+        }
+        break;
+      case "blur": {
+        if (this.element !== event.target || this.disablePopupAutohide) {
+          break;
+        }
+        this._closeAfterBlur = true;
+        // Send a ping-pong message to make sure that we wait for
+        // enough cycles to pass from the potential focusing of the
+        // search box to disable closing-after-blur.
+        this.actor.sendAsyncMessage("Forms:BlurDropDown-Ping", {});
+        break;
+      }
+      case "mozhidedropdown":
+        if (this.element === event.target) {
+          this.actor.sendAsyncMessage("Forms:HideDropDown", {});
+          this.uninit();
+        }
+        break;
+      case "transitionend":
+        if (
+          this.element === event.target &&
+          SUPPORTED_SELECT_PROPERTIES.includes(event.propertyName)
+        ) {
+          this._updateTimer.arm();
+        }
+        break;
+    }
+  },
+};
+
+function getComputedStyles(element) {
+  return element.ownerGlobal.getComputedStyle(element);
+}
+
+function supportedStyles(cs, supportedProps) {
+  let styles = {};
+  for (let property of supportedProps) {
+    styles[property] = cs.getPropertyValue(property);
+  }
+  return styles;
+}
+
+function supportedStylesEqual(styles, otherStyles) {
+  for (let property in styles) {
+    if (styles[property] !== otherStyles[property]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+function uniqueStylesIndex(cs, uniqueStyles) {
+  let styles = supportedStyles(cs, SUPPORTED_OPTION_OPTGROUP_PROPERTIES);
+  for (let i = uniqueStyles.length; i--; ) {
+    if (supportedStylesEqual(uniqueStyles[i], styles)) {
+      return i;
+    }
+  }
+  uniqueStyles.push(styles);
+  return uniqueStyles.length - 1;
+}
+
+function buildOptionListForChildren(node, uniqueStyles) {
+  let result = [];
+
+  for (let child of node.children) {
+    let className = ChromeUtils.getClassName(child);
+    let isOption = className == "HTMLOptionElement";
+    let isOptGroup = className == "HTMLOptGroupElement";
+    if (!isOption && !isOptGroup) {
+      continue;
+    }
+    if (child.hidden) {
+      continue;
+    }
+
+    // The option code-path should match HTMLOptionElement::GetRenderedLabel.
+    let textContent = isOptGroup
+      ? child.getAttribute("label")
+      : child.label || child.text;
+    if (textContent == null) {
+      textContent = "";
+    }
+
+    let cs = getComputedStyles(child);
+    let info = {
+      index: child.index,
+      isOptGroup,
+      textContent,
+      disabled: child.disabled,
+      display: cs.display,
+      tooltip: child.title,
+      children: isOptGroup
+        ? buildOptionListForChildren(child, uniqueStyles)
+        : [],
+      // Most options have the same style. In order to reduce the size of the
+      // IPC message, coalesce them in uniqueStyles.
+      styleIndex: uniqueStylesIndex(cs, uniqueStyles),
+    };
+    result.push(info);
+  }
+  return result;
+}
+
+// Hold the instance of SelectContentHelper created
+// when the dropdown list is opened. This variable helps
+// re-route the received message from SelectChild to SelectContentHelper object.
+let currentSelectContentHelper = new WeakMap();
+
+export class SelectChild extends JSWindowActorChild {
+  handleEvent(event) {
+    if (SelectContentHelper.open) {
+      // The SelectContentHelper object handles captured
+      // events when the <select> popup is open.
+      let contentHelper = currentSelectContentHelper.get(this);
+      if (contentHelper) {
+        contentHelper.handleEvent(event);
+      }
+      return;
+    }
+
+    switch (event.type) {
+      case "mozshowdropdown": {
+        let contentHelper = new SelectContentHelper(
+          event.target,
+          { isOpenedViaTouch: false },
+          this
+        );
+        currentSelectContentHelper.set(this, contentHelper);
+        break;
+      }
+
+      case "mozshowdropdown-sourcetouch": {
+        let contentHelper = new SelectContentHelper(
+          event.target,
+          { isOpenedViaTouch: true },
+          this
+        );
+        currentSelectContentHelper.set(this, contentHelper);
+        break;
+      }
+    }
+  }
+
+  receiveMessage(message) {
+    let contentHelper = currentSelectContentHelper.get(this);
+    if (contentHelper) {
+      contentHelper.receiveMessage(message);
+    }
+  }
+}
diff --git a/toolkit/actors/SelectParent.sys.mjs b/toolkit/actors/SelectParent.sys.mjs
new file mode 100644
index 0000000000..6c41903994
--- /dev/null
+++ b/toolkit/actors/SelectParent.sys.mjs
@@ -0,0 +1,798 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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 = {};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "DOM_FORMS_SELECTSEARCH",
+  "dom.forms.selectSearch",
+  false
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "CUSTOM_STYLING_ENABLED",
+  "dom.forms.select.customstyling",
+  false
+);
+
+// Maximum number of rows to display in the select dropdown.
+const MAX_ROWS = 20;
+
+// Minimum elements required to show select search
+const SEARCH_MINIMUM_ELEMENTS = 40;
+
+// The properties that we should respect only when the item is not active.
+const PROPERTIES_RESET_WHEN_ACTIVE = [
+  "color",
+  "background-color",
+  "text-shadow",
+];
+
+// Duplicated in SelectChild.jsm
+// Please keep these lists in sync.
+const SUPPORTED_OPTION_OPTGROUP_PROPERTIES = [
+  "direction",
+  "color",
+  "background-color",
+  "text-shadow",
+  "text-transform",
+  "font-family",
+  "font-weight",
+  "font-size",
+  "font-style",
+];
+
+const SUPPORTED_SELECT_PROPERTIES = [
+  ...SUPPORTED_OPTION_OPTGROUP_PROPERTIES,
+  "scrollbar-width",
+  "scrollbar-color",
+];
+
+export var SelectParentHelper = {
+  /**
+   * `populate` takes the `menulist` element and a list of `items` and generates
+   * a popup list of options.
+   *
+   * If `CUSTOM_STYLING_ENABLED` is set to `true`, the function will also
+   * style the select and its popup trying to prevent the text
+   * and background to end up in the same color.
+   *
+   * All `ua*` variables represent the color values for the default colors
+   * for their respective form elements used by the user agent.
+   * The `select*` variables represent the color values defined for the
+   * particular <select> element.
+   *
+   * The `customoptionstyling` attribute controls the application of
+   * `-moz-appearance` on the elements and is disabled if the element is
+   * defining its own background-color.
+   *
+   * @param {Element}        menulist
+   * @param {Array<Element>} items
+   * @param {Array<Object>}  uniqueItemStyles
+   * @param {Number}         selectedIndex
+   * @param {Number}         zoom
+   * @param {Boolean}        custom
+   * @param {Boolean}        isDarkBackground
+   * @param {Object}         uaStyle
+   * @param {Object}         selectStyle
+   */
+  populate(
+    menulist,
+    items,
+    uniqueItemStyles,
+    selectedIndex,
+    zoom,
+    custom,
+    isDarkBackground,
+    uaStyle,
+    selectStyle
+  ) {
+    let doc = menulist.ownerDocument;
+
+    // Clear the current contents of the popup
+    let menupopup = menulist.menupopup;
+    menupopup.textContent = "";
+
+    let stylesheet = menulist.querySelector("#ContentSelectDropdownStylesheet");
+    if (stylesheet) {
+      stylesheet.remove();
+    }
+
+    menupopup.setAttribute("style", "");
+    menupopup.style.colorScheme = isDarkBackground ? "dark" : "light";
+    menupopup.style.direction = selectStyle.direction;
+
+    stylesheet = doc.createElementNS("http://www.w3.org/1999/xhtml", "style");
+    stylesheet.setAttribute("id", "ContentSelectDropdownStylesheet");
+    stylesheet.hidden = true;
+    stylesheet = menulist.appendChild(stylesheet);
+
+    let sheet = stylesheet.sheet;
+
+    if (!custom) {
+      selectStyle = uaStyle;
+    }
+
+    if (selectStyle["background-color"] == "rgba(0, 0, 0, 0)") {
+      selectStyle["background-color"] = uaStyle["background-color"];
+    }
+
+    if (selectStyle.color == selectStyle["background-color"]) {
+      selectStyle.color = uaStyle.color;
+    }
+
+    // We ensure that we set the content background if the color changes as
+    // well, to prevent contrast issues.
+    let selectBackgroundSet =
+      selectStyle["background-color"] != uaStyle["background-color"] ||
+      selectStyle.color != uaStyle.color;
+
+    if (custom) {
+      if (selectStyle["text-shadow"] != "none") {
+        sheet.insertRule(
+          `#ContentSelectDropdown > menupopup > :is(menuitem, menucaption)[_moz-menuactive="true"] {
+          text-shadow: none;
+        }`,
+          0
+        );
+      }
+
+      for (let property of SUPPORTED_SELECT_PROPERTIES) {
+        let shouldSkip = (function () {
+          if (property == "direction") {
+            // Handled elsewhere.
+            return true;
+          }
+          if (!selectStyle[property]) {
+            return true;
+          }
+          if (property == "background-color") {
+            // This also depends on whether "color" is set.
+            return !selectBackgroundSet;
+          }
+          return selectStyle[property] == uaStyle[property];
+        })();
+
+        if (shouldSkip) {
+          continue;
+        }
+        let value = selectStyle[property];
+        if (property == "scrollbar-width") {
+          // This needs to actually apply to the relevant scrollbox, because
+          // scrollbar-width doesn't inherit.
+          property = "--content-select-scrollbar-width";
+        }
+        if (property == "color") {
+          property = "--panel-color";
+        }
+        menupopup.style.setProperty(property, value);
+      }
+      // Some webpages set the <select> backgroundColor to transparent,
+      // but they don't intend to change the popup to transparent.
+      // So we remove the backgroundColor and turn it into an image instead.
+      if (selectBackgroundSet) {
+        // We intentionally use the parsed color to prevent color
+        // values like `url(..)` being injected into the
+        // `background-image` property.
+        let parsedColor = menupopup.style.backgroundColor;
+        menupopup.style.setProperty(
+          "--content-select-background-image",
+          `linear-gradient(${parsedColor}, ${parsedColor})`
+        );
+        // Always drop the background color to avoid messing with the custom
+        // shadow on Windows 10 styling.
+        menupopup.style.backgroundColor = "";
+        // If the background is set, we also make sure we set the color, to
+        // prevent contrast issues.
+        menupopup.style.setProperty("--panel-color", selectStyle.color);
+
+        sheet.insertRule(
+          `#ContentSelectDropdown > menupopup > :is(menuitem, menucaption):not([_moz-menuactive="true"]) {
+            color: inherit;
+        }`,
+          0
+        );
+      }
+    }
+
+    for (let i = 0, len = uniqueItemStyles.length; i < len; ++i) {
+      sheet.insertRule(
+        `#ContentSelectDropdown .ContentSelectDropdown-item-${i} {}`,
+        0
+      );
+      let style = uniqueItemStyles[i];
+      let rule = sheet.cssRules[0].style;
+      rule.direction = style.direction;
+      rule.fontSize = zoom * parseFloat(style["font-size"], 10) + "px";
+
+      if (!custom) {
+        continue;
+      }
+      let optionBackgroundIsTransparent =
+        style["background-color"] == "rgba(0, 0, 0, 0)";
+      let optionBackgroundSet =
+        !optionBackgroundIsTransparent || style.color != selectStyle.color;
+
+      if (optionBackgroundIsTransparent && style.color != selectStyle.color) {
+        style["background-color"] = selectStyle["background-color"];
+      }
+
+      if (style.color == style["background-color"]) {
+        style.color = selectStyle.color;
+      }
+
+      let inactiveRule = null;
+      for (const property of SUPPORTED_OPTION_OPTGROUP_PROPERTIES) {
+        let shouldSkip = (function () {
+          if (property == "direction" || property == "font-size") {
+            // Handled elsewhere.
+            return true;
+          }
+          if (!style[property]) {
+            return true;
+          }
+          if (property == "background-color" || property == "color") {
+            // This also depends on whether "color" is set.
