Adding upstream version 115.7.0esr.upstream/115.7.0esr

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

1 files changed, 7229 insertions, 0 deletions

diff --git a/toolkit/mozapps/update/UpdateService.sys.mjs b/toolkit/mozapps/update/UpdateService.sys.mjs
new file mode 100644
index 0000000000..5e9caf090e
--- /dev/null
+++ b/toolkit/mozapps/update/UpdateService.sys.mjs
@@ -0,0 +1,7229 @@
+/* -*- Mode: javascript; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 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 { AUSTLMY } from "resource://gre/modules/UpdateTelemetry.sys.mjs";
+
+import {
+  Bits,
+  BitsRequest,
+  BitsUnknownError,
+  BitsVerificationError,
+} from "resource://gre/modules/Bits.sys.mjs";
+import { FileUtils } from "resource://gre/modules/FileUtils.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+  CertUtils: "resource://gre/modules/CertUtils.sys.mjs",
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+  UpdateUtils: "resource://gre/modules/UpdateUtils.sys.mjs",
+  WindowsRegistry: "resource://gre/modules/WindowsRegistry.sys.mjs",
+  ctypes: "resource://gre/modules/ctypes.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "AUS",
+  "@mozilla.org/updates/update-service;1",
+  "nsIApplicationUpdateService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "UM",
+  "@mozilla.org/updates/update-manager;1",
+  "nsIUpdateManager"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "CheckSvc",
+  "@mozilla.org/updates/update-checker;1",
+  "nsIUpdateChecker"
+);
+
+if (AppConstants.ENABLE_WEBDRIVER) {
+  XPCOMUtils.defineLazyServiceGetter(
+    lazy,
+    "Marionette",
+    "@mozilla.org/remote/marionette;1",
+    "nsIMarionette"
+  );
+
+  XPCOMUtils.defineLazyServiceGetter(
+    lazy,
+    "RemoteAgent",
+    "@mozilla.org/remote/agent;1",
+    "nsIRemoteAgent"
+  );
+} else {
+  lazy.Marionette = { running: false };
+  lazy.RemoteAgent = { running: false };
+}
+
+const UPDATESERVICE_CID = Components.ID(
+  "{B3C290A6-3943-4B89-8BBE-C01EB7B3B311}"
+);
+
+const PREF_APP_UPDATE_ALTUPDATEDIRPATH = "app.update.altUpdateDirPath";
+const PREF_APP_UPDATE_BACKGROUNDERRORS = "app.update.backgroundErrors";
+const PREF_APP_UPDATE_BACKGROUNDMAXERRORS = "app.update.backgroundMaxErrors";
+const PREF_APP_UPDATE_BITS_ENABLED = "app.update.BITS.enabled";
+const PREF_APP_UPDATE_CANCELATIONS = "app.update.cancelations";
+const PREF_APP_UPDATE_CANCELATIONS_OSX = "app.update.cancelations.osx";
+const PREF_APP_UPDATE_CANCELATIONS_OSX_MAX = "app.update.cancelations.osx.max";
+const PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_ENABLED =
+  "app.update.checkOnlyInstance.enabled";
+const PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_INTERVAL =
+  "app.update.checkOnlyInstance.interval";
+const PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_TIMEOUT =
+  "app.update.checkOnlyInstance.timeout";
+const PREF_APP_UPDATE_DISABLEDFORTESTING = "app.update.disabledForTesting";
+const PREF_APP_UPDATE_DOWNLOAD_ATTEMPTS = "app.update.download.attempts";
+const PREF_APP_UPDATE_DOWNLOAD_MAXATTEMPTS = "app.update.download.maxAttempts";
+const PREF_APP_UPDATE_ELEVATE_NEVER = "app.update.elevate.never";
+const PREF_APP_UPDATE_ELEVATE_VERSION = "app.update.elevate.version";
+const PREF_APP_UPDATE_ELEVATE_ATTEMPTS = "app.update.elevate.attempts";
+const PREF_APP_UPDATE_ELEVATE_MAXATTEMPTS = "app.update.elevate.maxAttempts";
+const PREF_APP_UPDATE_LANGPACK_ENABLED = "app.update.langpack.enabled";
+const PREF_APP_UPDATE_LANGPACK_TIMEOUT = "app.update.langpack.timeout";
+const PREF_APP_UPDATE_LOG = "app.update.log";
+const PREF_APP_UPDATE_LOG_FILE = "app.update.log.file";
+const PREF_APP_UPDATE_NOTIFYDURINGDOWNLOAD = "app.update.notifyDuringDownload";
+const PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_ENABLED =
+  "app.update.noWindowAutoRestart.enabled";
+const PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_DELAY_MS =
+  "app.update.noWindowAutoRestart.delayMs";
+const PREF_APP_UPDATE_PROMPTWAITTIME = "app.update.promptWaitTime";
+const PREF_APP_UPDATE_SERVICE_ENABLED = "app.update.service.enabled";
+const PREF_APP_UPDATE_SERVICE_ERRORS = "app.update.service.errors";
+const PREF_APP_UPDATE_SERVICE_MAXERRORS = "app.update.service.maxErrors";
+const PREF_APP_UPDATE_SOCKET_MAXERRORS = "app.update.socket.maxErrors";
+const PREF_APP_UPDATE_SOCKET_RETRYTIMEOUT = "app.update.socket.retryTimeout";
+const PREF_APP_UPDATE_STAGING_ENABLED = "app.update.staging.enabled";
+const PREF_APP_UPDATE_URL_DETAILS = "app.update.url.details";
+const PREF_NETWORK_PROXY_TYPE = "network.proxy.type";
+
+const URI_BRAND_PROPERTIES = "chrome://branding/locale/brand.properties";
+const URI_UPDATE_NS = "http://www.mozilla.org/2005/app-update";
+const URI_UPDATES_PROPERTIES =
+  "chrome://mozapps/locale/update/updates.properties";
+
+const KEY_EXECUTABLE = "XREExeF";
+const KEY_PROFILE_DIR = "ProfD";
+const KEY_UPDROOT = "UpdRootD";
+const KEY_OLD_UPDROOT = "OldUpdRootD";
+
+const DIR_UPDATES = "updates";
+const DIR_UPDATE_READY = "0";
+const DIR_UPDATE_DOWNLOADING = "downloading";
+
+const FILE_ACTIVE_UPDATE_XML = "active-update.xml";
+const FILE_BACKUP_UPDATE_LOG = "backup-update.log";
+const FILE_BT_RESULT = "bt.result";
+const FILE_LAST_UPDATE_LOG = "last-update.log";
+const FILE_UPDATES_XML = "updates.xml";
+const FILE_UPDATE_LOG = "update.log";
+const FILE_UPDATE_MAR = "update.mar";
+const FILE_UPDATE_STATUS = "update.status";
+const FILE_UPDATE_TEST = "update.test";
+const FILE_UPDATE_VERSION = "update.version";
+const FILE_UPDATE_MESSAGES = "update_messages.log";
+
+const STATE_NONE = "null";
+const STATE_DOWNLOADING = "downloading";
+const STATE_PENDING = "pending";
+const STATE_PENDING_SERVICE = "pending-service";
+const STATE_PENDING_ELEVATE = "pending-elevate";
+const STATE_APPLYING = "applying";
+const STATE_APPLIED = "applied";
+const STATE_APPLIED_SERVICE = "applied-service";
+const STATE_SUCCEEDED = "succeeded";
+const STATE_DOWNLOAD_FAILED = "download-failed";
+const STATE_FAILED = "failed";
+
+// BITS will keep retrying a download after transient errors, unless this much
+// time has passed since there has been download progress.
+// Similarly to ...POLL_RATE_MS below, we are much more aggressive when the user
+// is watching the download progress.
+const BITS_IDLE_NO_PROGRESS_TIMEOUT_SECS = 3600; // 1 hour
+const BITS_ACTIVE_NO_PROGRESS_TIMEOUT_SECS = 5;
+
+// These value control how frequently we get updates from the BITS client on
+// the progress made downloading. The difference between the two is that the
+// active interval is the one used when the user is watching. The idle interval
+// is the one used when no one is watching.
+const BITS_IDLE_POLL_RATE_MS = 1000;
+const BITS_ACTIVE_POLL_RATE_MS = 200;
+
+// The values below used by this code are from common/updatererrors.h
+const WRITE_ERROR = 7;
+const ELEVATION_CANCELED = 9;
+const SERVICE_UPDATER_COULD_NOT_BE_STARTED = 24;
+const SERVICE_NOT_ENOUGH_COMMAND_LINE_ARGS = 25;
+const SERVICE_UPDATER_SIGN_ERROR = 26;
+const SERVICE_UPDATER_COMPARE_ERROR = 27;
+const SERVICE_UPDATER_IDENTITY_ERROR = 28;
+const SERVICE_STILL_APPLYING_ON_SUCCESS = 29;
+const SERVICE_STILL_APPLYING_ON_FAILURE = 30;
+const SERVICE_UPDATER_NOT_FIXED_DRIVE = 31;
+const SERVICE_COULD_NOT_LOCK_UPDATER = 32;
+const SERVICE_INSTALLDIR_ERROR = 33;
+const WRITE_ERROR_ACCESS_DENIED = 35;
+const WRITE_ERROR_CALLBACK_APP = 37;
+const UNEXPECTED_STAGING_ERROR = 43;
+const DELETE_ERROR_STAGING_LOCK_FILE = 44;
+const SERVICE_COULD_NOT_COPY_UPDATER = 49;
+const SERVICE_STILL_APPLYING_TERMINATED = 50;
+const SERVICE_STILL_APPLYING_NO_EXIT_CODE = 51;
+const SERVICE_COULD_NOT_IMPERSONATE = 58;
+const WRITE_ERROR_FILE_COPY = 61;
+const WRITE_ERROR_DELETE_FILE = 62;
+const WRITE_ERROR_OPEN_PATCH_FILE = 63;
+const WRITE_ERROR_PATCH_FILE = 64;
+const WRITE_ERROR_APPLY_DIR_PATH = 65;
+const WRITE_ERROR_CALLBACK_PATH = 66;
+const WRITE_ERROR_FILE_ACCESS_DENIED = 67;
+const WRITE_ERROR_DIR_ACCESS_DENIED = 68;
+const WRITE_ERROR_DELETE_BACKUP = 69;
+const WRITE_ERROR_EXTRACT = 70;
+
+// Error codes 80 through 99 are reserved for UpdateService.jsm and are not
+// defined in common/updatererrors.h
+const ERR_UPDATER_CRASHED = 89;
+const ERR_OLDER_VERSION_OR_SAME_BUILD = 90;
+const ERR_UPDATE_STATE_NONE = 91;
+const ERR_CHANNEL_CHANGE = 92;
+const INVALID_UPDATER_STATE_CODE = 98;
+const INVALID_UPDATER_STATUS_CODE = 99;
+
+const SILENT_UPDATE_NEEDED_ELEVATION_ERROR = 105;
+const WRITE_ERROR_BACKGROUND_TASK_SHARING_VIOLATION = 106;
+
+// Array of write errors to simplify checks for write errors
+const WRITE_ERRORS = [
+  WRITE_ERROR,
+  WRITE_ERROR_ACCESS_DENIED,
+  WRITE_ERROR_CALLBACK_APP,
+  WRITE_ERROR_FILE_COPY,
+  WRITE_ERROR_DELETE_FILE,
+  WRITE_ERROR_OPEN_PATCH_FILE,
+  WRITE_ERROR_PATCH_FILE,
+  WRITE_ERROR_APPLY_DIR_PATH,
+  WRITE_ERROR_CALLBACK_PATH,
+  WRITE_ERROR_FILE_ACCESS_DENIED,
+  WRITE_ERROR_DIR_ACCESS_DENIED,
+  WRITE_ERROR_DELETE_BACKUP,
+  WRITE_ERROR_EXTRACT,
+  WRITE_ERROR_BACKGROUND_TASK_SHARING_VIOLATION,
+];
+
+// Array of write errors to simplify checks for service errors
+const SERVICE_ERRORS = [
+  SERVICE_UPDATER_COULD_NOT_BE_STARTED,
+  SERVICE_NOT_ENOUGH_COMMAND_LINE_ARGS,
+  SERVICE_UPDATER_SIGN_ERROR,
+  SERVICE_UPDATER_COMPARE_ERROR,
+  SERVICE_UPDATER_IDENTITY_ERROR,
+  SERVICE_STILL_APPLYING_ON_SUCCESS,
+  SERVICE_STILL_APPLYING_ON_FAILURE,
+  SERVICE_UPDATER_NOT_FIXED_DRIVE,
+  SERVICE_COULD_NOT_LOCK_UPDATER,
+  SERVICE_INSTALLDIR_ERROR,
+  SERVICE_COULD_NOT_COPY_UPDATER,
+  SERVICE_STILL_APPLYING_TERMINATED,
+  SERVICE_STILL_APPLYING_NO_EXIT_CODE,
+  SERVICE_COULD_NOT_IMPERSONATE,
+];
+
+// Custom update error codes
+const BACKGROUNDCHECK_MULTIPLE_FAILURES = 110;
+const NETWORK_ERROR_OFFLINE = 111;
+
+// Error codes should be < 1000. Errors above 1000 represent http status codes
+const HTTP_ERROR_OFFSET = 1000;
+
+// The is an HRESULT error that may be returned from the BITS interface
+// indicating that access was denied. Often, this error code is returned when
+// attempting to access a job created by a different user.
+const HRESULT_E_ACCESSDENIED = -2147024891;
+
+const DOWNLOAD_CHUNK_SIZE = 300000; // bytes
+
+// The number of consecutive failures when updating using the service before
+// setting the app.update.service.enabled preference to false.
+const DEFAULT_SERVICE_MAX_ERRORS = 10;
+
+// The number of consecutive socket errors to allow before falling back to
+// downloading a different MAR file or failing if already downloading the full.
+const DEFAULT_SOCKET_MAX_ERRORS = 10;
+
+// The number of milliseconds to wait before retrying a connection error.
+const DEFAULT_SOCKET_RETRYTIMEOUT = 2000;
+
+// Default maximum number of elevation cancelations per update version before
+// giving up.
+const DEFAULT_CANCELATIONS_OSX_MAX = 3;
+
+// This maps app IDs to their respective notification topic which signals when
+// the application's user interface has been displayed.
+const APPID_TO_TOPIC = {
+  // Firefox
+  "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}": "sessionstore-windows-restored",
+  // SeaMonkey
+  "{92650c4d-4b8e-4d2a-b7eb-24ecf4f6b63a}": "sessionstore-windows-restored",
+  // Thunderbird
+  "{3550f703-e582-4d05-9a08-453d09bdfdc6}": "mail-startup-done",
+};
+
+// The interval for the update xml write deferred task.
+const XML_SAVER_INTERVAL_MS = 200;
+
+// How long after a patch has downloaded should we wait for language packs to
+// update before proceeding anyway.
+const LANGPACK_UPDATE_DEFAULT_TIMEOUT = 300000;
+
+// Interval between rechecks for other instances after the initial check finds
+// at least one other instance.
+const ONLY_INSTANCE_CHECK_DEFAULT_POLL_INTERVAL_MS = 5 * 60 * 1000; // 5 minutes
+
+// Wait this long after detecting that another instance is running (having been
+// polling that entire time) before giving up and applying the update anyway.
+const ONLY_INSTANCE_CHECK_DEFAULT_TIMEOUT_MS = 6 * 60 * 60 * 1000; // 6 hours
+
+// The other instance check timeout can be overridden via a pref, but we limit
+// that value to this so that the pref can't effectively disable the feature.
+const ONLY_INSTANCE_CHECK_MAX_TIMEOUT_MS = 2 * 24 * 60 * 60 * 1000; // 2 days
+
+// Values to use when polling for staging. See `pollForStagingEnd` for more
+// details.
+const STAGING_POLLING_MIN_INTERVAL_MS = 15 * 1000; // 15 seconds
+const STAGING_POLLING_MAX_INTERVAL_MS = 5 * 60 * 1000; // 5 minutes
+const STAGING_POLLING_ATTEMPTS_PER_INTERVAL = 5;
+const STAGING_POLLING_MAX_DURATION_MS = 1 * 60 * 60 * 1000; // 1 hour
+
+var gUpdateMutexHandle = null;
+// This is the file stream used for the log file.
+var gLogfileOutputStream;
+// This value will be set to true if it appears that BITS is being used by
+// another user to download updates. We don't really want two users using BITS
+// at once. Computers with many users (ex: a school computer), should not end
+// up with dozens of BITS jobs.
+var gBITSInUseByAnotherUser = false;
+// The update service can be invoked as part of a standalone headless background
+// task.  In this context, when the background task kicks off an update
+// download, we don't want it to move on to staging. As soon as the download has
+// kicked off, the task begins shutting down and, even if the the download
+// completes incredibly quickly, we don't want staging to begin while we are
+// shutting down. That isn't a well tested scenario and it's possible that it
+// could leave us in a bad state.
+let gOnlyDownloadUpdatesThisSession = false;
+// This will be the backing for `nsIApplicationUpdateService.currentState`
+var gUpdateState = Ci.nsIApplicationUpdateService.STATE_IDLE;
+
+/**
+ * Simple container and constructor for a Promise and its resolve function.
+ */
+class SelfContainedPromise {
+  constructor() {
+    this.promise = new Promise(resolve => {
+      this.resolve = resolve;
+    });
+  }
+}
+
+// This will contain a `SelfContainedPromise` that will be used to back
+// `nsIApplicationUpdateService.stateTransition`.
+var gStateTransitionPromise = new SelfContainedPromise();
+
+XPCOMUtils.defineLazyGetter(lazy, "gLogEnabled", function aus_gLogEnabled() {
+  return (
+    Services.prefs.getBoolPref(PREF_APP_UPDATE_LOG, false) ||
+    Services.prefs.getBoolPref(PREF_APP_UPDATE_LOG_FILE, false)
+  );
+});
+
+XPCOMUtils.defineLazyGetter(
+  lazy,
+  "gLogfileEnabled",
+  function aus_gLogfileEnabled() {
+    return Services.prefs.getBoolPref(PREF_APP_UPDATE_LOG_FILE, false);
+  }
+);
+
+XPCOMUtils.defineLazyGetter(
+  lazy,
+  "gUpdateBundle",
+  function aus_gUpdateBundle() {
+    return Services.strings.createBundle(URI_UPDATES_PROPERTIES);
+  }
+);
+
+/**
+ * gIsBackgroundTaskMode will be true if Firefox is currently running as a
+ * background task. Otherwise it will be false.
+ */
+XPCOMUtils.defineLazyGetter(
+  lazy,
+  "gIsBackgroundTaskMode",
+  function aus_gCurrentlyRunningAsBackgroundTask() {
+    if (!("@mozilla.org/backgroundtasks;1" in Cc)) {
+      return false;
+    }
+    const bts = Cc["@mozilla.org/backgroundtasks;1"].getService(
+      Ci.nsIBackgroundTasks
+    );
+    if (!bts) {
+      return false;
+    }
+    return bts.isBackgroundTaskMode;
+  }
+);
+
+/**
+ * Changes `nsIApplicationUpdateService.currentState` and causes
+ * `nsIApplicationUpdateService.stateTransition` to resolve.
+ */
+function transitionState(newState) {
+  if (newState == gUpdateState) {
+    LOG("transitionState - Not transitioning state because it isn't changing.");
+    return;
+  }
+  LOG(
+    `transitionState - "${lazy.AUS.getStateName(gUpdateState)}" -> ` +
+      `"${lazy.AUS.getStateName(newState)}".`
+  );
+  gUpdateState = newState;
+  // Assign the new Promise before we resolve the old one just to make sure that
+  // anything that runs as a result of `resolve` doesn't end up waiting on the
+  // Promise that already resolved.
+  let oldStateTransitionPromise = gStateTransitionPromise;
+  gStateTransitionPromise = new SelfContainedPromise();
+  oldStateTransitionPromise.resolve();
+}
+
+/**
+ * When a plain JS object is passed through xpconnect the other side sees a
+ * wrapped version of the object instead of the real object. Since these two
+ * objects are different they act as different keys for Map and WeakMap. However
+ * xpconnect gives us a way to get the underlying JS object from the wrapper so
+ * this function returns the JS object regardless of whether passed the JS
+ * object or its wrapper for use in places where it is unclear which one you
+ * have.
+ */
+function unwrap(obj) {
+  return obj.wrappedJSObject ?? obj;
+}
+
+/**
+ * When an update starts to download (and if the feature is enabled) the add-ons
+ * manager starts downloading updated language packs for the new application
+ * version. A promise is used to track whether those updates are complete so the
+ * front-end is only notified that an application update is ready once the
+ * language pack updates have been staged.
+ *
+ * In order to be able to access that promise from various places in the update
+ * service they are cached in this map using the nsIUpdate object as a weak
+ * owner. Note that the key should always be the result of calling the above
+ * unwrap function on the nsIUpdate to ensure a consistent object is used as the
+ * key.
+ *
+ * When the language packs finish staging the nsIUpdate entriy is removed from
+ * this map so if the entry is still there then language pack updates are in
+ * progress.
+ */
+const LangPackUpdates = new WeakMap();
+
+/**
+ * When we're polling to see if other running instances of the application have
+ * exited, there's no need to ever start polling again in parallel. To prevent
+ * doing that, we keep track of the promise that resolves when polling completes
+ * and return that if a second simultaneous poll is requested, so that the
+ * multiple callers end up waiting for the same promise to resolve.
+ */
+let gOtherInstancePollPromise;
+
+/**
+ * Query the update sync manager to see if another instance of this same
+ * installation of this application is currently running, under the context of
+ * any operating system user (not just the current one).
+ * This function immediately returns the current, instantaneous status of any
+ * other instances.