+            return !optionBackgroundSet;
+          }
+          return style[property] == selectStyle[property];
+        })();
+        if (shouldSkip) {
+          continue;
+        }
+        if (PROPERTIES_RESET_WHEN_ACTIVE.includes(property)) {
+          if (!inactiveRule) {
+            sheet.insertRule(
+              `#ContentSelectDropdown .ContentSelectDropdown-item-${i}:not([_moz-menuactive="true"]) {}`,
+              0
+            );
+            inactiveRule = sheet.cssRules[0].style;
+          }
+          inactiveRule[property] = style[property];
+        } else {
+          rule[property] = style[property];
+        }
+      }
+      style.customStyling = selectBackgroundSet || optionBackgroundSet;
+    }
+
+    // We only set the `customoptionstyling` if the background has been
+    // manually set. This prevents the overlap between moz-appearance and
+    // background-color. `color` and `text-shadow` do not interfere with it.
+    if (custom && selectBackgroundSet) {
+      menulist.menupopup.setAttribute("customoptionstyling", "true");
+    } else {
+      menulist.menupopup.removeAttribute("customoptionstyling");
+    }
+
+    this._currentZoom = zoom;
+    this._currentMenulist = menulist;
+    this.populateChildren(menulist, items, uniqueItemStyles, selectedIndex);
+  },
+
+  open(browser, menulist, rect, isOpenedViaTouch, selectParentActor) {
+    this._actor = selectParentActor;
+    menulist.hidden = false;
+    this._currentBrowser = browser;
+    this._closedWithEnter = false;
+    this._selectRect = rect;
+    this._registerListeners(menulist.menupopup);
+
+    // Set the maximum height to show exactly MAX_ROWS items.
+    let menupopup = menulist.menupopup;
+    let firstItem = menupopup.firstElementChild;
+    while (firstItem && firstItem.hidden) {
+      firstItem = firstItem.nextElementSibling;
+    }
+
+    let win = menulist.ownerGlobal;
+    if (firstItem) {
+      let itemHeight = firstItem.getBoundingClientRect().height;
+
+      // Include the padding and border on the popup.
+      let cs = win.getComputedStyle(menupopup);
+      let bpHeight =
+        parseFloat(cs.borderTopWidth) +
+        parseFloat(cs.borderBottomWidth) +
+        parseFloat(cs.paddingTop) +
+        parseFloat(cs.paddingBottom);
+      menupopup.style.maxHeight = itemHeight * MAX_ROWS + bpHeight + "px";
+    }
+
+    menupopup.classList.toggle("isOpenedViaTouch", isOpenedViaTouch);
+
+    if (browser) {
+      browser.constrainPopup(menupopup);
+    } else {
+      menupopup.setConstraintRect(new win.DOMRect(0, 0, 0, 0));
+    }
+    menupopup.openPopupAtScreenRect(
+      AppConstants.platform == "macosx" ? "selection" : "after_start",
+      rect.left,
+      rect.top,
+      rect.width,
+      rect.height,
+      false,
+      false
+    );
+  },
+
+  hide(menulist, browser) {
+    if (this._currentBrowser == browser) {
+      menulist.menupopup.hidePopup();
+    }
+  },
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "mouseup":
+        function inRect(rect, x, y) {
+          return (
+            x >= rect.left &&
+            x <= rect.left + rect.width &&
+            y >= rect.top &&
+            y <= rect.top + rect.height
+          );
+        }
+
+        let x = event.screenX,
+          y = event.screenY;
+        let onAnchor =
+          !inRect(this._currentMenulist.menupopup.getOuterScreenRect(), x, y) &&
+          inRect(this._selectRect, x, y) &&
+          this._currentMenulist.menupopup.state == "open";
+        this._actor.sendAsyncMessage("Forms:MouseUp", { onAnchor });
+        break;
+
+      case "mouseover":
+        if (
+          !event.relatedTarget ||
+          !this._currentMenulist.contains(event.relatedTarget)
+        ) {
+          this._actor.sendAsyncMessage("Forms:MouseOver", {});
+        }
+        break;
+
+      case "mouseout":
+        if (
+          !event.relatedTarget ||
+          !this._currentMenulist.contains(event.relatedTarget)
+        ) {
+          this._actor.sendAsyncMessage("Forms:MouseOut", {});
+        }
+        break;
+
+      case "keydown":
+        if (event.keyCode == event.DOM_VK_RETURN) {
+          this._closedWithEnter = true;
+        }
+        break;
+
+      case "command":
+        if (event.target.hasAttribute("value")) {
+          this._actor.sendAsyncMessage("Forms:SelectDropDownItem", {
+            value: event.target.value,
+            closedWithEnter: this._closedWithEnter,
+          });
+        }
+        break;
+
+      case "fullscreen":
+        if (this._currentMenulist) {
+          this._currentMenulist.menupopup.hidePopup();
+        }
+        break;
+
+      case "popuphidden":
+        this._actor.sendAsyncMessage("Forms:DismissedDropDown", {});
+        let popup = event.target;
+        this._unregisterListeners(popup);
+        popup.parentNode.hidden = true;
+        this._currentBrowser = null;
+        this._currentMenulist = null;
+        this._selectRect = null;
+        this._currentZoom = 1;
+        this._actor = null;
+        break;
+    }
+  },
+
+  receiveMessage(browser, msg) {
+    // Sanity check - we'd better know what the currently opened menulist is,
+    // and what browser it belongs to...
+    if (!this._currentMenulist || this._currentBrowser != browser) {
+      return;
+    }
+
+    if (msg.name == "Forms:UpdateDropDown") {
+      let scrollBox = this._currentMenulist.menupopup.scrollBox.scrollbox;
+      let scrollTop = scrollBox.scrollTop;
+
+      let options = msg.data.options;
+      let selectedIndex = msg.data.selectedIndex;
+      this.populate(
+        this._currentMenulist,
+        options.options,
+        options.uniqueStyles,
+        selectedIndex,
+        this._currentZoom,
+        msg.data.custom && lazy.CUSTOM_STYLING_ENABLED,
+        msg.data.isDarkBackground,
+        msg.data.defaultStyle,
+        msg.data.style
+      );
+
+      // Restore scroll position to what it was prior to the update.
+      scrollBox.scrollTop = scrollTop;
+    } else if (msg.name == "Forms:BlurDropDown-Ping") {
+      this._actor.sendAsyncMessage("Forms:BlurDropDown-Pong", {});
+    }
+  },
+
+  _registerListeners(popup) {
+    popup.addEventListener("command", this);
+    popup.addEventListener("popuphidden", this);
+    popup.addEventListener("mouseover", this);
+    popup.addEventListener("mouseout", this);
+    popup.ownerGlobal.addEventListener("mouseup", this, true);
+    popup.ownerGlobal.addEventListener("keydown", this, true);
+    popup.ownerGlobal.addEventListener("fullscreen", this, true);
+  },
+
+  _unregisterListeners(popup) {
+    popup.removeEventListener("command", this);
+    popup.removeEventListener("popuphidden", this);
+    popup.removeEventListener("mouseover", this);
+    popup.removeEventListener("mouseout", this);
+    popup.ownerGlobal.removeEventListener("mouseup", this, true);
+    popup.ownerGlobal.removeEventListener("keydown", this, true);
+    popup.ownerGlobal.removeEventListener("fullscreen", this, true);
+  },
+
+  /**
+   * `populateChildren` creates all <menuitem> elements for the popup menu
+   * based on the list of <option> elements from the <select> element.
+   *
+   * It attempts to intelligently add per-item CSS rules if the single
+   * item values differ from the parent menu values and attempting to avoid
+   * ending up with the same color of text and background.
+   *
+   * @param {Element}        menulist
+   * @param {Array<Element>} options
+   * @param {Array<Object>}  uniqueOptionStyles
+   * @param {Number}         selectedIndex
+   * @param {Element}        parentElement
+   * @param {Boolean}        isGroupDisabled
+   * @param {Boolean}        addSearch
+   * @param {Number}         nthChildIndex
+   * @returns {Number}
+   */
+  populateChildren(
+    menulist,
+    options,
+    uniqueOptionStyles,
+    selectedIndex,
+    parentElement = null,
+    isGroupDisabled = false,
+    addSearch = true,
+    nthChildIndex = 1
+  ) {
+    let element = menulist.menupopup;
+
+    let ariaOwns = "";
+    for (let option of options) {
+      let isOptGroup = option.isOptGroup;
+      let item = element.ownerDocument.createXULElement(
+        isOptGroup ? "menucaption" : "menuitem"
+      );
+      if (isOptGroup) {
+        item.setAttribute("role", "group");
+      }
+      item.setAttribute("label", option.textContent);
+      item.className = `ContentSelectDropdown-item-${option.styleIndex}`;
+      item.hidden =
+        option.display == "none" || (parentElement && parentElement.hidden);
+      // Keep track of which options are hidden by page content, so we can avoid
+      // showing them on search input.
+      item.hiddenByContent = item.hidden;
+      item.setAttribute("tooltiptext", option.tooltip);
+
+      if (uniqueOptionStyles[option.styleIndex].customStyling) {
+        item.setAttribute("customoptionstyling", "true");
+      } else {
+        item.removeAttribute("customoptionstyling");
+      }
+
+      if (parentElement) {
+        // In the menupopup, the optgroup is a sibling of its contained options.
+        // For accessibility, we want to preserve the hierarchy such that the
+        // options are inside the optgroup. We do this using aria-owns on the
+        // parent.
+        item.id = "ContentSelectDropdownOption" + nthChildIndex;
+        item.setAttribute("aria-level", "2");
+        ariaOwns += item.id + " ";
+      }
+
+      element.appendChild(item);
+      nthChildIndex++;
+
+      // A disabled optgroup disables all of its child options.
+      let isDisabled = isGroupDisabled || option.disabled;
+      if (isDisabled) {
+        item.setAttribute("disabled", "true");
+      }
+
+      if (isOptGroup) {
+        nthChildIndex = this.populateChildren(
+          menulist,
+          option.children,
+          uniqueOptionStyles,
+          selectedIndex,
+          item,
+          isDisabled,
+          false,
+          nthChildIndex
+        );
+      } else {
+        if (option.index == selectedIndex) {
+          // We expect the parent element of the popup to be a <xul:menulist> that
+          // has the popuponly attribute set to "true". This is necessary in order
+          // for a <xul:menupopup> to act like a proper <html:select> dropdown, as
+          // the <xul:menulist> does things like remember state and set the
+          // _moz-menuactive attribute on the selected <xul:menuitem>.
+          menulist.selectedItem = item;
+
+          // It's hack time. In the event that we've re-populated the menulist due
+          // to a mutation in the <select> in content, that means that the -moz_activemenu
+          // may have been removed from the selected item. Since that's normally only
+          // set for the initially selected on popupshowing for the menulist, and we
+          // don't want to close and re-open the popup, we manually set it here.
+          menulist.activeChild = item;
+        }
+
+        item.setAttribute("value", option.index);
+
+        if (parentElement) {
+          item.classList.add("contentSelectDropdown-ingroup");
+        }
+      }
+    }
+
+    if (parentElement && ariaOwns) {
+      parentElement.setAttribute("aria-owns", ariaOwns);
+    }
+
+    // Check if search pref is enabled, if this is the first time iterating through
+    // the dropdown, and if the list is long enough for a search element to be added.