+ *
+ * @return true if at least one other instance is running, false if not
+ */
+function isOtherInstanceRunning(callback) {
+  const checkEnabled = Services.prefs.getBoolPref(
+    PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_ENABLED,
+    true
+  );
+  if (!checkEnabled) {
+    LOG("isOtherInstanceRunning - disabled by pref, skipping check");
+    return false;
+  }
+
+  try {
+    let syncManager = Cc[
+      "@mozilla.org/updates/update-sync-manager;1"
+    ].getService(Ci.nsIUpdateSyncManager);
+    return syncManager.isOtherInstanceRunning();
+  } catch (ex) {
+    LOG(`isOtherInstanceRunning - sync manager failed with exception: ${ex}`);
+    return false;
+  }
+}
+
+/**
+ * Query the update sync manager to see if another instance of this same
+ * installation of this application is currently running, under the context of
+ * any operating system user (not just the one running this instance).
+ * This function polls for the status of other instances continually
+ * (asynchronously) until either none exist or a timeout expires.
+ *
+ * @return a Promise that resolves with false if at any point during polling no
+ *         other instances can be found, or resolves with true if the timeout
+ *         expires when other instances are still running
+ */
+function waitForOtherInstances() {
+  // If we're already in the middle of a poll, reuse it rather than start again.
+  if (gOtherInstancePollPromise) {
+    return gOtherInstancePollPromise;
+  }
+
+  let timeout = Services.prefs.getIntPref(
+    PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_TIMEOUT,
+    ONLY_INSTANCE_CHECK_DEFAULT_TIMEOUT_MS
+  );
+  // Don't allow the pref to set a super high timeout and break this feature.
+  if (timeout > ONLY_INSTANCE_CHECK_MAX_TIMEOUT_MS) {
+    timeout = ONLY_INSTANCE_CHECK_MAX_TIMEOUT_MS;
+  }
+
+  let interval = Services.prefs.getIntPref(
+    PREF_APP_UPDATE_CHECK_ONLY_INSTANCE_INTERVAL,
+    ONLY_INSTANCE_CHECK_DEFAULT_POLL_INTERVAL_MS
+  );
+  // Don't allow an interval longer than the timeout.
+  interval = Math.min(interval, timeout);
+
+  let iterations = 0;
+  const maxIterations = Math.ceil(timeout / interval);
+
+  gOtherInstancePollPromise = new Promise(function (resolve, reject) {
+    let poll = function () {
+      iterations++;
+      if (!isOtherInstanceRunning()) {
+        LOG("waitForOtherInstances - no other instances found, exiting");
+        resolve(false);
+        gOtherInstancePollPromise = undefined;
+      } else if (iterations >= maxIterations) {
+        LOG(
+          "waitForOtherInstances - timeout expired while other instances " +
+            "are still running"
+        );
+        resolve(true);
+        gOtherInstancePollPromise = undefined;
+      } else if (iterations + 1 == maxIterations && timeout % interval != 0) {
+        // In case timeout isn't a multiple of interval, set the next timeout
+        // for the remainder of the time rather than for the usual interval.
+        lazy.setTimeout(poll, timeout % interval);
+      } else {
+        lazy.setTimeout(poll, interval);
+      }
+    };
+
+    LOG("waitForOtherInstances - beginning polling");
+    poll();
+  });
+
+  return gOtherInstancePollPromise;
+}
+
+/**
+ * Tests to make sure that we can write to a given directory.
+ *
+ * @param updateTestFile a test file in the directory that needs to be tested.
+ * @param createDirectory whether a test directory should be created.
+ * @throws if we don't have right access to the directory.
+ */
+function testWriteAccess(updateTestFile, createDirectory) {
+  const NORMAL_FILE_TYPE = Ci.nsIFile.NORMAL_FILE_TYPE;
+  const DIRECTORY_TYPE = Ci.nsIFile.DIRECTORY_TYPE;
+  if (updateTestFile.exists()) {
+    updateTestFile.remove(false);
+  }
+  updateTestFile.create(
+    createDirectory ? DIRECTORY_TYPE : NORMAL_FILE_TYPE,
+    createDirectory ? FileUtils.PERMS_DIRECTORY : FileUtils.PERMS_FILE
+  );
+  updateTestFile.remove(false);
+}
+
+/**
+ * Windows only function that closes a Win32 handle.
+ *
+ * @param handle The handle to close
+ */
+function closeHandle(handle) {
+  if (handle) {
+    let lib = lazy.ctypes.open("kernel32.dll");
+    let CloseHandle = lib.declare(
+      "CloseHandle",
+      lazy.ctypes.winapi_abi,
+      lazy.ctypes.int32_t /* success */,
+      lazy.ctypes.void_t.ptr
+    ); /* handle */
+    CloseHandle(handle);
+    lib.close();
+  }
+}
+
+/**
+ * Windows only function that creates a mutex.
+ *
+ * @param  aName
+ *         The name for the mutex.
+ * @param  aAllowExisting
+ *         If false the function will close the handle and return null.
+ * @return The Win32 handle to the mutex.
+ */
+function createMutex(aName, aAllowExisting = true) {
+  if (AppConstants.platform != "win") {
+    throw Components.Exception("", Cr.NS_ERROR_NOT_IMPLEMENTED);
+  }
+
+  const INITIAL_OWN = 1;
+  const ERROR_ALREADY_EXISTS = 0xb7;
+  let lib = lazy.ctypes.open("kernel32.dll");
+  let CreateMutexW = lib.declare(
+    "CreateMutexW",
+    lazy.ctypes.winapi_abi,
+    lazy.ctypes.void_t.ptr /* return handle */,
+    lazy.ctypes.void_t.ptr /* security attributes */,
+    lazy.ctypes.int32_t /* initial owner */,
+    lazy.ctypes.char16_t.ptr
+  ); /* name */
+
+  let handle = CreateMutexW(null, INITIAL_OWN, aName);
+  let alreadyExists = lazy.ctypes.winLastError == ERROR_ALREADY_EXISTS;
+  if (handle && !handle.isNull() && !aAllowExisting && alreadyExists) {
+    closeHandle(handle);
+    handle = null;
+  }
+  lib.close();
+
+  if (handle && handle.isNull()) {
+    handle = null;
+  }
+
+  return handle;
+}
+
+/**
+ * Windows only function that determines a unique mutex name for the
+ * installation.
+ *
+ * @param aGlobal
+ *        true if the function should return a global mutex. A global mutex is
+ *        valid across different sessions.
+ * @return Global mutex path
+ */
+function getPerInstallationMutexName(aGlobal = true) {
+  if (AppConstants.platform != "win") {
+    throw Components.Exception("", Cr.NS_ERROR_NOT_IMPLEMENTED);
+  }
+
+  let hasher = Cc["@mozilla.org/security/hash;1"].createInstance(
+    Ci.nsICryptoHash
+  );
+  hasher.init(hasher.SHA1);
+
+  let exeFile = Services.dirsvc.get(KEY_EXECUTABLE, Ci.nsIFile);
+
+  let converter = Cc[
+    "@mozilla.org/intl/scriptableunicodeconverter"
+  ].createInstance(Ci.nsIScriptableUnicodeConverter);
+  converter.charset = "UTF-8";
+  var data = converter.convertToByteArray(exeFile.path.toLowerCase());
+
+  hasher.update(data, data.length);
+  return (
+    (aGlobal ? "Global\\" : "") + "MozillaUpdateMutex-" + hasher.finish(true)
+  );
+}
+
+/**
+ * Whether or not the current instance has the update mutex. The update mutex
+ * gives protection against 2 applications from the same installation updating:
+ * 1) Running multiple profiles from the same installation path
+ * 2) Two applications running in 2 different user sessions from the same path
+ *
+ * @return true if this instance holds the update mutex
+ */
+function hasUpdateMutex() {
+  if (AppConstants.platform != "win") {
+    return true;
+  }
+  if (!gUpdateMutexHandle) {
+    gUpdateMutexHandle = createMutex(getPerInstallationMutexName(true), false);
+  }
+  return !!gUpdateMutexHandle;
+}
+
+/**
+ * Determines whether or not all descendants of a directory are writeable.
+ * Note: Does not check the root directory itself for writeability.
+ *
+ * @return true if all descendants are writeable, false otherwise
+ */
+function areDirectoryEntriesWriteable(aDir) {
+  let items = aDir.directoryEntries;
+  while (items.hasMoreElements()) {
+    let item = items.nextFile;
+    if (!item.isWritable()) {
+      LOG("areDirectoryEntriesWriteable - unable to write to " + item.path);
+      return false;
+    }
+    if (item.isDirectory() && !areDirectoryEntriesWriteable(item)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+/**
+ * OSX only function to determine if the user requires elevation to be able to
+ * write to the application bundle.
+ *
+ * @return true if elevation is required, false otherwise
+ */
+function getElevationRequired() {
+  if (AppConstants.platform != "macosx") {
+    return false;
+  }
+
+  try {
+    // Recursively check that the application bundle (and its descendants) can
+    // be written to.
+    LOG(
+      "getElevationRequired - recursively testing write access on " +
+        getInstallDirRoot().path
+    );
+    if (
+      !getInstallDirRoot().isWritable() ||
+      !areDirectoryEntriesWriteable(getInstallDirRoot())
+    ) {
+      LOG(
+        "getElevationRequired - unable to write to application bundle, " +
+          "elevation required"
+      );
+      return true;
+    }
+  } catch (ex) {
+    LOG(
+      "getElevationRequired - unable to write to application bundle, " +
+        "elevation required. Exception: " +
+        ex
+    );
+    return true;
+  }
+  LOG(
+    "getElevationRequired - able to write to application bundle, elevation " +
+      "not required"
+  );
+  return false;
+}
+
+/**
+ * A promise that resolves when language packs are downloading or if no language
+ * packs are being downloaded.
+ */
+function promiseLangPacksUpdated(update) {
+  let promise = LangPackUpdates.get(unwrap(update));
+  if (promise) {
+    LOG(
+      "promiseLangPacksUpdated - waiting for language pack updates to stage."
+    );
+    return promise;
+  }
+
+  // In case callers rely on a promise just return an already resolved promise.
+  return Promise.resolve();
+}
+
+/**
+ * Determines whether or not an update can be applied. This is always true on
+ * Windows when the service is used. On Mac OS X and Linux, if the user has
+ * write access to the update directory this will return true because on OSX we
+ * offer users the option to perform an elevated update when necessary and on
+ * Linux the update directory is located in the application directory.
+ *
+ * @return true if an update can be applied, false otherwise
+ */
+function getCanApplyUpdates() {
+  try {
+    // Check if it is possible to write to the update directory so clients won't
+    // repeatedly try to apply an update without the ability to complete the
+    // update process which requires write access to the update directory.
+    let updateTestFile = getUpdateFile([FILE_UPDATE_TEST]);
+    LOG("getCanApplyUpdates - testing write access " + updateTestFile.path);
+    testWriteAccess(updateTestFile, false);
+  } catch (e) {
+    LOG(
+      "getCanApplyUpdates - unable to apply updates without write " +
+        "access to the update directory. Exception: " +
+        e
+    );
+    return false;
+  }
+
+  if (AppConstants.platform == "macosx") {
+    LOG(
+      "getCanApplyUpdates - bypass the write since elevation can be used " +
+        "on Mac OS X"
+    );
+    return true;
+  }
+
+  if (shouldUseService()) {
+    LOG(
+      "getCanApplyUpdates - bypass the write checks because the Windows " +
+        "Maintenance Service can be used"
+    );
+    return true;
+  }
+
+  try {
+    if (AppConstants.platform == "win") {
+      // On Windows when the maintenance service isn't used updates can still be
+      // performed in a location requiring admin privileges by the client
+      // accepting a UAC prompt from an elevation request made by the updater.
+      // Whether the client can elevate (e.g. has a split token) is determined
+      // in nsXULAppInfo::GetUserCanElevate which is located in nsAppRunner.cpp.
+      let userCanElevate = Services.appinfo.QueryInterface(
+        Ci.nsIWinAppHelper
+      ).userCanElevate;
+      if (lazy.gIsBackgroundTaskMode) {
+        LOG(
+          "getCanApplyUpdates - in background task mode, assuming user can't elevate"
+        );
+        userCanElevate = false;
+      }
+      if (!userCanElevate) {
+        // if we're unable to create the test file this will throw an exception.
+        let appDirTestFile = getAppBaseDir();
+        appDirTestFile.append(FILE_UPDATE_TEST);
+        LOG("getCanApplyUpdates - testing write access " + appDirTestFile.path);
+        if (appDirTestFile.exists()) {
+          appDirTestFile.remove(false);
+        }
+        appDirTestFile.create(
+          Ci.nsIFile.NORMAL_FILE_TYPE,
+          FileUtils.PERMS_FILE
+        );
+        appDirTestFile.remove(false);
+      }
+    }
+  } catch (e) {
+    LOG("getCanApplyUpdates - unable to apply updates. Exception: " + e);
+    // No write access to the installation directory
+    return false;
+  }
+
+  LOG("getCanApplyUpdates - able to apply updates");
+  return true;
+}
+
+/**
+ * Whether or not the application can stage an update for the current session.
+ * These checks are only performed once per session due to using a lazy getter.
+ *
+ * @return true if updates can be staged for this session.
+ */
+XPCOMUtils.defineLazyGetter(
+  lazy,
+  "gCanStageUpdatesSession",
+  function aus_gCSUS() {
+    if (getElevationRequired()) {
+      LOG(
+        "gCanStageUpdatesSession - unable to stage updates because elevation " +
+          "is required."
+      );
+      return false;
+    }
+
+    try {
+      let updateTestFile;
+      if (AppConstants.platform == "macosx") {
+        updateTestFile = getUpdateFile([FILE_UPDATE_TEST]);
+      } else {
+        updateTestFile = getInstallDirRoot();
+        updateTestFile.append(FILE_UPDATE_TEST);
+      }
+      LOG(
+        "gCanStageUpdatesSession - testing write access " + updateTestFile.path
+      );
+      testWriteAccess(updateTestFile, true);
+      if (AppConstants.platform != "macosx") {
+        // On all platforms except Mac, we need to test the parent directory as
+        // well, as we need to be able to move files in that directory during the
+        // replacing step.
+        updateTestFile = getInstallDirRoot().parent;
+        updateTestFile.append(FILE_UPDATE_TEST);
+        LOG(
+          "gCanStageUpdatesSession - testing write access " +
+            updateTestFile.path
+        );
+        updateTestFile.createUnique(
+          Ci.nsIFile.DIRECTORY_TYPE,
+          FileUtils.PERMS_DIRECTORY
+        );
+        updateTestFile.remove(false);
+      }
+    } catch (e) {
+      LOG("gCanStageUpdatesSession - unable to stage updates. Exception: " + e);
+      // No write privileges
+      return false;
+    }
+
+    LOG("gCanStageUpdatesSession - able to stage updates");
+    return true;
+  }
+);
+
+/**
+ * Whether or not the application can stage an update.
+ *
+ * @param {boolean} [transient] Whether transient factors such as the update
+ *        mutex should be considered.
+ * @return true if updates can be staged.
+ */
+function getCanStageUpdates(transient = true) {
+  // If staging updates are disabled, then just bail out!
+  if (!Services.prefs.getBoolPref(PREF_APP_UPDATE_STAGING_ENABLED, false)) {
+    LOG(
+      "getCanStageUpdates - staging updates is disabled by preference " +
+        PREF_APP_UPDATE_STAGING_ENABLED
+    );
+    return false;
+  }
+
+  if (AppConstants.platform == "win" && shouldUseService()) {
+    // No need to perform directory write checks, the maintenance service will
+    // be able to write to all directories.
+    LOG("getCanStageUpdates - able to stage updates using the service");
+    return true;
+  }
+
+  if (transient && !hasUpdateMutex()) {
+    LOG(
+      "getCanStageUpdates - unable to apply updates because another " +
+        "instance of the application is already handling updates for this " +
+        "installation."
+    );
+    return false;
+  }
+
+  return lazy.gCanStageUpdatesSession;
+}
+
+/*
+ * Whether or not the application can use BITS to download updates.
+ *
+ * @param {boolean} [transient] Whether transient factors such as the update
+ *        mutex should be considered.
+ * @return A string with one of these values:
+ *           CanUseBits
+ *           NoBits_NotWindows
+ *           NoBits_FeatureOff
+ *           NoBits_Pref
+ *           NoBits_Proxy
+ *           NoBits_OtherUser
+ *         These strings are directly compatible with the categories for
+ *         UPDATE_CAN_USE_BITS_EXTERNAL and UPDATE_CAN_USE_BITS_NOTIFY telemetry
+ *         probes. If this function is made to return other values, they should
+ *         also be added to the labels lists for those probes in Histograms.json
+ */
+function getCanUseBits(transient = true) {
+  if (AppConstants.platform != "win") {
+    LOG("getCanUseBits - Not using BITS because this is not Windows");
+    return "NoBits_NotWindows";
+  }
+  if (!AppConstants.MOZ_BITS_DOWNLOAD) {
+    LOG("getCanUseBits - Not using BITS because the feature is disabled");
+    return "NoBits_FeatureOff";
+  }
+
+  if (!Services.prefs.getBoolPref(PREF_APP_UPDATE_BITS_ENABLED, true)) {
+    LOG("getCanUseBits - Not using BITS. Disabled by pref.");
+    return "NoBits_Pref";
+  }
+  // Firefox support for passing proxies to BITS is still rudimentary.
+  // For now, disable BITS support on configurations that are not using the
+  // standard system proxy.
+  let defaultProxy = Ci.nsIProtocolProxyService.PROXYCONFIG_SYSTEM;
+  if (
+    Services.prefs.getIntPref(PREF_NETWORK_PROXY_TYPE, defaultProxy) !=
+      defaultProxy &&
+    !Cu.isInAutomation
+  ) {
+    LOG("getCanUseBits - Not using BITS because of proxy usage");
+    return "NoBits_Proxy";
+  }
+  if (transient && gBITSInUseByAnotherUser) {
+    LOG("getCanUseBits - Not using BITS. Already in use by another user");
+    return "NoBits_OtherUser";
+  }
+  LOG("getCanUseBits - BITS can be used to download updates");
+  return "CanUseBits";
+}
+
+/**
+ * Logs a string to the error console. If enabled, also logs to the update
+ * messages file.
+ * @param   string
+ *          The string to write to the error console.
+ */
+function LOG(string) {
+  if (lazy.gLogEnabled) {
+    dump("*** AUS:SVC " + string + "\n");
+    if (!Cu.isInAutomation) {
+      Services.console.logStringMessage("AUS:SVC " + string);
+    }
+
+    if (lazy.gLogfileEnabled) {
+      if (!gLogfileOutputStream) {
+        let logfile = Services.dirsvc.get(KEY_PROFILE_DIR, Ci.nsIFile);
+        logfile.append(FILE_UPDATE_MESSAGES);
+        gLogfileOutputStream = FileUtils.openAtomicFileOutputStream(logfile);
+      }
+
+      try {
+        let encoded = new TextEncoder().encode(string + "\n");
+        gLogfileOutputStream.write(encoded, encoded.length);
+        gLogfileOutputStream.flush();
+      } catch (e) {
+        dump("*** AUS:SVC Unable to write to messages file: " + e + "\n");
+        Services.console.logStringMessage(
+          "AUS:SVC Unable to write to messages file: " + e
+        );
+      }
+    }
+  }
+}
+
+/**
+ * Gets the specified directory at the specified hierarchy under the
+ * update root directory and creates it if it doesn't exist.
+ * @param   pathArray
+ *          An array of path components to locate beneath the directory
+ *          specified by |key|
+ * @return  nsIFile object for the location specified.
+ */
+function getUpdateDirCreate(pathArray) {
+  if (Cu.isInAutomation) {
+    // This allows tests to use an alternate updates directory so they can test
+    // startup behavior.
+    const MAGIC_TEST_ROOT_PREFIX = "<test-root>";
+    const PREF_TEST_ROOT = "mochitest.testRoot";
+    let alternatePath = Services.prefs.getCharPref(
+      PREF_APP_UPDATE_ALTUPDATEDIRPATH,
+      null
+    );
+    if (alternatePath && alternatePath.startsWith(MAGIC_TEST_ROOT_PREFIX)) {
+      let testRoot = Services.prefs.getCharPref(PREF_TEST_ROOT);
+      let relativePath = alternatePath.substring(MAGIC_TEST_ROOT_PREFIX.length);
+      if (AppConstants.platform == "win") {
+        relativePath = relativePath.replace(/\//g, "\\");
+      }
+      alternatePath = testRoot + relativePath;
+      let updateDir = Cc["@mozilla.org/file/local;1"].createInstance(
+        Ci.nsIFile
+      );
+      updateDir.initWithPath(alternatePath);
+      for (let i = 0; i < pathArray.length; ++i) {
+        updateDir.append(pathArray[i]);
+      }
+      return updateDir;
+    }
+  }
+
+  return FileUtils.getDir(KEY_UPDROOT, pathArray, true);
+}
+
+/**
+ * Gets the application base directory.
+ *
+ * @return  nsIFile object for the application base directory.
+ */
+function getAppBaseDir() {
+  return Services.dirsvc.get(KEY_EXECUTABLE, Ci.nsIFile).parent;
+}
+
+/**
+ * Gets the root of the installation directory which is the application
+ * bundle directory on Mac OS X and the location of the application binary
+ * on all other platforms.
+ *
+ * @return nsIFile object for the directory
+ */
+function getInstallDirRoot() {
+  let dir = getAppBaseDir();
+  if (AppConstants.platform == "macosx") {
+    // On macOS, the executable is stored under Contents/MacOS.
+    dir = dir.parent.parent;
+  }
+  return dir;
+}
+
+/**
+ * Gets the file at the specified hierarchy under the update root directory.
+ * @param   pathArray
+ *          An array of path components to locate beneath the directory
+ *          specified by |key|. The last item in this array must be the
+ *          leaf name of a file.