+    if (
+      lazy.DOM_FORMS_SELECTSEARCH &&
+      addSearch &&
+      element.childElementCount > SEARCH_MINIMUM_ELEMENTS
+    ) {
+      // Add a search text field as the first element of the dropdown
+      let searchbox = element.ownerDocument.createXULElement("search-textbox");
+      searchbox.className = "contentSelectDropdown-searchbox";
+      searchbox.addEventListener("input", this.onSearchInput);
+      searchbox.addEventListener("focus", this.onSearchFocus.bind(this));
+      searchbox.addEventListener("blur", this.onSearchBlur);
+      searchbox.addEventListener("command", this.onSearchInput);
+
+      // Handle special keys for exiting search
+      searchbox.addEventListener(
+        "keydown",
+        event => {
+          this.onSearchKeydown(event, menulist);
+        },
+        true
+      );
+
+      element.insertBefore(searchbox, element.children[0]);
+    }
+
+    return nthChildIndex;
+  },
+
+  onSearchKeydown(event, menulist) {
+    if (event.defaultPrevented) {
+      return;
+    }
+
+    let searchbox = event.currentTarget;
+    switch (event.key) {
+      case "Escape":
+        searchbox.parentElement.hidePopup();
+        break;
+      case "ArrowDown":
+      case "Enter":
+      case "Tab":
+        searchbox.blur();
+        if (
+          searchbox.nextElementSibling.localName == "menuitem" &&
+          !searchbox.nextElementSibling.hidden
+        ) {
+          menulist.activeChild = searchbox.nextElementSibling;
+        } else {
+          let currentOption = searchbox.nextElementSibling;
+          while (
+            currentOption &&
+            (currentOption.localName != "menuitem" || currentOption.hidden)
+          ) {
+            currentOption = currentOption.nextElementSibling;
+          }
+          if (currentOption) {
+            menulist.activeChild = currentOption;
+          } else {
+            searchbox.focus();
+          }
+        }
+        break;
+      default:
+        return;
+    }
+    event.preventDefault();
+  },
+
+  onSearchInput(event) {
+    let searchObj = event.currentTarget;
+
+    // Get input from search field, set to all lower case for comparison
+    let input = searchObj.value.toLowerCase();
+    // Get all items in dropdown (could be options or optgroups)
+    let menupopup = searchObj.parentElement;
+    let menuItems = menupopup.querySelectorAll("menuitem, menucaption");
+
+    // Flag used to detect any group headers with no visible options.
+    // These group headers should be hidden.
+    let allHidden = true;
+    // Keep a reference to the previous group header (menucaption) to go back
+    // and set to hidden if all options within are hidden.
+    let prevCaption = null;
+
+    for (let currentItem of menuItems) {
+      // Make sure we don't show any options that were hidden by page content
+      if (!currentItem.hiddenByContent) {
+        // Get label and tooltip (title) from option and change to
+        // lower case for comparison
+        let itemLabel = currentItem.getAttribute("label").toLowerCase();
+        let itemTooltip = currentItem.getAttribute("title").toLowerCase();
+
+        // If search input is empty, all options should be shown
+        if (!input) {
+          currentItem.hidden = false;
+        } else if (currentItem.localName == "menucaption") {
+          if (prevCaption != null) {
+            prevCaption.hidden = allHidden;
+          }
+          prevCaption = currentItem;
+          allHidden = true;
+        } else {
+          if (
+            !currentItem.classList.contains("contentSelectDropdown-ingroup") &&
+            currentItem.previousElementSibling.classList.contains(
+              "contentSelectDropdown-ingroup"
+            )
+          ) {
+            if (prevCaption != null) {
+              prevCaption.hidden = allHidden;
+            }
+            prevCaption = null;
+            allHidden = true;
+          }
+          if (itemLabel.includes(input) || itemTooltip.includes(input)) {
+            currentItem.hidden = false;
+            allHidden = false;
+          } else {
+            currentItem.hidden = true;
+          }
+        }
+        if (prevCaption != null) {
+          prevCaption.hidden = allHidden;
+        }
+      }
+    }
+  },
+
+  onSearchFocus(event) {
+    let menupopup = event.target.closest("menupopup");
+    menupopup.parentElement.activeChild = null;
+    menupopup.setAttribute("ignorekeys", "true");
+    this._actor.sendAsyncMessage("Forms:SearchFocused", {});
+  },
+
+  onSearchBlur(event) {
+    let menupopup = event.target.closest("menupopup");
+    menupopup.setAttribute(
+      "ignorekeys",
+      AppConstants.platform == "win" ? "shortcuts" : "false"
+    );
+  },
+};
+
+export class SelectParent extends JSWindowActorParent {
+  get relevantBrowser() {
+    return this.browsingContext.top.embedderElement;
+  }
+
+  get _document() {
+    return this.browsingContext.topChromeWindow.document;
+  }
+
+  get _menulist() {
+    return this._document.getElementById("ContentSelectDropdown");
+  }
+
+  _createMenulist() {
+    let document = this._document;
+    let menulist = document.createXULElement("menulist");
+    menulist.setAttribute("id", "ContentSelectDropdown");
+    menulist.setAttribute("popuponly", "true");
+    menulist.setAttribute("hidden", "true");
+
+    let popup = menulist.appendChild(document.createXULElement("menupopup"));
+    popup.setAttribute("id", "ContentSelectDropdownPopup");
+    popup.setAttribute("activateontab", "true");
+    popup.setAttribute("position", "after_start");
+    popup.setAttribute("level", "parent");
+    if (AppConstants.platform == "win") {
+      popup.setAttribute("consumeoutsideclicks", "false");
+      popup.setAttribute("ignorekeys", "shortcuts");
+    }
+
+    let container =
+      document.getElementById("mainPopupSet") ||
+      document.querySelector("popupset") ||
+      document.documentElement.appendChild(
+        document.createXULElement("popupset")
+      );
+
+    container.appendChild(menulist);
+    return menulist;
+  }
+
+  receiveMessage(message) {
+    switch (message.name) {
+      case "Forms:ShowDropDown": {
+        let menulist = this._menulist || this._createMenulist();
+
+        let data = message.data;
+
+        SelectParentHelper.populate(
+          menulist,
+          data.options.options,
+          data.options.uniqueStyles,
+          data.selectedIndex,
+          // We only want to apply the full zoom. The text zoom is already
+          // applied in the font-size.
+          this.browsingContext.fullZoom,
+          data.custom && lazy.CUSTOM_STYLING_ENABLED,
+          data.isDarkBackground,
+          data.defaultStyle,
+          data.style
+        );
+        SelectParentHelper.open(
+          this.relevantBrowser,
+          menulist,
+          data.rect,
+          data.isOpenedViaTouch,
+          this
+        );
+        break;
+      }
+
+      case "Forms:HideDropDown": {
+        SelectParentHelper.hide(this._menulist, this.relevantBrowser);
+        break;
+      }
+
+      default:
+        SelectParentHelper.receiveMessage(this.relevantBrowser, message);
+    }
+  }
+}
diff --git a/toolkit/actors/TestProcessActorChild.jsm b/toolkit/actors/TestProcessActorChild.jsm
new file mode 100644
index 0000000000..20b7382624
--- /dev/null
+++ b/toolkit/actors/TestProcessActorChild.jsm
@@ -0,0 +1,59 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var EXPORTED_SYMBOLS = ["TestProcessActorChild"];
+
+class TestProcessActorChild extends JSProcessActorChild {
+  constructor() {
+    super();
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "toChild":
+        aMessage.data.toChild = true;
+        this.sendAsyncMessage("toParent", aMessage.data);
+        break;
+      case "asyncAdd":
+        let { a, b } = aMessage.data;
+        return new Promise(resolve => {
+          resolve({ result: a + b });
+        });
+      case "error":
+        return Promise.reject(new SyntaxError(aMessage.data.message));
+      case "exception":
+        return Promise.reject(
+          Components.Exception(aMessage.data.message, aMessage.data.result)
+        );
+      case "done":
+        this.done(aMessage.data);
+        break;
+    }
+
+    return undefined;
+  }
+
+  observe(subject, topic, data) {
+    this.lastObserved = { subject, topic, data };
+  }
+
+  show() {
+    return "TestProcessActorChild";
+  }
+
+  didDestroy() {
+    Services.obs.notifyObservers(
+      this,
+      "test-js-content-actor-diddestroy",
+      true
+    );
+  }
+}
diff --git a/toolkit/actors/TestProcessActorChild.sys.mjs b/toolkit/actors/TestProcessActorChild.sys.mjs
new file mode 100644
index 0000000000..b9631ff9d9
--- /dev/null
+++ b/toolkit/actors/TestProcessActorChild.sys.mjs
@@ -0,0 +1,56 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class TestProcessActorChild extends JSProcessActorChild {
+  constructor() {
+    super();
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "toChild":
+        aMessage.data.toChild = true;
+        this.sendAsyncMessage("toParent", aMessage.data);
+        break;
+      case "asyncAdd":
+        let { a, b } = aMessage.data;
+        return new Promise(resolve => {
+          resolve({ result: a + b });
+        });
+      case "error":
+        return Promise.reject(new SyntaxError(aMessage.data.message));
+      case "exception":
+        return Promise.reject(
+          Components.Exception(aMessage.data.message, aMessage.data.result)
+        );
+      case "done":
+        this.done(aMessage.data);
+        break;
+    }
+
+    return undefined;
+  }
+
+  observe(subject, topic, data) {
+    this.lastObserved = { subject, topic, data };
+  }
+
+  show() {
+    return "TestProcessActorChild";
+  }
+
+  didDestroy() {
+    Services.obs.notifyObservers(
+      this,
+      "test-js-content-actor-diddestroy",
+      true
+    );
+  }
+}
diff --git a/toolkit/actors/TestProcessActorParent.jsm b/toolkit/actors/TestProcessActorParent.jsm
new file mode 100644
index 0000000000..dfa1fc1df0
--- /dev/null
+++ b/toolkit/actors/TestProcessActorParent.jsm
@@ -0,0 +1,41 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var EXPORTED_SYMBOLS = ["TestProcessActorParent"];
+
+class TestProcessActorParent extends JSProcessActorParent {
+  constructor() {
+    super();
+    this.wrappedJSObject = this;
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "init":
+        aMessage.data.initial = true;
+        this.sendAsyncMessage("toChild", aMessage.data);
+        break;
+      case "toParent":
+        aMessage.data.toParent = true;
+        this.sendAsyncMessage("done", aMessage.data);
+        break;
+      case "asyncMul":
+        let { a, b } = aMessage.data;
+        return { result: a * b };
+    }
+
+    return undefined;
+  }
+
+  show() {
+    return "TestProcessActorParent";
+  }
+}
diff --git a/toolkit/actors/TestProcessActorParent.sys.mjs b/toolkit/actors/TestProcessActorParent.sys.mjs
new file mode 100644
index 0000000000..c86bd8ad84
--- /dev/null
+++ b/toolkit/actors/TestProcessActorParent.sys.mjs
@@ -0,0 +1,38 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class TestProcessActorParent extends JSProcessActorParent {
+  constructor() {
+    super();
+    this.wrappedJSObject = this;
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "init":
+        aMessage.data.initial = true;
+        this.sendAsyncMessage("toChild", aMessage.data);
+        break;
+      case "toParent":
+        aMessage.data.toParent = true;
+        this.sendAsyncMessage("done", aMessage.data);
+        break;
+      case "asyncMul":
+        let { a, b } = aMessage.data;
+        return { result: a * b };
+    }
+
+    return undefined;
+  }
+
+  show() {
+    return "TestProcessActorParent";
+  }
+}
diff --git a/toolkit/actors/TestWindowChild.jsm b/toolkit/actors/TestWindowChild.jsm
new file mode 100644
index 0000000000..127c03fa0e
--- /dev/null
+++ b/toolkit/actors/TestWindowChild.jsm
@@ -0,0 +1,102 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var EXPORTED_SYMBOLS = ["TestWindowChild"];
+
+var docShellThunks = new Map();
+
+class TestWindowChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    this.sawActorCreated = false;
+
+    try {
+      void this.contentWindow;
+    } catch (e) {
+      this.uninitializedGetterError = e;
+    }
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "toChild":
+        aMessage.data.toChild = true;
+        this.sendAsyncMessage("toParent", aMessage.data);
+        break;
+      case "asyncAdd":
+        let { a, b } = aMessage.data;
+        return new Promise(resolve => {
+          resolve({ result: a + b });
+        });
+      case "error":
+        return Promise.reject(new SyntaxError(aMessage.data.message));
+      case "exception":
+        return Promise.reject(
+          Components.Exception(aMessage.data.message, aMessage.data.result)
+        );
+      case "done":
+        this.done(aMessage.data);
+        break;
+      case "noncloneReply":
+        // Return a value which is non-cloneable, like a WindowProxy.