+ * @return  nsIFile object for the file specified. The file is NOT created
+ *          if it does not exist, however all required directories along
+ *          the way are.
+ */
+function getUpdateFile(pathArray) {
+  let file = getUpdateDirCreate(pathArray.slice(0, -1));
+  file.append(pathArray[pathArray.length - 1]);
+  return file;
+}
+
+/**
+ * This function is designed to let us slightly clean up the mapping between
+ * strings and error codes. So that instead of having:
+ *   check_error-2147500036=Connection aborted
+ *   check_error-2152398850=Connection aborted
+ * We can have:
+ *   check_error-connection_aborted=Connection aborted
+ * And map both of those error codes to it.
+ */
+function maybeMapErrorCode(code) {
+  switch (code) {
+    case Cr.NS_BINDING_ABORTED:
+    case Cr.NS_ERROR_ABORT:
+      return "connection_aborted";
+  }
+  return code;
+}
+
+/**
+ * Returns human readable status text from the updates.properties bundle
+ * based on an error code
+ * @param   code
+ *          The error code to look up human readable status text for
+ * @param   defaultCode
+ *          The default code to look up should human readable status text
+ *          not exist for |code|
+ * @return  A human readable status text string
+ */
+function getStatusTextFromCode(code, defaultCode) {
+  code = maybeMapErrorCode(code);
+
+  let reason;
+  try {
+    reason = lazy.gUpdateBundle.GetStringFromName("check_error-" + code);
+    LOG(
+      "getStatusTextFromCode - transfer error: " + reason + ", code: " + code
+    );
+  } catch (e) {
+    defaultCode = maybeMapErrorCode(defaultCode);
+
+    // Use the default reason
+    reason = lazy.gUpdateBundle.GetStringFromName("check_error-" + defaultCode);
+    LOG(
+      "getStatusTextFromCode - transfer error: " +
+        reason +
+        ", default code: " +
+        defaultCode
+    );
+  }
+  return reason;
+}
+
+/**
+ * Get the Ready Update directory. This is the directory that an update
+ * should reside in after download has completed but before it has been
+ * installed and cleaned up.
+ * @return The ready updates directory, as a nsIFile object
+ */
+function getReadyUpdateDir() {
+  return getUpdateDirCreate([DIR_UPDATES, DIR_UPDATE_READY]);
+}
+
+/**
+ * Get the Downloading Update directory. This is the directory that an update
+ * should reside in during download. Once download is completed, it will be
+ * moved to the Ready Update directory.
+ * @return The downloading update directory, as a nsIFile object
+ */
+function getDownloadingUpdateDir() {
+  return getUpdateDirCreate([DIR_UPDATES, DIR_UPDATE_DOWNLOADING]);
+}
+
+/**
+ * Reads the update state from the update.status file in the specified
+ * directory.
+ * @param   dir
+ *          The dir to look for an update.status file in
+ * @return  The status value of the update.
+ */
+function readStatusFile(dir) {
+  let statusFile = dir.clone();
+  statusFile.append(FILE_UPDATE_STATUS);
+  let status = readStringFromFile(statusFile) || STATE_NONE;
+  LOG("readStatusFile - status: " + status + ", path: " + statusFile.path);
+  return status;
+}
+
+/**
+ * Reads the binary transparency result file from the given directory.
+ * Removes the file if it is present (so don't call this twice and expect a
+ * result the second time).
+ * @param   dir
+ *          The dir to look for an update.bt file in
+ * @return  A error code from verifying binary transparency information or null
+ *          if the file was not present (indicating there was no error).
+ */
+function readBinaryTransparencyResult(dir) {
+  let binaryTransparencyResultFile = dir.clone();
+  binaryTransparencyResultFile.append(FILE_BT_RESULT);
+  let result = readStringFromFile(binaryTransparencyResultFile);
+  LOG(
+    "readBinaryTransparencyResult - result: " +
+      result +
+      ", path: " +
+      binaryTransparencyResultFile.path
+  );
+  // If result is non-null, the file exists. We should remove it to avoid
+  // double-reporting this result.
+  if (result) {
+    binaryTransparencyResultFile.remove(false);
+  }
+  return result;
+}
+
+/**
+ * Writes the current update operation/state to a file in the patch
+ * directory, indicating to the patching system that operations need
+ * to be performed.
+ * @param   dir
+ *          The patch directory where the update.status file should be
+ *          written.
+ * @param   state
+ *          The state value to write.
+ */
+function writeStatusFile(dir, state) {
+  let statusFile = dir.clone();
+  statusFile.append(FILE_UPDATE_STATUS);
+  writeStringToFile(statusFile, state);
+}
+
+/**
+ * Writes the update's application version to a file in the patch directory. If
+ * the update doesn't provide application version information via the
+ * appVersion attribute the string "null" will be written to the file.
+ * This value is compared during startup (in nsUpdateDriver.cpp) to determine if
+ * the update should be applied. Note that this won't provide protection from
+ * downgrade of the application for the nightly user case where the application
+ * version doesn't change.
+ * @param   dir
+ *          The patch directory where the update.version file should be
+ *          written.
+ * @param   version
+ *          The version value to write. Will be the string "null" when the
+ *          update doesn't provide the appVersion attribute in the update xml.
+ */
+function writeVersionFile(dir, version) {
+  let versionFile = dir.clone();
+  versionFile.append(FILE_UPDATE_VERSION);
+  writeStringToFile(versionFile, version);
+}
+
+/**
+ * Determines if the service should be used to attempt an update
+ * or not.
+ *
+ * @return  true if the service should be used for updates.
+ */
+function shouldUseService() {
+  // This function will return true if the mantenance service should be used if
+  // all of the following conditions are met:
+  // 1) This build was done with the maintenance service enabled
+  // 2) The maintenance service is installed
+  // 3) The pref for using the service is enabled
+  if (
+    !AppConstants.MOZ_MAINTENANCE_SERVICE ||
+    !isServiceInstalled() ||
+    !Services.prefs.getBoolPref(PREF_APP_UPDATE_SERVICE_ENABLED, false)
+  ) {
+    LOG("shouldUseService - returning false");
+    return false;
+  }
+
+  LOG("shouldUseService - returning true");
+  return true;
+}
+
+/**
+ * Determines if the service is is installed.
+ *
+ * @return  true if the service is installed.
+ */
+function isServiceInstalled() {
+  if (!AppConstants.MOZ_MAINTENANCE_SERVICE || AppConstants.platform != "win") {
+    LOG("isServiceInstalled - returning false");
+    return false;
+  }
+
+  let installed = 0;
+  try {
+    let wrk = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+      Ci.nsIWindowsRegKey
+    );
+    wrk.open(
+      wrk.ROOT_KEY_LOCAL_MACHINE,
+      "SOFTWARE\\Mozilla\\MaintenanceService",
+      wrk.ACCESS_READ | wrk.WOW64_64
+    );
+    installed = wrk.readIntValue("Installed");
+    wrk.close();
+  } catch (e) {}
+  installed = installed == 1; // convert to bool
+  LOG("isServiceInstalled - returning " + installed);
+  return installed;
+}
+
+/**
+ * Gets the appropriate pending update state. Returns STATE_PENDING_SERVICE,
+ * STATE_PENDING_ELEVATE, or STATE_PENDING.
+ */
+function getBestPendingState() {
+  if (shouldUseService()) {
+    return STATE_PENDING_SERVICE;
+  } else if (getElevationRequired()) {
+    return STATE_PENDING_ELEVATE;
+  }
+  return STATE_PENDING;
+}
+
+/**
+ * Removes the contents of the ready update directory and rotates the update
+ * logs when present. If the update.log exists in the patch directory this will
+ * move the last-update.log if it exists to backup-update.log in the parent
+ * directory of the patch directory and then move the update.log in the patch
+ * directory to last-update.log in the parent directory of the patch directory.
+ *
+ * @param aRemovePatchFiles (optional, defaults to true)
+ *        When true the update's patch directory contents are removed.
+ */
+function cleanUpReadyUpdateDir(aRemovePatchFiles = true) {
+  let updateDir;
+  try {
+    updateDir = getReadyUpdateDir();
+  } catch (e) {
+    LOG(
+      "cleanUpReadyUpdateDir - unable to get the updates patch directory. " +
+        "Exception: " +
+        e
+    );
+    return;
+  }
+
+  // Preserve the last update log file for debugging purposes.
+  let updateLogFile = updateDir.clone();
+  updateLogFile.append(FILE_UPDATE_LOG);
+  if (updateLogFile.exists()) {
+    let dir = updateDir.parent;
+    let logFile = dir.clone();
+    logFile.append(FILE_LAST_UPDATE_LOG);
+    if (logFile.exists()) {
+      try {
+        logFile.moveTo(dir, FILE_BACKUP_UPDATE_LOG);
+      } catch (e) {
+        LOG(
+          "cleanUpReadyUpdateDir - failed to rename file " +
+            logFile.path +
+            " to " +
+            FILE_BACKUP_UPDATE_LOG
+        );
+      }
+    }
+
+    try {
+      updateLogFile.moveTo(dir, FILE_LAST_UPDATE_LOG);
+    } catch (e) {
+      LOG(
+        "cleanUpReadyUpdateDir - failed to rename file " +
+          updateLogFile.path +
+          " to " +
+          FILE_LAST_UPDATE_LOG
+      );
+    }
+  }
+
+  if (aRemovePatchFiles) {
+    let dirEntries = updateDir.directoryEntries;
+    while (dirEntries.hasMoreElements()) {
+      let file = dirEntries.nextFile;
+      // Now, recursively remove this file.  The recursive removal is needed for
+      // Mac OSX because this directory will contain a copy of updater.app,
+      // which is itself a directory and the MozUpdater directory on platforms
+      // other than Windows.
+      try {
+        file.remove(true);
+      } catch (e) {
+        LOG("cleanUpReadyUpdateDir - failed to remove file " + file.path);
+      }
+    }
+  }
+}
+
+/**
+ * Removes the contents of the update download directory.
+ *
+ */
+function cleanUpDownloadingUpdateDir() {
+  let updateDir;
+  try {
+    updateDir = getDownloadingUpdateDir();
+  } catch (e) {
+    LOG(
+      "cleanUpDownloadUpdatesDir - unable to get the updates patch " +
+        "directory. Exception: " +
+        e
+    );
+    return;
+  }
+
+  let dirEntries = updateDir.directoryEntries;
+  while (dirEntries.hasMoreElements()) {
+    let file = dirEntries.nextFile;
+    // Now, recursively remove this file.
+    try {
+      file.remove(true);
+    } catch (e) {
+      LOG("cleanUpDownloadUpdatesDir - failed to remove file " + file.path);
+    }
+  }
+}
+
+/**
+ * Clean up the updates list and the directory that contains the update that
+ * is ready to be installed.
+ *
+ * Note - This function causes a state transition to either STATE_DOWNLOADING
+ *        or STATE_NONE, depending on whether an update download is in progress.
+ */
+function cleanupReadyUpdate() {
+  // Move the update from the Active Update list into the Past Updates list.