+        return this.contentWindow;
+      case "storeActor":
+        docShellThunks.set(this.docShell, this);
+        break;
+      case "checkActor": {
+        let actor = docShellThunks.get(this.docShell);
+        docShellThunks.delete(this.docShell);
+
+        let contentWindow;
+        let error;
+        try {
+          contentWindow = actor.contentWindow;
+        } catch (e) {
+          error = e;
+        }
+        if (error) {
+          return {
+            status: "error",
+            errorType: error.name,
+          };
+        }
+        return {
+          status: "success",
+          valueIsNull: contentWindow === null,
+        };
+      }
+    }
+
+    return undefined;
+  }
+
+  handleEvent(aEvent) {
+    this.sendAsyncMessage("event", { type: aEvent.type });
+  }
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "audio-playback":
+        this.done({ subject, topic, data });
+        break;
+      default:
+        this.lastObserved = { subject, topic, data };
+        break;
+    }
+  }
+
+  show() {
+    return "TestWindowChild";
+  }
+
+  didDestroy() {
+    Services.obs.notifyObservers(this, "test-js-window-actor-diddestroy", true);
+  }
+}
diff --git a/toolkit/actors/TestWindowChild.sys.mjs b/toolkit/actors/TestWindowChild.sys.mjs
new file mode 100644
index 0000000000..5df64c932c
--- /dev/null
+++ b/toolkit/actors/TestWindowChild.sys.mjs
@@ -0,0 +1,99 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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/. */
+
+var docShellThunks = new Map();
+
+export class TestWindowChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    this.sawActorCreated = false;
+
+    try {
+      void this.contentWindow;
+    } catch (e) {
+      this.uninitializedGetterError = e;
+    }
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "toChild":
+        aMessage.data.toChild = true;
+        this.sendAsyncMessage("toParent", aMessage.data);
+        break;
+      case "asyncAdd":
+        let { a, b } = aMessage.data;
+        return new Promise(resolve => {
+          resolve({ result: a + b });
+        });
+      case "error":
+        return Promise.reject(new SyntaxError(aMessage.data.message));
+      case "exception":
+        return Promise.reject(
+          Components.Exception(aMessage.data.message, aMessage.data.result)
+        );
+      case "done":
+        this.done(aMessage.data);
+        break;
+      case "noncloneReply":
+        // Return a value which is non-cloneable, like a WindowProxy.
+        return this.contentWindow;
+      case "storeActor":
+        docShellThunks.set(this.docShell, this);
+        break;
+      case "checkActor": {
+        let actor = docShellThunks.get(this.docShell);
+        docShellThunks.delete(this.docShell);
+
+        let contentWindow;
+        let error;
+        try {
+          contentWindow = actor.contentWindow;
+        } catch (e) {
+          error = e;
+        }
+        if (error) {
+          return {
+            status: "error",
+            errorType: error.name,
+          };
+        }
+        return {
+          status: "success",
+          valueIsNull: contentWindow === null,
+        };
+      }
+    }
+
+    return undefined;
+  }
+
+  handleEvent(aEvent) {
+    this.sendAsyncMessage("event", { type: aEvent.type });
+  }
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "audio-playback":
+        this.done({ subject, topic, data });
+        break;
+      default:
+        this.lastObserved = { subject, topic, data };
+        break;
+    }
+  }
+
+  show() {
+    return "TestWindowChild";
+  }
+
+  didDestroy() {
+    Services.obs.notifyObservers(this, "test-js-window-actor-diddestroy", true);
+  }
+}
diff --git a/toolkit/actors/TestWindowParent.jsm b/toolkit/actors/TestWindowParent.jsm
new file mode 100644
index 0000000000..5c368d3321
--- /dev/null
+++ b/toolkit/actors/TestWindowParent.jsm
@@ -0,0 +1,49 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var EXPORTED_SYMBOLS = ["TestWindowParent"];
+
+class TestWindowParent extends JSWindowActorParent {
+  constructor() {
+    super();
+    this.wrappedJSObject = this;
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "init":
+        aMessage.data.initial = true;
+        this.sendAsyncMessage("toChild", aMessage.data);
+        break;
+      case "toParent":
+        aMessage.data.toParent = true;
+        this.sendAsyncMessage("done", aMessage.data);
+        break;
+      case "asyncMul":
+        let { a, b } = aMessage.data;
+        return { result: a * b };
+
+      case "event":
+        Services.obs.notifyObservers(
+          this,
+          "test-js-window-actor-parent-event",
+          aMessage.data.type
+        );
+        break;
+    }
+
+    return undefined;
+  }
+
+  show() {
+    return "TestWindowParent";
+  }
+}
diff --git a/toolkit/actors/TestWindowParent.sys.mjs b/toolkit/actors/TestWindowParent.sys.mjs
new file mode 100644
index 0000000000..d7d0c2b547
--- /dev/null
+++ b/toolkit/actors/TestWindowParent.sys.mjs
@@ -0,0 +1,46 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class TestWindowParent extends JSWindowActorParent {
+  constructor() {
+    super();
+    this.wrappedJSObject = this;
+    this.sawActorCreated = false;
+  }
+
+  actorCreated() {
+    this.sawActorCreated = true;
+  }
+
+  receiveMessage(aMessage) {
+    switch (aMessage.name) {
+      case "init":
+        aMessage.data.initial = true;
+        this.sendAsyncMessage("toChild", aMessage.data);
+        break;
+      case "toParent":
+        aMessage.data.toParent = true;
+        this.sendAsyncMessage("done", aMessage.data);
+        break;
+      case "asyncMul":
+        let { a, b } = aMessage.data;
+        return { result: a * b };
+
+      case "event":
+        Services.obs.notifyObservers(
+          this,
+          "test-js-window-actor-parent-event",
+          aMessage.data.type
+        );
+        break;
+    }
+
+    return undefined;
+  }
+
+  show() {
+    return "TestWindowParent";
+  }
+}
diff --git a/toolkit/actors/ThumbnailsChild.sys.mjs b/toolkit/actors/ThumbnailsChild.sys.mjs
new file mode 100644
index 0000000000..9ab6fc3b08
--- /dev/null
+++ b/toolkit/actors/ThumbnailsChild.sys.mjs
@@ -0,0 +1,60 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* 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, {
+  PageThumbUtils: "resource://gre/modules/PageThumbUtils.sys.mjs",
+});
+
+export class ThumbnailsChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    switch (message.name) {
+      case "Browser:Thumbnail:ContentInfo": {
+        let [width, height] = lazy.PageThumbUtils.getContentSize(
+          this.contentWindow
+        );
+        return { width, height };
+      }
+      case "Browser:Thumbnail:CheckState": {
+        /**
+         * Remote isSafeForCapture request handler for PageThumbs.
+         */
+        return new Promise(resolve =>
+          Services.tm.idleDispatchToMainThread(() => {
+            if (!this.manager) {
+              // If we have no manager, our actor has been destroyed, which
+              // means we can't respond, and trying to touch
+              // `this.contentWindow` or `this.browsingContext` will throw.
+              // The `sendQuery` call in the parent will already have been
+              // rejected when the actor was destroyed, so there's no need to
+              // reject our promise or log an additional error.
+              return;
+            }
+
+            let result = lazy.PageThumbUtils.shouldStoreContentThumbnail(
+              this.contentWindow,
+              this.browsingContext.docShell
+            );
+            resolve(result);
+          })
+        );
+      }
+      case "Browser:Thumbnail:GetOriginalURL": {
+        /**
+         * Remote GetOriginalURL request handler for PageThumbs.
+         */
+        let channel = this.browsingContext.docShell.currentDocumentChannel;
+        let channelError = lazy.PageThumbUtils.isChannelErrorResponse(channel);
+        let originalURL;
+        try {
+          originalURL = channel.originalURI.spec;
+        } catch (ex) {}
+        return { channelError, originalURL };
+      }
+    }
+    return undefined;
+  }
+}
diff --git a/toolkit/actors/UAWidgetsChild.sys.mjs b/toolkit/actors/UAWidgetsChild.sys.mjs
new file mode 100644
index 0000000000..6f4244ffe9
--- /dev/null
+++ b/toolkit/actors/UAWidgetsChild.sys.mjs
@@ -0,0 +1,236 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class UAWidgetsChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    this.widgets = new WeakMap();
+    this.prefsCache = new Map();
+    this.observedPrefs = [];
+
+    // Bug 1570744 - JSWindowActorChild's cannot be used as nsIObserver's
+    // directly, so we create a new function here instead to act as our
+    // nsIObserver, which forwards the notification to the observe method.
+    this.observerFunction = (subject, topic, data) => {
+      this.observe(subject, topic, data);
+    };
+  }
+
+  didDestroy() {
+    for (let pref in this.observedPrefs) {
+      Services.prefs.removeObserver(pref, this.observerFunction);
+    }
+  }
+
+  unwrap(obj) {
+    return Cu.isXrayWrapper(obj) ? obj.wrappedJSObject : obj;
+  }
+
+  handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case "UAWidgetSetupOrChange":
+        this.setupOrNotifyWidget(aEvent.target);
+        break;
+      case "UAWidgetTeardown":
+        this.teardownWidget(aEvent.target);
+        break;
+    }
+  }
+
+  setupOrNotifyWidget(aElement) {
+    if (!this.widgets.has(aElement)) {
+      this.setupWidget(aElement);
+      return;
+    }
+
+    let { widget } = this.widgets.get(aElement);
+
+    if (typeof widget.onchange == "function") {
+      if (
+        this.unwrap(aElement.openOrClosedShadowRoot) !=
+        this.unwrap(widget.shadowRoot)
+      ) {
+        console.error(
+          "Getting a UAWidgetSetupOrChange event without the ShadowRoot. " +
+            "Torn down already?"
+        );
+        return;
+      }
+      try {
+        widget.onchange();
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  setupWidget(aElement) {
+    let uri;
+    let widgetName;
+    // Use prefKeys to optionally send a list of preferences to forward to
+    // the UAWidget. The UAWidget will receive those preferences as key-value
+    // pairs as the second argument to its constructor. Updates to those prefs
+    // can be observed by implementing an optional onPrefChange method for the
+    // UAWidget that receives the changed pref name as the first argument, and
+    // the updated value as the second.
+    let prefKeys = [];
+    switch (aElement.localName) {
+      case "video":
+      case "audio":
+        uri = "chrome://global/content/elements/videocontrols.js";
+        widgetName = "VideoControlsWidget";
+        prefKeys = [
+          "media.videocontrols.picture-in-picture.enabled",
+          "media.videocontrols.picture-in-picture.video-toggle.enabled",
+          "media.videocontrols.picture-in-picture.video-toggle.always-show",
+          "media.videocontrols.picture-in-picture.video-toggle.min-video-secs",
+          "media.videocontrols.picture-in-picture.video-toggle.position",
+          "media.videocontrols.picture-in-picture.video-toggle.has-used",
+          "media.videocontrols.keyboard-tab-to-all-controls",
+          "media.videocontrols.picture-in-picture.respect-disablePictureInPicture",
+        ];
+        break;
+      case "input":
+        uri = "chrome://global/content/elements/datetimebox.js";
+        widgetName = "DateTimeBoxWidget";
+        break;
+      case "marquee":
+        uri = "chrome://global/content/elements/marquee.js";
+        widgetName = "MarqueeWidget";
+        break;
+      case "img":
+        uri = "chrome://global/content/elements/textrecognition.js";
+        widgetName = "TextRecognitionWidget";
+    }
+
+    if (!uri || !widgetName) {
+      console.error(
+        "Getting a UAWidgetSetupOrChange event on undefined element."
+      );
+      return;
+    }
+
+    let shadowRoot = aElement.openOrClosedShadowRoot;
+    if (!shadowRoot) {
+      console.error(
+        "Getting a UAWidgetSetupOrChange event without the Shadow Root. " +
+          "Torn down already?"