+  if (lazy.UM.readyUpdate) {
+    LOG("cleanupReadyUpdate - Clearing readyUpdate");
+    lazy.UM.addUpdateToHistory(lazy.UM.readyUpdate);
+    lazy.UM.readyUpdate = null;
+  }
+  lazy.UM.saveUpdates();
+
+  let readyUpdateDir = getReadyUpdateDir();
+  let shouldSetDownloadingStatus =
+    lazy.UM.downloadingUpdate ||
+    readStatusFile(readyUpdateDir) == STATE_DOWNLOADING;
+
+  // Now trash the ready update directory, since we're done with it
+  cleanUpReadyUpdateDir();
+
+  // We need to handle two similar cases here.
+  // The first is where we clean up the ready updates directory while we are in
+  // the downloading state. In this case, we remove the update.status file that
+  // says we are downloading, even though we should remain in that state.
+  // The second case is when we clean up a ready update, but there is also a
+  // downloading update (in which case the update status file's state will
+  // reflect the state of the ready update, not the downloading one). In that
+  // case, instead of reverting to STATE_NONE (which is what we do by removing
+  // the status file), we should set our state to downloading.
+  if (shouldSetDownloadingStatus) {
+    LOG("cleanupReadyUpdate - Transitioning back to downloading state.");
+    transitionState(Ci.nsIApplicationUpdateService.STATE_DOWNLOADING);
+    writeStatusFile(readyUpdateDir, STATE_DOWNLOADING);
+  }
+}
+
+/**
+ * Clean up updates list and the directory that the currently downloading update
+ * is downloaded to.
+ *
+ * Note - This function may cause a state transition. If the current state is
+ *        STATE_DOWNLOADING, this will cause it to change to STATE_NONE.
+ */
+function cleanupDownloadingUpdate() {
+  // Move the update from the Active Update list into the Past Updates list.
+  if (lazy.UM.downloadingUpdate) {
+    LOG("cleanupDownloadingUpdate - Clearing downloadingUpdate.");
+    lazy.UM.addUpdateToHistory(lazy.UM.downloadingUpdate);
+    lazy.UM.downloadingUpdate = null;
+  }
+  lazy.UM.saveUpdates();
+
+  // Now trash the update download directory, since we're done with it
+  cleanUpDownloadingUpdateDir();
+
+  // If the update status file says we are downloading, we should remove that
+  // too, since we aren't doing that anymore.
+  let readyUpdateDir = getReadyUpdateDir();
+  let status = readStatusFile(readyUpdateDir);
+  if (status == STATE_DOWNLOADING) {
+    let statusFile = readyUpdateDir.clone();
+    statusFile.append(FILE_UPDATE_STATUS);
+    statusFile.remove();
+  }
+}
+
+/**
+ * Clean up updates list, the ready update directory, and the downloading update
+ * directory.
+ *
+ * This is more efficient than calling
+ *   cleanupReadyUpdate();
+ *   cleanupDownloadingUpdate();
+ * because those need some special handling of the update status file to make
+ * sure that, for example, cleaning up a ready update doesn't make us forget
+ * that we are downloading an update. When we cleanup both updates, we don't
+ * need to worry about things like that.
+ *
+ * Note - This function causes a state transition to STATE_NONE.
+ */
+function cleanupActiveUpdates() {
+  // Move the update from the Active Update list into the Past Updates list.
+  if (lazy.UM.readyUpdate) {
+    LOG("cleanupActiveUpdates - Clearing readyUpdate");
+    lazy.UM.addUpdateToHistory(lazy.UM.readyUpdate);
+    lazy.UM.readyUpdate = null;
+  }
+  if (lazy.UM.downloadingUpdate) {
+    LOG("cleanupActiveUpdates - Clearing downloadingUpdate.");
+    lazy.UM.addUpdateToHistory(lazy.UM.downloadingUpdate);
+    lazy.UM.downloadingUpdate = null;
+  }
+  lazy.UM.saveUpdates();
+
+  // Now trash both active update directories, since we're done with them
+  cleanUpReadyUpdateDir();
+  cleanUpDownloadingUpdateDir();
+}
+
+/**
+ * Writes a string of text to a file.  A newline will be appended to the data
+ * written to the file.  This function only works with ASCII text.
+ * @param file An nsIFile indicating what file to write to.
+ * @param text A string containing the text to write to the file.
+ * @return true on success, false on failure.
+ */
+function writeStringToFile(file, text) {
+  try {
+    let fos = FileUtils.openSafeFileOutputStream(file);
+    text += "\n";
+    fos.write(text, text.length);
+    FileUtils.closeSafeFileOutputStream(fos);
+  } catch (e) {
+    LOG(`writeStringToFile - Failed to write to file: "${file}". Error: ${e}"`);
+    return false;
+  }
+  return true;
+}
+
+function readStringFromInputStream(inputStream) {
+  var sis = Cc["@mozilla.org/scriptableinputstream;1"].createInstance(
+    Ci.nsIScriptableInputStream
+  );
+  sis.init(inputStream);
+  var text = sis.read(sis.available());
+  sis.close();
+  if (text && text[text.length - 1] == "\n") {
+    text = text.slice(0, -1);
+  }
+  return text;
+}
+
+/**
+ * Reads a string of text from a file.  A trailing newline will be removed
+ * before the result is returned.  This function only works with ASCII text.
+ */
+function readStringFromFile(file) {
+  if (!file.exists()) {
+    LOG("readStringFromFile - file doesn't exist: " + file.path);
+    return null;
+  }
+  var fis = Cc["@mozilla.org/network/file-input-stream;1"].createInstance(
+    Ci.nsIFileInputStream
+  );
+  fis.init(file, FileUtils.MODE_RDONLY, FileUtils.PERMS_FILE, 0);
+  return readStringFromInputStream(fis);
+}
+
+/**
+ * Attempts to recover from an update error. If successful, `true` will be
+ * returned and AUS.currentState will be transitioned.
+ */
+function handleUpdateFailure(update) {
+  if (WRITE_ERRORS.includes(update.errorCode)) {
+    LOG(
+      "handleUpdateFailure - Failure is a write error. Setting state to pending"
+    );
+    writeStatusFile(getReadyUpdateDir(), (update.state = STATE_PENDING));
+    transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+    return true;
+  }
+
+  if (update.errorCode == SILENT_UPDATE_NEEDED_ELEVATION_ERROR) {
+    // There's no need to count attempts and escalate: it's expected that the
+    // background update task will try to update and fail due to required
+    // elevation repeatedly if, for example, the maintenance service is not
+    // available (or not functioning) and the installation requires privileges
+    // to update.
+
+    let bestState = getBestPendingState();
+    LOG(
+      "handleUpdateFailure - witnessed SILENT_UPDATE_NEEDED_ELEVATION_ERROR, " +
+        "returning to " +
+        bestState
+    );
+    writeStatusFile(getReadyUpdateDir(), (update.state = bestState));
+
+    transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+    // Return true to indicate a recoverable error.
+    return true;
+  }
+
+  if (update.errorCode == ELEVATION_CANCELED) {
+    let elevationAttempts = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_ELEVATE_ATTEMPTS,
+      0
+    );
+    elevationAttempts++;
+    Services.prefs.setIntPref(
+      PREF_APP_UPDATE_ELEVATE_ATTEMPTS,
+      elevationAttempts
+    );
+    let maxAttempts = Math.min(
+      Services.prefs.getIntPref(PREF_APP_UPDATE_ELEVATE_MAXATTEMPTS, 2),
+      10
+    );
+
+    if (elevationAttempts > maxAttempts) {
+      LOG(
+        "handleUpdateFailure - notifying observers of error. " +
+          "topic: update-error, status: elevation-attempts-exceeded"
+      );
+      Services.obs.notifyObservers(
+        update,
+        "update-error",
+        "elevation-attempts-exceeded"
+      );
+    } else {
+      LOG(
+        "handleUpdateFailure - notifying observers of error. " +
+          "topic: update-error, status: elevation-attempt-failed"
+      );
+      Services.obs.notifyObservers(
+        update,
+        "update-error",
+        "elevation-attempt-failed"
+      );
+    }
+
+    let cancelations = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_CANCELATIONS,
+      0
+    );
+    cancelations++;
+    Services.prefs.setIntPref(PREF_APP_UPDATE_CANCELATIONS, cancelations);
+    if (AppConstants.platform == "macosx") {
+      let osxCancelations = Services.prefs.getIntPref(
+        PREF_APP_UPDATE_CANCELATIONS_OSX,
+        0
+      );
+      osxCancelations++;
+      Services.prefs.setIntPref(
+        PREF_APP_UPDATE_CANCELATIONS_OSX,
+        osxCancelations
+      );
+      let maxCancels = Services.prefs.getIntPref(
+        PREF_APP_UPDATE_CANCELATIONS_OSX_MAX,
+        DEFAULT_CANCELATIONS_OSX_MAX
+      );
+      // Prevent the preference from setting a value greater than 5.
+      maxCancels = Math.min(maxCancels, 5);
+      if (osxCancelations >= maxCancels) {
+        LOG(
+          "handleUpdateFailure - Too many OSX cancellations. Cleaning up " +
+            "ready update."
+        );
+        cleanupReadyUpdate();
+        return false;
+      }
+      LOG(
+        `handleUpdateFailure - OSX cancellation. Trying again by setting ` +
+          `status to "${STATE_PENDING_ELEVATE}".`
+      );
+      writeStatusFile(
+        getReadyUpdateDir(),
+        (update.state = STATE_PENDING_ELEVATE)
+      );
+      update.statusText =
+        lazy.gUpdateBundle.GetStringFromName("elevationFailure");
+    } else {
+      LOG(
+        "handleUpdateFailure - Failure because elevation was cancelled. " +
+          "again by setting status to pending."
+      );
+      writeStatusFile(getReadyUpdateDir(), (update.state = STATE_PENDING));
+    }
+    transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+    return true;
+  }
+
+  if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CANCELATIONS)) {
+    Services.prefs.clearUserPref(PREF_APP_UPDATE_CANCELATIONS);
+  }
+  if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CANCELATIONS_OSX)) {
+    Services.prefs.clearUserPref(PREF_APP_UPDATE_CANCELATIONS_OSX);
+  }
+
+  if (SERVICE_ERRORS.includes(update.errorCode)) {
+    var failCount = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_SERVICE_ERRORS,
+      0
+    );
+    var maxFail = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_SERVICE_MAXERRORS,
+      DEFAULT_SERVICE_MAX_ERRORS
+    );
+    // Prevent the preference from setting a value greater than 10.
+    maxFail = Math.min(maxFail, 10);
+    // As a safety, when the service reaches maximum failures, it will
+    // disable itself and fallback to using the normal update mechanism
+    // without the service.
+    if (failCount >= maxFail) {
+      Services.prefs.setBoolPref(PREF_APP_UPDATE_SERVICE_ENABLED, false);
+      Services.prefs.clearUserPref(PREF_APP_UPDATE_SERVICE_ERRORS);
+    } else {
+      failCount++;
+      Services.prefs.setIntPref(PREF_APP_UPDATE_SERVICE_ERRORS, failCount);
+    }
+
+    LOG(
+      "handleUpdateFailure - Got a service error. Try to update without the " +
+        "service by setting the state to pending."