+      );
+      return;
+    }
+
+    let isSystemPrincipal = aElement.nodePrincipal.isSystemPrincipal;
+    let sandbox = isSystemPrincipal
+      ? Object.create(null)
+      : Cu.getUAWidgetScope(aElement.nodePrincipal);
+
+    if (!sandbox[widgetName]) {
+      Services.scriptloader.loadSubScript(uri, sandbox);
+    }
+
+    let prefs = Cu.cloneInto(
+      this.getPrefsForUAWidget(widgetName, prefKeys),
+      sandbox
+    );
+
+    let widget = new sandbox[widgetName](shadowRoot, prefs);
+    if (!isSystemPrincipal) {
+      widget = widget.wrappedJSObject;
+    }
+    if (this.unwrap(widget.shadowRoot) != this.unwrap(shadowRoot)) {
+      console.error("Widgets should expose their shadow root.");
+    }
+    this.widgets.set(aElement, { widget, widgetName });
+    try {
+      widget.onsetup();
+    } catch (ex) {
+      console.error(ex);
+    }
+  }
+
+  teardownWidget(aElement) {
+    if (!this.widgets.has(aElement)) {
+      return;
+    }
+    let { widget } = this.widgets.get(aElement);
+    if (typeof widget.teardown == "function") {
+      try {
+        widget.teardown();
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+    this.widgets.delete(aElement);
+  }
+
+  getPrefsForUAWidget(aWidgetName, aPrefKeys) {
+    let result = this.prefsCache.get(aWidgetName);
+    if (result) {
+      return result;
+    }
+
+    result = {};
+    for (let key of aPrefKeys) {
+      result[key] = this.getPref(key);
+      this.observePref(key);
+    }
+
+    this.prefsCache.set(aWidgetName, result);
+    return result;
+  }
+
+  observePref(prefKey) {
+    Services.prefs.addObserver(prefKey, this.observerFunction);
+    this.observedPrefs.push(prefKey);
+  }
+
+  getPref(prefKey) {
+    switch (Services.prefs.getPrefType(prefKey)) {
+      case Ci.nsIPrefBranch.PREF_BOOL: {
+        return Services.prefs.getBoolPref(prefKey);
+      }
+      case Ci.nsIPrefBranch.PREF_INT: {
+        return Services.prefs.getIntPref(prefKey);
+      }
+      case Ci.nsIPrefBranch.PREF_STRING: {
+        return Services.prefs.getStringPref(prefKey);
+      }
+    }
+
+    return undefined;
+  }
+
+  observe(subject, topic, data) {
+    if (topic == "nsPref:changed") {
+      for (let [widgetName, prefCache] of this.prefsCache) {
+        if (prefCache.hasOwnProperty(data)) {
+          let newValue = this.getPref(data);
+          prefCache[data] = newValue;
+
+          this.notifyWidgetsOnPrefChange(widgetName, data, newValue);
+        }
+      }
+    }
+  }
+
+  notifyWidgetsOnPrefChange(nameOfWidgetToNotify, prefKey, newValue) {
+    let elements = ChromeUtils.nondeterministicGetWeakMapKeys(this.widgets);
+    for (let element of elements) {
+      if (!Cu.isDeadWrapper(element) && element.isConnected) {
+        let { widgetName, widget } = this.widgets.get(element);
+        if (widgetName == nameOfWidgetToNotify) {
+          if (typeof widget.onPrefChange == "function") {
+            try {
+              widget.onPrefChange(prefKey, newValue);
+            } catch (ex) {
+              console.error(ex);
+            }
+          }
+        }
+      }
+    }
+  }
+}
diff --git a/toolkit/actors/UnselectedTabHoverChild.sys.mjs b/toolkit/actors/UnselectedTabHoverChild.sys.mjs
new file mode 100644
index 0000000000..1cf1ddb9fd
--- /dev/null
+++ b/toolkit/actors/UnselectedTabHoverChild.sys.mjs
@@ -0,0 +1,20 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class UnselectedTabHoverChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    Services.obs.notifyObservers(
+      this.contentWindow,
+      "unselected-tab-hover",
+      message.data.hovered
+    );
+  }
+
+  handleEvent(event) {
+    this.sendAsyncMessage("UnselectedTabHover:Toggle", {
+      enable: event.type == "UnselectedTabHover:Enable",
+    });
+  }
+}
diff --git a/toolkit/actors/UnselectedTabHoverParent.sys.mjs b/toolkit/actors/UnselectedTabHoverParent.sys.mjs
new file mode 100644
index 0000000000..797cbd8af2
--- /dev/null
+++ b/toolkit/actors/UnselectedTabHoverParent.sys.mjs
@@ -0,0 +1,15 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export class UnselectedTabHoverParent extends JSWindowActorParent {
+  receiveMessage(message) {
+    const topBrowsingContext = this.manager.browsingContext.top;
+    const browser = topBrowsingContext.embedderElement;
+    if (!browser) {
+      return;
+    }
+    browser.shouldHandleUnselectedTabHover = message.data.enable;
+  }
+}
diff --git a/toolkit/actors/ViewSourceChild.sys.mjs b/toolkit/actors/ViewSourceChild.sys.mjs
new file mode 100644
index 0000000000..4c573865b7
--- /dev/null
+++ b/toolkit/actors/ViewSourceChild.sys.mjs
@@ -0,0 +1,346 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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, {
+  ViewSourcePageChild: "resource://gre/actors/ViewSourcePageChild.sys.mjs",
+});
+
+export class ViewSourceChild extends JSWindowActorChild {
+  receiveMessage(message) {
+    let data = message.data;
+    switch (message.name) {
+      case "ViewSource:LoadSource":
+        this.viewSource(data.URL, data.outerWindowID, data.lineNumber);
+        break;
+      case "ViewSource:LoadSourceWithSelection":
+        this.viewSourceWithSelection(
+          data.URL,
+          data.drawSelection,
+          data.baseURI
+        );
+        break;
+      case "ViewSource:GetSelection":
+        let selectionDetails;
+        try {
+          selectionDetails = this.getSelection(this.document.ownerGlobal);
+        } catch (e) {}
+        return selectionDetails;
+    }
+
+    return undefined;
+  }
+
+  /**
+   * Called when the parent sends a message to view some source code.
+   *
+   * @param URL (required)
+   *        The URL string of the source to be shown.
+   * @param outerWindowID (optional)
+   *        The outerWindowID of the content window that has hosted
+   *        the document, in case we want to retrieve it from the network
+   *        cache.
+   * @param lineNumber (optional)
+   *        The line number to focus as soon as the source has finished
+   *        loading.
+   */
+  viewSource(URL, outerWindowID, lineNumber) {
+    let otherDocShell;
+    let forceEncodingDetection = false;
+
+    if (outerWindowID) {
+      let contentWindow = Services.wm.getOuterWindowWithId(outerWindowID);
+      if (contentWindow) {
+        otherDocShell = contentWindow.docShell;
+
+        forceEncodingDetection = contentWindow.windowUtils.docCharsetIsForced;
+      }
+    }
+
+    this.loadSource(URL, otherDocShell, lineNumber, forceEncodingDetection);
+  }
+
+  /**
+   * Loads a view source selection showing the given view-source url and
+   * highlight the selection.
+   *
+   * @param uri view-source uri to show
+   * @param drawSelection true to highlight the selection
+   * @param baseURI base URI of the original document
+   */
+  viewSourceWithSelection(uri, drawSelection, baseURI) {
+    // This isn't ideal, but set a global in the view source page actor
+    // that indicates that a selection should be drawn. It will be read
+    // when by the page's pageshow listener. This should work as the
+    // view source page is always loaded in the same process.
+    lazy.ViewSourcePageChild.setNeedsDrawSelection(drawSelection);
+
+    // all our content is held by the data:URI and URIs are internally stored as utf-8 (see nsIURI.idl)
+    let loadFlags = Ci.nsIWebNavigation.LOAD_FLAGS_NONE;
+    let webNav = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+    let loadURIOptions = {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      loadFlags,
+      baseURI: Services.io.newURI(baseURI),
+    };
+    webNav.fixupAndLoadURIString(uri, loadURIOptions);
+  }
+
+  /**
+   * Common utility function used by both the current and deprecated APIs
+   * for loading source.
+   *
+   * @param URL (required)
+   *        The URL string of the source to be shown.
+   * @param otherDocShell (optional)
+   *        The docshell of the content window that is hosting the document.
+   * @param lineNumber (optional)
+   *        The line number to focus as soon as the source has finished
+   *        loading.
+   * @param forceEncodingDetection (optional)
+   *        Force autodetection of the character encoding.
+   */
+  loadSource(URL, otherDocShell, lineNumber, forceEncodingDetection) {
+    const viewSrcURL = "view-source:" + URL;
+
+    if (forceEncodingDetection) {
+      this.docShell.forceEncodingDetection();
+    }
+
+    if (lineNumber) {
+      lazy.ViewSourcePageChild.setInitialLineNumber(lineNumber);
+    }
+
+    if (!otherDocShell) {
+      this.loadSourceFromURL(viewSrcURL);
+      return;
+    }
+
+    try {
+      let pageLoader = this.docShell.QueryInterface(Ci.nsIWebPageDescriptor);
+      pageLoader.loadPageAsViewSource(otherDocShell, viewSrcURL);
+    } catch (e) {
+      // We were not able to load the source from the network cache.
+      this.loadSourceFromURL(viewSrcURL);
+    }
+  }
+
+  /**
+   * Load some URL in the browser.
+   *
+   * @param URL
+   *        The URL string to load.
+   */
+  loadSourceFromURL(URL) {
+    let loadFlags = Ci.nsIWebNavigation.LOAD_FLAGS_NONE;
+    let webNav = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+    let loadURIOptions = {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      loadFlags,
+    };
+    webNav.fixupAndLoadURIString(URL, loadURIOptions);
+  }
+
+  /**
+   * A helper to get a path like FIXptr, but with an array instead of the
+   * "tumbler" notation.
+   * See FIXptr: http://lists.w3.org/Archives/Public/www-xml-linking-comments/2001AprJun/att-0074/01-NOTE-FIXptr-20010425.htm
+   */
+  getPath(ancestor, node) {
+    var n = node;
+    var p = n.parentNode;
+    if (n == ancestor || !p) {
+      return null;
+    }
+    var path = [];
+    if (!path) {
+      return null;
+    }
+    do {
+      for (var i = 0; i < p.childNodes.length; i++) {
+        if (p.childNodes.item(i) == n) {
+          path.push(i);
+          break;
+        }
+      }
+      n = p;
+      p = n.parentNode;
+    } while (n != ancestor && p);
+    return path;
+  }
+
+  getSelection(global) {
+    const { content } = global;
+
+    // These are markers used to delimit the selection during processing. They
+    // are removed from the final rendering.
+    // We use noncharacter Unicode codepoints to minimize the risk of clashing
+    // with anything that might legitimately be present in the document.
+    // U+FDD0..FDEF <noncharacters>
+    const MARK_SELECTION_START = "\uFDD0";
+    const MARK_SELECTION_END = "\uFDEF";
+
+    var focusedWindow = Services.focus.focusedWindow || content;
+    var selection = focusedWindow.getSelection();
+
+    var range = selection.getRangeAt(0);
+    var ancestorContainer = range.commonAncestorContainer;
+    var doc = ancestorContainer.ownerDocument;
+
+    var startContainer = range.startContainer;
+    var endContainer = range.endContainer;
+    var startOffset = range.startOffset;
+    var endOffset = range.endOffset;
+
+    // let the ancestor be an element
+    var Node = doc.defaultView.Node;
+    if (
+      ancestorContainer.nodeType == Node.TEXT_NODE ||
+      ancestorContainer.nodeType == Node.CDATA_SECTION_NODE
+    ) {
+      ancestorContainer = ancestorContainer.parentNode;
+    }
+
+    // for selectAll, let's use the entire document, including <html>...</html>
+    // @see nsDocumentViewer::SelectAll() for how selectAll is implemented
+    try {
+      if (ancestorContainer == doc.body) {
+        ancestorContainer = doc.documentElement;
+      }
+    } catch (e) {}
+
+    // each path is a "child sequence" (a.k.a. "tumbler") that
+    // descends from the ancestor down to the boundary point
+    var startPath = this.getPath(ancestorContainer, startContainer);
+    var endPath = this.getPath(ancestorContainer, endContainer);
+
+    // clone the fragment of interest and reset everything to be relative to it
+    // note: it is with the clone that we operate/munge from now on.  Also note
+    // that we clone into a data document to prevent images in the fragment from
+    // loading and the like.  The use of importNode here, as opposed to adoptNode,
+    // is _very_ important.
+    // XXXbz wish there were a less hacky way to create an untrusted document here
+    var isHTML = doc.createElement("div").tagName == "DIV";
+    var dataDoc = isHTML
+      ? ancestorContainer.ownerDocument.implementation.createHTMLDocument("")
+      : ancestorContainer.ownerDocument.implementation.createDocument(
+          "",
+          "",
+          null
+        );
+    ancestorContainer = dataDoc.importNode(ancestorContainer, true);
+    startContainer = ancestorContainer;
+    endContainer = ancestorContainer;
+
+    // Only bother with the selection if it can be remapped. Don't mess with
+    // leaf elements (such as <isindex>) that secretly use anynomous content
+    // for their display appearance.