+    );
+    writeStatusFile(getReadyUpdateDir(), (update.state = STATE_PENDING));
+    transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+    return true;
+  }
+
+  if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_SERVICE_ERRORS)) {
+    Services.prefs.clearUserPref(PREF_APP_UPDATE_SERVICE_ERRORS);
+  }
+
+  return false;
+}
+
+/**
+ * Return the first UpdatePatch with the given type.
+ * @param   update
+ *          A nsIUpdate object to search through for a patch of the desired
+ *          type.
+ * @param   patch_type
+ *          The type of the patch ("complete" or "partial")
+ * @return  A nsIUpdatePatch object matching the type specified
+ */
+function getPatchOfType(update, patch_type) {
+  for (var i = 0; i < update.patchCount; ++i) {
+    var patch = update.getPatchAt(i);
+    if (patch && patch.type == patch_type) {
+      return patch;
+    }
+  }
+  return null;
+}
+
+/**
+ * Fall back to downloading a complete update in case an update has failed.
+ *
+ * This will transition `AUS.currentState` to `STATE_DOWNLOADING` if there is
+ * another patch to download, or `STATE_IDLE` if there is not.
+ */
+async function handleFallbackToCompleteUpdate() {
+  // If we failed to install an update, we need to fall back to a complete
+  // update. If the install directory has been modified, more partial updates
+  // will fail for the same reason. Since we only download partial updates
+  // while there is already an update downloaded, we don't have to check the
+  // downloading update, we can be confident that we are not downloading the
+  // right thing at the moment.
+
+  // The downloading update will be newer than the ready update, so use that
+  // update, if it exists.
+  let update = lazy.UM.downloadingUpdate || lazy.UM.readyUpdate;
+  if (!update) {
+    LOG(
+      "handleFallbackToCompleteUpdate - Unable to find an update to fall " +
+        "back to."
+    );
+    return;
+  }
+
+  LOG(
+    "handleFallbackToCompleteUpdate - Cleaning up active updates in " +
+      "preparation of falling back to complete update."
+  );
+  await lazy.AUS.stopDownload();
+  cleanupActiveUpdates();
+
+  if (!update.selectedPatch) {
+    // If we don't have a partial patch selected but a partial is available,
+    // _selectPatch() will download that instead of the complete patch.
+    let patch = getPatchOfType(update, "partial");
+    if (patch) {
+      patch.selected = true;
+    }
+  }
+
+  update.statusText = lazy.gUpdateBundle.GetStringFromName("patchApplyFailure");
+  var oldType = update.selectedPatch ? update.selectedPatch.type : "complete";
+  if (update.selectedPatch && oldType == "partial" && update.patchCount == 2) {
+    // Partial patch application failed, try downloading the complete
+    // update in the background instead.
+    LOG(
+      "handleFallbackToCompleteUpdate - install of partial patch " +
+        "failed, downloading complete patch"
+    );
+    var success = await lazy.AUS.downloadUpdate(update);
+    if (!success) {
+      LOG(
+        "handleFallbackToCompleteUpdate - Starting complete patch download " +
+          "failed. Cleaning up downloading patch."
+      );
+      cleanupDownloadingUpdate();
+    }
+  } else {
+    LOG(
+      "handleFallbackToCompleteUpdate - install of complete or " +
+        "only one patch offered failed. Notifying observers. topic: " +
+        "update-error, status: unknown, " +
+        "update.patchCount: " +
+        update.patchCount +
+        ", " +
+        "oldType: " +
+        oldType
+    );
+    transitionState(Ci.nsIApplicationUpdateService.STATE_IDLE);
+    Services.obs.notifyObservers(update, "update-error", "unknown");
+  }
+}
+
+function pingStateAndStatusCodes(aUpdate, aStartup, aStatus) {
+  let patchType = AUSTLMY.PATCH_UNKNOWN;
+  if (aUpdate && aUpdate.selectedPatch && aUpdate.selectedPatch.type) {
+    if (aUpdate.selectedPatch.type == "complete") {
+      patchType = AUSTLMY.PATCH_COMPLETE;
+    } else if (aUpdate.selectedPatch.type == "partial") {
+      patchType = AUSTLMY.PATCH_PARTIAL;
+    }
+  }
+
+  let suffix = patchType + "_" + (aStartup ? AUSTLMY.STARTUP : AUSTLMY.STAGE);
+  let stateCode = 0;
+  let parts = aStatus.split(":");
+  if (parts.length) {
+    switch (parts[0]) {
+      case STATE_NONE:
+        stateCode = 2;
+        break;
+      case STATE_DOWNLOADING:
+        stateCode = 3;
+        break;
+      case STATE_PENDING:
+        stateCode = 4;
+        break;
+      case STATE_PENDING_SERVICE:
+        stateCode = 5;
+        break;
+      case STATE_APPLYING:
+        stateCode = 6;
+        break;
+      case STATE_APPLIED:
+        stateCode = 7;
+        break;
+      case STATE_APPLIED_SERVICE:
+        stateCode = 9;
+        break;
+      case STATE_SUCCEEDED:
+        stateCode = 10;
+        break;
+      case STATE_DOWNLOAD_FAILED:
+        stateCode = 11;
+        break;
+      case STATE_FAILED:
+        stateCode = 12;
+        break;
+      case STATE_PENDING_ELEVATE:
+        stateCode = 13;
+        break;
+      // Note: Do not use stateCode 14 here. It is defined in
+      // UpdateTelemetry.jsm
+      default:
+        stateCode = 1;
+    }
+
+    if (parts.length > 1) {
+      let statusErrorCode = INVALID_UPDATER_STATE_CODE;
+      if (parts[0] == STATE_FAILED) {
+        statusErrorCode = parseInt(parts[1]) || INVALID_UPDATER_STATUS_CODE;
+      }
+      AUSTLMY.pingStatusErrorCode(suffix, statusErrorCode);
+    }
+  }
+  let binaryTransparencyResult = readBinaryTransparencyResult(
+    getReadyUpdateDir()
+  );
+  if (binaryTransparencyResult) {
+    AUSTLMY.pingBinaryTransparencyResult(
+      suffix,
+      parseInt(binaryTransparencyResult)
+    );
+  }
+  AUSTLMY.pingStateCode(suffix, stateCode);
+}
+
+/**
+ * This returns true if the passed update is the same version or older than the
+ * version and build ID values passed. Otherwise it returns false.
+ */
+function updateIsAtLeastAsOldAs(update, version, buildID) {
+  if (!update || !update.appVersion || !update.buildID) {
+    return false;
+  }
+  let versionComparison = Services.vc.compare(update.appVersion, version);
+  return (
+    versionComparison < 0 ||
+    (versionComparison == 0 && update.buildID == buildID)
+  );
+}
+
+/**
+ * This returns true if the passed update is the same version or older than
+ * currently installed Firefox version.
+ */
+function updateIsAtLeastAsOldAsCurrentVersion(update) {
+  return updateIsAtLeastAsOldAs(
+    update,
+    Services.appinfo.version,
+    Services.appinfo.appBuildID
+  );
+}
+
+/**
+ * This returns true if the passed update is the same version or older than
+ * the update that we have already downloaded (UpdateManager.readyUpdate).
+ * Returns false if no update has already been downloaded.
+ */
+function updateIsAtLeastAsOldAsReadyUpdate(update) {
+  if (
+    !lazy.UM.readyUpdate ||
+    !lazy.UM.readyUpdate.appVersion ||
+    !lazy.UM.readyUpdate.buildID
+  ) {
+    return false;
+  }
+  return updateIsAtLeastAsOldAs(
+    update,
+    lazy.UM.readyUpdate.appVersion,
+    lazy.UM.readyUpdate.buildID
+  );
+}
+
+/**
+ * This function determines whether the error represented by the passed error
+ * code could potentially be recovered from or bypassed by updating without
+ * using the Maintenance Service (i.e. by showing a UAC prompt).
+ * We don't really want to show a UAC prompt, but it's preferable over the
+ * manual update doorhanger. So this function effectively distinguishes between
+ * which of those we should do if update staging failed. (The updater
+ * automatically falls back if the Maintenance Services fails, so this function
+ * doesn't handle that case)
+ *
+ * @param   An integer error code from the update.status file. Should be one of
+ *          the codes enumerated in updatererrors.h.
+ * @returns true if the code represents a Maintenance Service specific error.
+ *          Otherwise, false.
+ */
+function isServiceSpecificErrorCode(errorCode) {
+  return (
+    (errorCode >= 24 && errorCode <= 33) || (errorCode >= 49 && errorCode <= 58)
+  );
+}
+
+/**
+ * This function determines whether the error represented by the passed error
+ * code is the result of the updater failing to allocate memory. This is
+ * relevant when staging because, since Firefox is also running, we may not be
+ * able to allocate much memory. Thus, if we fail to stage an update, we may
+ * succeed at updating without staging.
+ *
+ * @param   An integer error code from the update.status file. Should be one of
+ *          the codes enumerated in updatererrors.h.
+ * @returns true if the code represents a memory allocation error.
+ *          Otherwise, false.
+ */
+function isMemoryAllocationErrorCode(errorCode) {
+  return errorCode >= 10 && errorCode <= 14;
+}
+
+/**
+ * Normally when staging, `nsUpdateProcessor::WaitForProcess` waits for the
+ * staging process to complete by watching for its PID to terminate.
+ * However, there are less ideal situations. Notably, we might start the browser
+ * and find that update staging appears to already be in-progress. If that
+ * happens, we really want to pick up the update process from STATE_STAGING,
+ * but we don't really have any way of keeping an eye on the staging process
+ * other than to just poll the status file.
+ *
+ * Like `nsUpdateProcessor`, this calls `nsIUpdateManager.refreshUpdateStatus`
+ * after polling completes (regardless of result).
+ *
+ * It is also important to keep in mind that the updater might have crashed
+ * during staging, meaning that the status file will never change, no matter how
+ * long we keep polling. So we need to set an upper bound on how long we are
+ * willing to poll for.
+ *
+ * There are three situations that we want to avoid.
+ * (1) We don't want to set the poll interval too long. A user might be watching
+ * the user interface and waiting to restart to install the update. A long poll
+ * interval will cause them to have to wait longer than necessary. Especially
+ * since the expected total staging time is not that long.
+ * (2) We don't want to give up polling too early and give up on an update that
+ * will ultimately succeed.
+ * (3) We don't want to use a rapid polling interval over a long duration.
+ *
+ * To avoid these situations, we will start with a short polling interval, but
+ * will increase it the longer that we have to wait. Then if we hit the upper
+ * bound of polling, we will give up.
+ */
+function pollForStagingEnd() {
+  let pollingIntervalMs = STAGING_POLLING_MIN_INTERVAL_MS;
+  // Number of times to poll before increasing the polling interval.