+    var canDrawSelection = ancestorContainer.hasChildNodes();
+    var tmpNode;
+    if (canDrawSelection) {
+      var i;
+      for (i = startPath ? startPath.length - 1 : -1; i >= 0; i--) {
+        startContainer = startContainer.childNodes.item(startPath[i]);
+      }
+      for (i = endPath ? endPath.length - 1 : -1; i >= 0; i--) {
+        endContainer = endContainer.childNodes.item(endPath[i]);
+      }
+
+      // add special markers to record the extent of the selection
+      // note: |startOffset| and |endOffset| are interpreted either as
+      // offsets in the text data or as child indices (see the Range spec)
+      // (here, munging the end point first to keep the start point safe...)
+      if (
+        endContainer.nodeType == Node.TEXT_NODE ||
+        endContainer.nodeType == Node.CDATA_SECTION_NODE
+      ) {
+        // do some extra tweaks to try to avoid the view-source output to look like
+        // ...<tag>]... or ...]</tag>... (where ']' marks the end of the selection).
+        // To get a neat output, the idea here is to remap the end point from:
+        // 1. ...<tag>]...   to   ...]<tag>...
+        // 2. ...]</tag>...  to   ...</tag>]...
+        if (
+          (endOffset > 0 && endOffset < endContainer.data.length) ||
+          !endContainer.parentNode ||
+          !endContainer.parentNode.parentNode
+        ) {
+          endContainer.insertData(endOffset, MARK_SELECTION_END);
+        } else {
+          tmpNode = dataDoc.createTextNode(MARK_SELECTION_END);
+          endContainer = endContainer.parentNode;
+          if (endOffset === 0) {
+            endContainer.parentNode.insertBefore(tmpNode, endContainer);
+          } else {
+            endContainer.parentNode.insertBefore(
+              tmpNode,
+              endContainer.nextSibling
+            );
+          }
+        }
+      } else {
+        tmpNode = dataDoc.createTextNode(MARK_SELECTION_END);
+        endContainer.insertBefore(
+          tmpNode,
+          endContainer.childNodes.item(endOffset)
+        );
+      }
+
+      if (
+        startContainer.nodeType == Node.TEXT_NODE ||
+        startContainer.nodeType == Node.CDATA_SECTION_NODE
+      ) {
+        // do some extra tweaks to try to avoid the view-source output to look like
+        // ...<tag>[... or ...[</tag>... (where '[' marks the start of the selection).
+        // To get a neat output, the idea here is to remap the start point from:
+        // 1. ...<tag>[...   to   ...[<tag>...
+        // 2. ...[</tag>...  to   ...</tag>[...
+        if (
+          (startOffset > 0 && startOffset < startContainer.data.length) ||
+          !startContainer.parentNode ||
+          !startContainer.parentNode.parentNode ||
+          startContainer != startContainer.parentNode.lastChild
+        ) {
+          startContainer.insertData(startOffset, MARK_SELECTION_START);
+        } else {
+          tmpNode = dataDoc.createTextNode(MARK_SELECTION_START);
+          startContainer = startContainer.parentNode;
+          if (startOffset === 0) {
+            startContainer.parentNode.insertBefore(tmpNode, startContainer);
+          } else {
+            startContainer.parentNode.insertBefore(
+              tmpNode,
+              startContainer.nextSibling
+            );
+          }
+        }
+      } else {
+        tmpNode = dataDoc.createTextNode(MARK_SELECTION_START);
+        startContainer.insertBefore(
+          tmpNode,
+          startContainer.childNodes.item(startOffset)
+        );
+      }
+    }
+
+    // now extract and display the syntax highlighted source
+    tmpNode = dataDoc.createElementNS("http://www.w3.org/1999/xhtml", "div");
+    tmpNode.appendChild(ancestorContainer);
+
+    return {
+      URL:
+        (isHTML
+          ? "view-source:data:text/html;charset=utf-8,"
+          : "view-source:data:application/xml;charset=utf-8,") +
+        encodeURIComponent(tmpNode.innerHTML),
+      drawSelection: canDrawSelection,
+      baseURI: doc.baseURI,
+    };
+  }
+
+  get wrapLongLines() {
+    return Services.prefs.getBoolPref("view_source.wrap_long_lines");
+  }
+}
diff --git a/toolkit/actors/ViewSourcePageChild.sys.mjs b/toolkit/actors/ViewSourcePageChild.sys.mjs
new file mode 100644
index 0000000000..2f5c53cc2e
--- /dev/null
+++ b/toolkit/actors/ViewSourcePageChild.sys.mjs
@@ -0,0 +1,474 @@
+/* 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 BUNDLE_URL = "chrome://global/locale/viewSource.properties";
+
+// These are markers used to delimit the selection during processing. They
+// are removed from the final rendering.
+// We use noncharacter Unicode codepoints to minimize the risk of clashing
+// with anything that might legitimately be present in the document.
+// U+FDD0..FDEF <noncharacters>
+const MARK_SELECTION_START = "\uFDD0";
+const MARK_SELECTION_END = "\uFDEF";
+
+/**
+ * When showing selection source, chrome will construct a page fragment to
+ * show, and then instruct content to draw a selection after load.  This is
+ * set true when there is a pending request to draw selection.
+ */
+let gNeedsDrawSelection = false;
+
+/**
+ * Start at a specific line number.
+ */
+let gInitialLineNumber = -1;
+
+export class ViewSourcePageChild extends JSWindowActorChild {
+  constructor() {
+    super();
+
+    XPCOMUtils.defineLazyGetter(this, "bundle", function () {
+      return Services.strings.createBundle(BUNDLE_URL);
+    });
+  }
+
+  static setNeedsDrawSelection(value) {
+    gNeedsDrawSelection = value;
+  }
+
+  static setInitialLineNumber(value) {
+    gInitialLineNumber = value;
+  }
+
+  receiveMessage(msg) {
+    switch (msg.name) {
+      case "ViewSource:GoToLine":
+        this.goToLine(msg.data.lineNumber);
+        break;
+      case "ViewSource:IsWrapping":
+        return this.isWrapping;
+      case "ViewSource:IsSyntaxHighlighting":
+        return this.isSyntaxHighlighting;
+      case "ViewSource:ToggleWrapping":
+        this.toggleWrapping();
+        break;
+      case "ViewSource:ToggleSyntaxHighlighting":
+        this.toggleSyntaxHighlighting();
+        break;
+    }
+    return undefined;
+  }
+
+  /**
+   * Any events should get handled here, and should get dispatched to
+   * a specific function for the event type.
+   */
+  handleEvent(event) {
+    switch (event.type) {
+      case "pageshow":
+        this.onPageShow(event);
+        break;
+      case "click":
+        this.onClick(event);
+        break;
+    }
+  }
+
+  /**
+   * A shortcut to the nsISelectionController for the content.
+   */
+  get selectionController() {
+    return this.docShell
+      .QueryInterface(Ci.nsIInterfaceRequestor)
+      .getInterface(Ci.nsISelectionDisplay)
+      .QueryInterface(Ci.nsISelectionController);
+  }
+
+  /**
+   * A shortcut to the nsIWebBrowserFind for the content.
+   */
+  get webBrowserFind() {
+    return this.docShell
+      .QueryInterface(Ci.nsIInterfaceRequestor)
+      .getInterface(Ci.nsIWebBrowserFind);
+  }
+
+  /**
+   * This handler is for click events from:
+   *   * error page content, which can show up if the user attempts to view the
+   *     source of an attack page.
+   */
+  onClick(event) {
+    let target = event.originalTarget;
+
+    // Don't trust synthetic events
+    if (!event.isTrusted || event.target.localName != "button") {
+      return;
+    }
+
+    let errorDoc = target.ownerDocument;
+
+    if (/^about:blocked/.test(errorDoc.documentURI)) {
+      // The event came from a button on a malware/phishing block page
+
+      if (target == errorDoc.getElementById("goBackButton")) {
+        // Instead of loading some safe page, just close the window
+        this.sendAsyncMessage("ViewSource:Close");
+      }
+    }
+  }
+
+  /**
+   * Handler for the pageshow event.
+   *
+   * @param event
+   *        The pageshow event being handled.
+   */
+  onPageShow(event) {
+    // If we need to draw the selection, wait until an actual view source page
+    // has loaded, instead of about:blank.
+    if (
+      gNeedsDrawSelection &&
+      this.document.documentURI.startsWith("view-source:")
+    ) {
+      gNeedsDrawSelection = false;
+      this.drawSelection();
+    }
+
+    if (gInitialLineNumber >= 0) {
+      this.goToLine(gInitialLineNumber);
+      gInitialLineNumber = -1;
+    }
+  }
+
+  /**
+   * Attempts to go to a particular line in the source code being
+   * shown. If it succeeds in finding the line, it will fire a
+   * "ViewSource:GoToLine:Success" message, passing up an object
+   * with the lineNumber we just went to. If it cannot find the line,
+   * it will fire a "ViewSource:GoToLine:Failed" message.
+   *
+   * @param lineNumber
+   *        The line number to attempt to go to.
+   */
+  goToLine(lineNumber) {
+    let body = this.document.body;
+
+    // The source document is made up of a number of pre elements with
+    // id attributes in the format <pre id="line123">, meaning that
+    // the first line in the pre element is number 123.
+    // Do binary search to find the pre element containing the line.
+    // However, in the plain text case, we have only one pre without an
+    // attribute, so assume it begins on line 1.
+    let pre;
+    for (let lbound = 0, ubound = body.childNodes.length; ; ) {
+      let middle = (lbound + ubound) >> 1;
+      pre = body.childNodes[middle];
+
+      let firstLine = pre.id ? parseInt(pre.id.substring(4)) : 1;
+
+      if (lbound == ubound - 1) {
+        break;
+      }
+
+      if (lineNumber >= firstLine) {
+        lbound = middle;
+      } else {
+        ubound = middle;
+      }
+    }
+
+    let result = {};
+    let found = this.findLocation(pre, lineNumber, null, -1, false, result);
+
+    if (!found) {
+      this.sendAsyncMessage("ViewSource:GoToLine:Failed");
+      return;
+    }
+
+    let selection = this.document.defaultView.getSelection();
+    selection.removeAllRanges();
+
+    // In our case, the range's startOffset is after "\n" on the previous line.
+    // Tune the selection at the beginning of the next line and do some tweaking
+    // to position the focusNode and the caret at the beginning of the line.
+    selection.interlinePosition = true;
+
+    selection.addRange(result.range);
+
+    if (!selection.isCollapsed) {
+      selection.collapseToEnd();
+
+      let offset = result.range.startOffset;
+      let node = result.range.startContainer;
+      if (offset < node.data.length) {
+        // The same text node spans across the "\n", just focus where we were.
+        selection.extend(node, offset);
+      } else {
+        // There is another tag just after the "\n", hook there. We need
+        // to focus a safe point because there are edgy cases such as
+        // <span>...\n</span><span>...</span> vs.
+        // <span>...\n<span>...</span></span><span>...</span>
+        node = node.nextSibling
+          ? node.nextSibling
+          : node.parentNode.nextSibling;
+        selection.extend(node, 0);
+      }
+    }
+
+    let selCon = this.selectionController;
+    selCon.setDisplaySelection(Ci.nsISelectionController.SELECTION_ON);
+    selCon.setCaretVisibilityDuringSelection(true);
+
+    // Scroll the beginning of the line into view.
+    selCon.scrollSelectionIntoView(
+      Ci.nsISelectionController.SELECTION_NORMAL,
+      Ci.nsISelectionController.SELECTION_FOCUS_REGION,
+      true
+    );
+
+    this.sendAsyncMessage("ViewSource:GoToLine:Success", { lineNumber });
+  }
+
+  /**
+   * Some old code from the original view source implementation. Original
+   * documentation follows:
+   *
+   * "Loops through the text lines in the pre element. The arguments are either
+   *  (pre, line) or (node, offset, interlinePosition). result is an out
+   *  argument. If (pre, line) are specified (and node == null), result.range is
+   *  a range spanning the specified line. If the (node, offset,
+   *  interlinePosition) are specified, result.line and result.col are the line
+   *  and column number of the specified offset in the specified node relative to
+   *  the whole file."
+   */
+  findLocation(pre, lineNumber, node, offset, interlinePosition, result) {
+    if (node && !pre) {
+      // Look upwards to find the current pre element.
+      // eslint-disable-next-line no-empty
+      for (pre = node; pre.nodeName != "PRE"; pre = pre.parentNode) {}
+    }
+
+    // The source document is made up of a number of pre elements with
+    // id attributes in the format <pre id="line123">, meaning that
+    // the first line in the pre element is number 123.
+    // However, in the plain text case, there is only one <pre> without an id,
+    // so assume line 1.
+    let curLine = pre.id ? parseInt(pre.id.substring(4)) : 1;
+
+    // Walk through each of the text nodes and count newlines.
+    let treewalker = this.document.createTreeWalker(
+      pre,
+      NodeFilter.SHOW_TEXT,
+      null
+    );
+
+    // The column number of the first character in the current text node.
+    let firstCol = 1;
+
+    let found = false;
+    for (
+      let textNode = treewalker.firstChild();
+      textNode && !found;
+      textNode = treewalker.nextNode()
+    ) {
+      // \r is not a valid character in the DOM, so we only check for \n.
+      let lineArray = textNode.data.split(/\n/);
+      let lastLineInNode = curLine + lineArray.length - 1;
+
+      // Check if we can skip the text node without further inspection.
+      if (node ? textNode != node : lastLineInNode < lineNumber) {
+        if (lineArray.length > 1) {
+          firstCol = 1;
+        }
+        firstCol += lineArray[lineArray.length - 1].length;
+        curLine = lastLineInNode;
+        continue;
+      }
+
+      // curPos is the offset within the current text node of the first
+      // character in the current line.
+      for (
+        var i = 0, curPos = 0;
+        i < lineArray.length;
+        curPos += lineArray[i++].length + 1
+      ) {
+        if (i > 0) {
+          curLine++;
+        }
+
+        if (node) {
+          if (offset >= curPos && offset <= curPos + lineArray[i].length) {
+            // If we are right after the \n of a line and interlinePosition is
+            // false, the caret looks as if it were at the end of the previous
+            // line, so we display that line and column instead.
+
+            if (i > 0 && offset == curPos && !interlinePosition) {
+              result.line = curLine - 1;
+              var prevPos = curPos - lineArray[i - 1].length;
+              result.col = (i == 1 ? firstCol : 1) + offset - prevPos;
+            } else {
+              result.line = curLine;
+              result.col = (i == 0 ? firstCol : 1) + offset - curPos;
+            }
+            found = true;
+
+            break;
+          }
+        } else if (curLine == lineNumber && !("range" in result)) {
+          result.range = this.document.createRange();
+          result.range.setStart(textNode, curPos);
+
+          // This will always be overridden later, except when we look for
+          // the very last line in the file (this is the only line that does
+          // not end with \n).
+          result.range.setEndAfter(pre.lastChild);
+        } else if (curLine == lineNumber + 1) {
+          result.range.setEnd(textNode, curPos - 1);
+          found = true;
+          break;
+        }
+      }
+    }
+
+    return found || "range" in result;
+  }
+
+  /**
+   * @return {boolean} whether the "wrap" class exists on the document body.
+   */
+  get isWrapping() {
+    return this.document.body.classList.contains("wrap");
+  }
+
+  /**
+   * @return {boolean} whether the "highlight" class exists on the document body.
+   */
+  get isSyntaxHighlighting() {
+    return this.document.body.classList.contains("highlight");
+  }
+
+  /**
+   * Toggles the "wrap" class on the document body, which sets whether
+   * or not long lines are wrapped.  Notifies parent to update the pref.
+   */
+  toggleWrapping() {
+    let body = this.document.body;
+    let state = body.classList.toggle("wrap");
+    this.sendAsyncMessage("ViewSource:StoreWrapping", { state });
+  }
+
+  /**
+   * Toggles the "highlight" class on the document body, which sets whether
+   * or not syntax highlighting is displayed.  Notifies parent to update the
+   * pref.
+   */
+  toggleSyntaxHighlighting() {
+    let body = this.document.body;
+    let state = body.classList.toggle("highlight");
+    this.sendAsyncMessage("ViewSource:StoreSyntaxHighlighting", { state });
+  }
+
+  /**
+   * Using special markers left in the serialized source, this helper makes the
+   * underlying markup of the selected fragment to automatically appear as
+   * selected on the inflated view-source DOM.
+   */
+  drawSelection() {
+    this.document.title = this.bundle.GetStringFromName(
+      "viewSelectionSourceTitle"
+    );
+
+    // find the special selection markers that we added earlier, and
+    // draw the selection between the two...
+    var findService = null;
+    try {
+      // get the find service which stores the global find state
+      findService = Cc["@mozilla.org/find/find_service;1"].getService(
+        Ci.nsIFindService
+      );
+    } catch (e) {}
+    if (!findService) {
+      return;
+    }
+
+    // cache the current global find state
+    var matchCase = findService.matchCase;
+    var entireWord = findService.entireWord;
+    var wrapFind = findService.wrapFind;
+    var findBackwards = findService.findBackwards;
+    var searchString = findService.searchString;
+    var replaceString = findService.replaceString;
+
+    // setup our find instance
+    var findInst = this.webBrowserFind;
+    findInst.matchCase = true;
+    findInst.entireWord = false;
+    findInst.wrapFind = true;
+    findInst.findBackwards = false;
+
+    // ...lookup the start mark
+    findInst.searchString = MARK_SELECTION_START;
+    var startLength = MARK_SELECTION_START.length;
+    findInst.findNext();
+
+    var selection = this.document.defaultView.getSelection();
+    if (!selection.rangeCount) {
+      return;
+    }
+
+    var range = selection.getRangeAt(0);
+
+    var startContainer = range.startContainer;
+    var startOffset = range.startOffset;
+
+    // ...lookup the end mark
+    findInst.searchString = MARK_SELECTION_END;
+    var endLength = MARK_SELECTION_END.length;
+    findInst.findNext();
+
+    var endContainer = selection.anchorNode;
+    var endOffset = selection.anchorOffset;
+
+    // reset the selection that find has left
+    selection.removeAllRanges();
+
+    // delete the special markers now...
+    endContainer.deleteData(endOffset, endLength);
+    startContainer.deleteData(startOffset, startLength);
+    if (startContainer == endContainer) {
+      endOffset -= startLength;
+    } // has shrunk if on same text node...
+    range.setEnd(endContainer, endOffset);
+
+    // show the selection and scroll it into view
+    selection.addRange(range);
+    // the default behavior of the selection is to scroll at the end of
+    // the selection, whereas in this situation, it is more user-friendly
+    // to scroll at the beginning. So we override the default behavior here
+    try {
+      this.selectionController.scrollSelectionIntoView(
+        Ci.nsISelectionController.SELECTION_NORMAL,
+        Ci.nsISelectionController.SELECTION_ANCHOR_REGION,
+        true
+      );
+    } catch (e) {}
+
+    // restore the current find state
+    findService.matchCase = matchCase;
+    findService.entireWord = entireWord;
+    findService.wrapFind = wrapFind;
+    findService.findBackwards = findBackwards;
+    findService.searchString = searchString;
+    findService.replaceString = replaceString;
+
+    findInst.matchCase = matchCase;
+    findInst.entireWord = entireWord;
+    findInst.wrapFind = wrapFind;
+    findInst.findBackwards = findBackwards;
+    findInst.searchString = searchString;
+  }
+}
diff --git a/toolkit/actors/ViewSourcePageParent.sys.mjs b/toolkit/actors/ViewSourcePageParent.sys.mjs
new file mode 100644
index 0000000000..cc45b248cc
--- /dev/null
+++ b/toolkit/actors/ViewSourcePageParent.sys.mjs
@@ -0,0 +1,165 @@
+// -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+
+/* 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 BUNDLE_URL = "chrome://global/locale/viewSource.properties";
+
+/**
+ * ViewSourcePageParent manages the view source <browser> from the chrome side.
+ */
+export class ViewSourcePageParent extends JSWindowActorParent {
+  constructor() {
+    super();
+
+    /**
+     * Holds the value of the last line found via the "Go to line"
+     * command, to pre-populate the prompt the next time it is
+     * opened.
+     */
+    this.lastLineFound = null;
+  }
+
+  /**
+   * Anything added to the messages array will get handled here, and should
+   * get dispatched to a specific function for the message name.
+   */
+  receiveMessage(message) {
+    let data = message.data;
+
+    switch (message.name) {
+      case "ViewSource:PromptAndGoToLine":
+        this.promptAndGoToLine();
+        break;
+      case "ViewSource:GoToLine:Success":
+        this.onGoToLineSuccess(data.lineNumber);
+        break;
+      case "ViewSource:GoToLine:Failed":
+        this.onGoToLineFailed();
+        break;
+      case "ViewSource:StoreWrapping":
+        this.storeWrapping(data.state);
+        break;
+      case "ViewSource:StoreSyntaxHighlighting":
+        this.storeSyntaxHighlighting(data.state);
+        break;
+    }
+  }
+
+  /**
+   * A getter for the view source string bundle.
+   */
+  get bundle() {
+    if (this._bundle) {
+      return this._bundle;
+    }
+    return (this._bundle = Services.strings.createBundle(BUNDLE_URL));
+  }
+
+  /**
+   * Opens the "Go to line" prompt for a user to hop to a particular line
+   * of the source code they're viewing. This will keep prompting until the
+   * user either cancels out of the prompt, or enters a valid line number.
+   */
+  promptAndGoToLine() {
+    let input = { value: this.lastLineFound };
+    let window = Services.wm.getMostRecentWindow(null);
+
+    let ok = Services.prompt.prompt(
+      window,
+      this.bundle.GetStringFromName("goToLineTitle"),
+      this.bundle.GetStringFromName("goToLineText"),
+      input,
+      null,
+      { value: 0 }
+    );
+
+    if (!ok) {
+      return;
+    }
+
+    let line = parseInt(input.value, 10);
+
+    if (!(line > 0)) {
+      Services.prompt.alert(
+        window,
+        this.bundle.GetStringFromName("invalidInputTitle"),
+        this.bundle.GetStringFromName("invalidInputText")
+      );
+      this.promptAndGoToLine();
+    } else {
+      this.goToLine(line);
+    }
+  }
+
+  /**
+   * Go to a particular line of the source code. This act is asynchronous.
+   *
+   * @param lineNumber
+   *        The line number to try to go to to.
+   */
+  goToLine(lineNumber) {
+    this.sendAsyncMessage("ViewSource:GoToLine", { lineNumber });
+  }
+
+  /**
+   * Called when the frame script reports that a line was successfully gotten
+   * to.
+   *
+   * @param lineNumber
+   *        The line number that we successfully got to.
+   */
+  onGoToLineSuccess(lineNumber) {
+    // We'll pre-populate the "Go to line" prompt with this value the next
+    // time it comes up.
+    this.lastLineFound = lineNumber;
+  }
+
+  /**
+   * Called when the child reports that we failed to go to a particular
+   * line. This informs the user that their selection was likely out of range,
+   * and then reprompts the user to try again.
+   */
+  onGoToLineFailed() {
+    let window = Services.wm.getMostRecentWindow(null);
+    Services.prompt.alert(
+      window,
+      this.bundle.GetStringFromName("outOfRangeTitle"),
+      this.bundle.GetStringFromName("outOfRangeText")
+    );
+    this.promptAndGoToLine();
+  }
+
+  /**
+   * @return {boolean} the wrapping state
+   */
+  queryIsWrapping() {
+    return this.sendQuery("ViewSource:IsWrapping");
+  }
+
+  /**
+   * @return {boolean} the syntax highlighting state
+   */
+  queryIsSyntaxHighlighting() {
+    return this.sendQuery("ViewSource:IsSyntaxHighlighting");
+  }
+
+  /**
+   * Update the wrapping pref based on the child's current state.
+   * @param state
+   *        Whether wrapping is currently enabled in the child.
+   */
+  storeWrapping(state) {
+    Services.prefs.setBoolPref("view_source.wrap_long_lines", state);
+  }
+
+  /**
+   * Update the syntax highlighting pref based on the child's current state.
+   * @param state
+   *        Whether syntax highlighting is currently enabled in the child.
+   */
+  storeSyntaxHighlighting(state) {
+    Services.prefs.setBoolPref("view_source.syntax_highlight", state);
+  }
+}
diff --git a/toolkit/actors/WebChannelChild.sys.mjs b/toolkit/actors/WebChannelChild.sys.mjs
new file mode 100644
index 0000000000..563f2566a0
--- /dev/null
+++ b/toolkit/actors/WebChannelChild.sys.mjs
@@ -0,0 +1,132 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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 { ContentDOMReference } from "resource://gre/modules/ContentDOMReference.sys.mjs";
+
+// Preference containing the list (space separated) of origins that are
+// allowed to send non-string values through a WebChannel, mainly for
+// backwards compatability. See bug 1238128 for more information.
+const URL_WHITELIST_PREF = "webchannel.allowObject.urlWhitelist";
+
+let _cachedWhitelist = null;
+
+const CACHED_PREFS = {};
+XPCOMUtils.defineLazyPreferenceGetter(
+  CACHED_PREFS,
+  "URL_WHITELIST",
+  URL_WHITELIST_PREF,
+  "",
+  // Null this out so we update it.
+  () => (_cachedWhitelist = null)
+);
+
+export class WebChannelChild extends JSWindowActorChild {
+  handleEvent(event) {
+    if (event.type === "WebChannelMessageToChrome") {
+      return this._onMessageToChrome(event);
+    }
+    return undefined;
+  }
+
+  receiveMessage(msg) {
+    if (msg.name === "WebChannelMessageToContent") {
+      return this._onMessageToContent(msg);
+    }
+    return undefined;
+  }
+
+  _getWhitelistedPrincipals() {
+    if (!_cachedWhitelist) {
+      let urls = CACHED_PREFS.URL_WHITELIST.split(/\s+/);
+      _cachedWhitelist = urls.map(origin =>
+        Services.scriptSecurityManager.createContentPrincipalFromOrigin(origin)
+      );
+    }
+    return _cachedWhitelist;
+  }
+
+  _onMessageToChrome(e) {
+    // If target is window then we want the document principal, otherwise fallback to target itself.
+    let principal = e.target.nodePrincipal
+      ? e.target.nodePrincipal
+      : e.target.document.nodePrincipal;
+
+    if (e.detail) {
+      if (typeof e.detail != "string") {
+        // Check if the principal is one of the ones that's allowed to send
+        // non-string values for e.detail.  They're whitelisted by site origin,
+        // so we compare on originNoSuffix in order to avoid other origin attributes
+        // that are not relevant here, such as containers or private browsing.
+        let objectsAllowed = this._getWhitelistedPrincipals().some(
+          whitelisted => principal.originNoSuffix == whitelisted.originNoSuffix
+        );
+        if (!objectsAllowed) {
+          console.error(
+            "WebChannelMessageToChrome sent with an object from a non-whitelisted principal"
+          );
+          return;
+        }
+      }
+
+      let eventTarget =
+        e.target instanceof Ci.nsIDOMWindow
+          ? null
+          : ContentDOMReference.get(e.target);
+      this.sendAsyncMessage("WebChannelMessageToChrome", {
+        contentData: e.detail,
+        eventTarget,
+        principal,
+      });
+    } else {
+      console.error("WebChannel message failed. No message detail.");
+    }
+  }
+
+  _onMessageToContent(msg) {
+    if (msg.data && this.contentWindow) {
+      // msg.objects.eventTarget will be defined if sending a response to
+      // a WebChannelMessageToChrome event. An unsolicited send
+      // may not have an eventTarget defined, in this case send to the
+      // main content window.
+      let { eventTarget, principal } = msg.data;
+      if (!eventTarget) {
+        eventTarget = this.contentWindow;
+      } else {
+        eventTarget = ContentDOMReference.resolve(eventTarget);
+      }
+      if (!eventTarget) {
+        console.error("WebChannel message failed. No target.");
+        return;
+      }
+
+      // Use nodePrincipal if available, otherwise fallback to document principal.
+      let targetPrincipal =
+        eventTarget instanceof Ci.nsIDOMWindow
+          ? eventTarget.document.nodePrincipal
+          : eventTarget.nodePrincipal;
+
+      if (principal.subsumes(targetPrincipal)) {
+        let targetWindow = this.contentWindow;
+        eventTarget.dispatchEvent(
+          new targetWindow.CustomEvent("WebChannelMessageToContent", {
+            detail: Cu.cloneInto(
+              {
+                id: msg.data.id,
+                message: msg.data.message,
+              },
+              targetWindow
+            ),
+          })
+        );
+      } else {
+        console.error("WebChannel message failed. Principal mismatch.");
+      }
+    } else {
+      console.error("WebChannel message failed. No message data.");
+    }
+  }
+}
diff --git a/toolkit/actors/WebChannelParent.sys.mjs b/toolkit/actors/WebChannelParent.sys.mjs
new file mode 100644
index 0000000000..98b3ad093a
--- /dev/null
+++ b/toolkit/actors/WebChannelParent.sys.mjs
@@ -0,0 +1,92 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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 { WebChannelBroker } from "resource://gre/modules/WebChannel.sys.mjs";
+
+const ERRNO_MISSING_PRINCIPAL = 1;
+const ERRNO_NO_SUCH_CHANNEL = 2;
+
+export class WebChannelParent extends JSWindowActorParent {
+  receiveMessage(msg) {
+    let data = msg.data.contentData;
+    let sendingContext = {
+      browsingContext: this.browsingContext,
+      browser: this.browsingContext.top.embedderElement,
+      eventTarget: msg.data.eventTarget,
+      principal: msg.data.principal,
+    };
+    // data must be a string except for a few legacy origins allowed by browser-content.js.
+    if (typeof data == "string") {
+      try {
+        data = JSON.parse(data);
+      } catch (e) {
+        console.error("Failed to parse WebChannel data as a JSON object");
+        return;
+      }
+    }
+
+    if (data && data.id) {
+      if (!msg.data.principal) {
+        this._sendErrorEventToContent(
+          data.id,
+          sendingContext,
+          ERRNO_MISSING_PRINCIPAL,
+          "Message principal missing"
+        );
+      } else {
+        let validChannelFound = WebChannelBroker.tryToDeliver(
+          data,
+          sendingContext
+        );
+
+        // if no valid origins send an event that there is no such valid channel
+        if (!validChannelFound) {
+          this._sendErrorEventToContent(
+            data.id,
+            sendingContext,
+            ERRNO_NO_SUCH_CHANNEL,
+            "No Such Channel"
+          );
+        }
+      }
+    } else {
+      console.error("WebChannel channel id missing");
+    }
+  }
+
+  /**
+   *
+   * @param id {String}
+   *        The WebChannel id to include in the message
+   * @param sendingContext {Object}
+   *        Message sending context
+   * @param [errorMsg] {String}
+   *        Error message
+   * @private
+   */
+  _sendErrorEventToContent(id, sendingContext, errorNo, errorMsg) {
+    let { eventTarget, principal } = sendingContext;
+
+    errorMsg = errorMsg || "Web Channel Parent error";
+
+    let { currentWindowGlobal = null } = this.browsingContext;
+    if (currentWindowGlobal) {
+      currentWindowGlobal
+        .getActor("WebChannel")
+        .sendAsyncMessage("WebChannelMessageToContent", {
+          id,
+          message: {
+            errno: errorNo,
+            error: errorMsg,
+          },
+          eventTarget,
+          principal,
+        });
+    } else {
+      console.error("Failed to send a WebChannel error. Target invalid.");
+    }
+    console.error(id.toString() + " error message. ", errorMsg);
+  }
+}
diff --git a/toolkit/actors/docs/picture-in-picture-child-video-wrapper-api.rst b/toolkit/actors/docs/picture-in-picture-child-video-wrapper-api.rst
new file mode 100644
index 0000000000..2e33741539
--- /dev/null
+++ b/toolkit/actors/docs/picture-in-picture-child-video-wrapper-api.rst
@@ -0,0 +1,6 @@
+.. _picture_in_picture_child_video_wrapper_api:
+
+PictureInPictureChildVideoWrapper Reference
+===========================================
+.. js:autoclass:: PictureInPictureChildVideoWrapper
+  :members:
diff --git a/toolkit/actors/moz.build b/toolkit/actors/moz.build
new file mode 100644
index 0000000000..a0a987a5e6
--- /dev/null
+++ b/toolkit/actors/moz.build
@@ -0,0 +1,85 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Toolkit", "General")
+
+with Files("AutoScroll*.jsm"):
+    BUG_COMPONENT = ("Core", "Panning and Zooming")
+
+with Files("Finder*.jsm"):
+    BUG_COMPONENT = ("Toolkit", "Find Toolbar")
+
+with Files("KeyPressEventModelCheckerChild.sys.mjs"):
+    BUG_COMPONENT = ("Core", "DOM: Events")
+
+with Files("PictureInPictureChild.sys.mjs"):
+    BUG_COMPONENT = ("Toolkit", "Picture-in-Picture")
+
+SPHINX_TREES["actors"] = "docs"
+
+TESTING_JS_MODULES += [
+    "TestProcessActorChild.jsm",
+    "TestProcessActorChild.sys.mjs",
+    "TestProcessActorParent.jsm",
+    "TestProcessActorParent.sys.mjs",
+    "TestWindowChild.jsm",
+    "TestWindowChild.sys.mjs",
+    "TestWindowParent.jsm",
+    "TestWindowParent.sys.mjs",
+]
+
+FINAL_TARGET_FILES.actors += [
+    "AboutHttpsOnlyErrorChild.sys.mjs",
+    "AboutHttpsOnlyErrorParent.sys.mjs",
+    "AudioPlaybackChild.sys.mjs",
+    "AudioPlaybackParent.sys.mjs",
+    "AutoCompleteChild.sys.mjs",
+    "AutoCompleteParent.sys.mjs",
+    "AutoplayChild.sys.mjs",
+    "AutoplayParent.sys.mjs",
+    "AutoScrollChild.sys.mjs",
+    "AutoScrollParent.sys.mjs",
+    "BackgroundThumbnailsChild.sys.mjs",
+    "BrowserElementChild.sys.mjs",
+    "BrowserElementParent.sys.mjs",
+    "ClipboardReadPasteChild.sys.mjs",
+    "ClipboardReadPasteParent.sys.mjs",
+    "ContentMetaChild.sys.mjs",
+    "ContentMetaParent.sys.mjs",
+    "ControllersChild.sys.mjs",
+    "ControllersParent.sys.mjs",
+    "DateTimePickerChild.sys.mjs",
+    "DateTimePickerParent.sys.mjs",
+    "ExtFindChild.sys.mjs",
+    "FindBarChild.sys.mjs",
+    "FindBarParent.sys.mjs",
+    "FinderChild.sys.mjs",
+    "InlineSpellCheckerChild.sys.mjs",
+    "InlineSpellCheckerParent.sys.mjs",
+    "KeyPressEventModelCheckerChild.sys.mjs",
+    "NetErrorChild.sys.mjs",
+    "NetErrorParent.sys.mjs",
+    "PictureInPictureChild.sys.mjs",
+    "PopupBlockingChild.sys.mjs",
+    "PopupBlockingParent.sys.mjs",
+    "PrintingChild.sys.mjs",
+    "PrintingParent.sys.mjs",
+    "PrintingSelectionChild.sys.mjs",
+    "PurgeSessionHistoryChild.sys.mjs",
+    "RemotePageChild.sys.mjs",
+    "SelectChild.sys.mjs",
+    "SelectParent.sys.mjs",
+    "ThumbnailsChild.sys.mjs",
+    "UAWidgetsChild.sys.mjs",
+    "UnselectedTabHoverChild.sys.mjs",
+    "UnselectedTabHoverParent.sys.mjs",
+    "ViewSourceChild.sys.mjs",
+    "ViewSourcePageChild.sys.mjs",
+    "ViewSourcePageParent.sys.mjs",
+    "WebChannelChild.sys.mjs",
+    "WebChannelParent.sys.mjs",
+]