+  let pollAttemptsAtIntervalRemaining = STAGING_POLLING_ATTEMPTS_PER_INTERVAL;
+  let timeElapsedMs = 0;
+
+  let pollingFn = () => {
+    pollAttemptsAtIntervalRemaining -= 1;
+    // This isn't a perfectly accurate way of keeping time, but it does nicely
+    // sidestep dealing with issues of (non)monotonic time.
+    timeElapsedMs += pollingIntervalMs;
+
+    if (timeElapsedMs >= STAGING_POLLING_MAX_DURATION_MS) {
+      lazy.UM.refreshUpdateStatus();
+      return;
+    }
+
+    if (readStatusFile(getReadyUpdateDir()) != STATE_APPLYING) {
+      lazy.UM.refreshUpdateStatus();
+      return;
+    }
+
+    if (pollAttemptsAtIntervalRemaining <= 0) {
+      pollingIntervalMs = Math.min(
+        pollingIntervalMs * 2,
+        STAGING_POLLING_MAX_INTERVAL_MS
+      );
+      pollAttemptsAtIntervalRemaining = STAGING_POLLING_ATTEMPTS_PER_INTERVAL;
+    }
+
+    lazy.setTimeout(pollingFn, pollingIntervalMs);
+  };
+
+  lazy.setTimeout(pollingFn, pollingIntervalMs);
+}
+
+/**
+ * Update Patch
+ * @param   patch
+ *          A <patch> element to initialize this object with
+ * @throws if patch has a size of 0
+ * @constructor
+ */
+function UpdatePatch(patch) {
+  this._properties = {};
+  this.errorCode = 0;
+  this.finalURL = null;
+  this.state = STATE_NONE;
+
+  for (let i = 0; i < patch.attributes.length; ++i) {
+    var attr = patch.attributes.item(i);
+    // If an undefined value is saved to the xml file it will be a string when
+    // it is read from the xml file.
+    if (attr.value == "undefined") {
+      continue;
+    }
+    switch (attr.name) {
+      case "xmlns":
+        // Don't save the XML namespace.
+        break;
+      case "selected":
+        this.selected = attr.value == "true";
+        break;
+      case "size":
+        if (0 == parseInt(attr.value)) {
+          LOG("UpdatePatch:init - 0-sized patch!");
+          throw Components.Exception("", Cr.NS_ERROR_ILLEGAL_VALUE);
+        }
+        this[attr.name] = attr.value;
+        break;
+      case "errorCode":
+        if (attr.value) {
+          let val = parseInt(attr.value);
+          // This will evaluate to false if the value is 0 but that's ok since
+          // this.errorCode is set to the default of 0 above.
+          if (val) {
+            this.errorCode = val;
+          }
+        }
+        break;
+      case "finalURL":
+      case "state":
+      case "type":
+      case "URL":
+        this[attr.name] = attr.value;
+        break;
+      default:
+        if (!this._attrNames.includes(attr.name)) {
+          // Set nsIPropertyBag properties that were read from the xml file.
+          this.setProperty(attr.name, attr.value);
+        }
+        break;
+    }
+  }
+}
+UpdatePatch.prototype = {
+  // nsIUpdatePatch attribute names used to prevent nsIWritablePropertyBag from
+  // over writing nsIUpdatePatch attributes.
+  _attrNames: [
+    "errorCode",
+    "finalURL",
+    "selected",
+    "size",
+    "state",
+    "type",
+    "URL",
+  ],
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  serialize: function UpdatePatch_serialize(updates) {
+    var patch = updates.createElementNS(URI_UPDATE_NS, "patch");
+    patch.setAttribute("size", this.size);
+    patch.setAttribute("type", this.type);
+    patch.setAttribute("URL", this.URL);
+    // Don't write an errorCode if it evaluates to false since 0 is the same as
+    // no error code.
+    if (this.errorCode) {
+      patch.setAttribute("errorCode", this.errorCode);
+    }
+    // finalURL is not available until after the download has started
+    if (this.finalURL) {
+      patch.setAttribute("finalURL", this.finalURL);
+    }
+    // The selected patch is the only patch that should have this attribute.
+    if (this.selected) {
+      patch.setAttribute("selected", this.selected);
+    }
+    if (this.state != STATE_NONE) {
+      patch.setAttribute("state", this.state);
+    }
+
+    for (let [name, value] of Object.entries(this._properties)) {
+      if (value.present && !this._attrNames.includes(name)) {
+        patch.setAttribute(name, value.data);
+      }
+    }
+    return patch;
+  },
+
+  /**
+   * See nsIWritablePropertyBag.idl
+   */
+  setProperty: function UpdatePatch_setProperty(name, value) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdatePatch) " +
+          "when calling method: [nsIWritablePropertyBag::setProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    this._properties[name] = { data: value, present: true };
+  },
+
+  /**
+   * See nsIWritablePropertyBag.idl
+   */
+  deleteProperty: function UpdatePatch_deleteProperty(name) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdatePatch) " +
+          "when calling method: [nsIWritablePropertyBag::deleteProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    if (name in this._properties) {
+      this._properties[name].present = false;
+    } else {
+      throw Components.Exception("", Cr.NS_ERROR_FAILURE);
+    }
+  },
+
+  /**
+   * See nsIPropertyBag.idl
+   *
+   * Note: this only contains the nsIPropertyBag name / value pairs and not the
+   *       nsIUpdatePatch name / value pairs.
+   */
+  get enumerator() {
+    return this.enumerate();
+  },
+
+  *enumerate() {
+    // An nsISupportsInterfacePointer is used so creating an array using
+    // Array.from will retain the QueryInterface for nsIProperty.
+    let ip = Cc["@mozilla.org/supports-interface-pointer;1"].createInstance(
+      Ci.nsISupportsInterfacePointer
+    );
+    let qi = ChromeUtils.generateQI(["nsIProperty"]);
+    for (let [name, value] of Object.entries(this._properties)) {
+      if (value.present && !this._attrNames.includes(name)) {
+        // The nsIPropertyBag enumerator returns a nsISimpleEnumerator whose
+        // elements are nsIProperty objects. Calling QueryInterface for
+        // nsIProperty on the object doesn't return to the caller an object that
+        // is already queried to nsIProperty but do it just in case it is fixed
+        // at some point.
+        ip.data = { name, value: value.data, QueryInterface: qi };
+        yield ip.data.QueryInterface(Ci.nsIProperty);
+      }
+    }
+  },
+
+  /**
+   * See nsIPropertyBag.idl
+   *
+   * Note: returns null instead of throwing when the property doesn't exist to
+   *       simplify code and to silence warnings in debug builds.
+   */
+  getProperty: function UpdatePatch_getProperty(name) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdatePatch) " +
+          "when calling method: [nsIWritablePropertyBag::getProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    if (name in this._properties && this._properties[name].present) {
+      return this._properties[name].data;
+    }
+    return null;
+  },
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIUpdatePatch",
+    "nsIPropertyBag",
+    "nsIWritablePropertyBag",
+  ]),
+};
+
+/**
+ * Update
+ * Implements nsIUpdate
+ * @param   update
+ *          An <update> element to initialize this object with
+ * @throws if the update contains no patches
+ * @constructor
+ */
+function Update(update) {
+  this._patches = [];
+  this._properties = {};
+  this.isCompleteUpdate = false;
+  this.channel = "default";
+  this.promptWaitTime = Services.prefs.getIntPref(
+    PREF_APP_UPDATE_PROMPTWAITTIME,
+    43200
+  );
+  this.unsupported = false;
+
+  // Null <update>, assume this is a message container and do no
+  // further initialization
+  if (!update) {
+    return;
+  }
+
+  for (let i = 0; i < update.childNodes.length; ++i) {
+    let patchElement = update.childNodes.item(i);
+    if (
+      patchElement.nodeType != patchElement.ELEMENT_NODE ||
+      patchElement.localName != "patch"
+    ) {
+      continue;
+    }
+
+    let patch;
+    try {
+      patch = new UpdatePatch(patchElement);
+    } catch (e) {
+      continue;
+    }
+    this._patches.push(patch);
+  }
+
+  if (!this._patches.length && !update.hasAttribute("unsupported")) {
+    throw Components.Exception("", Cr.NS_ERROR_ILLEGAL_VALUE);
+  }
+
+  // Set the installDate value with the current time. If the update has an
+  // installDate attribute this will be replaced with that value if it doesn't
+  // equal 0.
+  this.installDate = new Date().getTime();
+  this.patchCount = this._patches.length;
+
+  for (let i = 0; i < update.attributes.length; ++i) {
+    let attr = update.attributes.item(i);
+    if (attr.name == "xmlns" || attr.value == "undefined") {
+      // Don't save the XML namespace or undefined values.
+      // If an undefined value is saved to the xml file it will be a string when
+      // it is read from the xml file.
+      continue;
+    } else if (attr.name == "detailsURL") {
+      this.detailsURL = attr.value;
+    } else if (attr.name == "installDate" && attr.value) {
+      let val = parseInt(attr.value);
+      if (val) {
+        this.installDate = val;
+      }
+    } else if (attr.name == "errorCode" && attr.value) {
+      let val = parseInt(attr.value);
+      if (val) {
+        // Set the value of |_errorCode| instead of |errorCode| since
+        // selectedPatch won't be available at this point and normally the
+        // nsIUpdatePatch will provide the errorCode.
+        this._errorCode = val;
+      }
+    } else if (attr.name == "isCompleteUpdate") {
+      this.isCompleteUpdate = attr.value == "true";
+    } else if (attr.name == "promptWaitTime") {
+      if (!isNaN(attr.value)) {
+        this.promptWaitTime = parseInt(attr.value);
+      }
+    } else if (attr.name == "unsupported") {
+      this.unsupported = attr.value == "true";
+    } else {
+      switch (attr.name) {
+        case "appVersion":
+        case "buildID":
+        case "channel":
+        case "displayVersion":
+        case "elevationFailure":
+        case "name":
+        case "previousAppVersion":
+        case "serviceURL":
+        case "statusText":
+        case "type":
+          this[attr.name] = attr.value;
+          break;
+        default:
+          if (!this._attrNames.includes(attr.name)) {
+            // Set nsIPropertyBag properties that were read from the xml file.
+            this.setProperty(attr.name, attr.value);
+          }
+          break;
+      }
+    }
+  }
+
+  if (!this.previousAppVersion) {
+    this.previousAppVersion = Services.appinfo.version;
+  }
+
+  if (!this.elevationFailure) {
+    this.elevationFailure = false;
+  }
+
+  if (!this.detailsURL) {
+    try {
+      // Try using a default details URL supplied by the distribution
+      // if the update XML does not supply one.
+      this.detailsURL = Services.urlFormatter.formatURLPref(
+        PREF_APP_UPDATE_URL_DETAILS
+      );
+    } catch (e) {
+      this.detailsURL = "";
+    }
+  }
+
+  if (!this.displayVersion) {
+    this.displayVersion = this.appVersion;
+  }
+
+  if (!this.name) {
+    // When the update doesn't provide a name fallback to using
+    // "<App Name> <Update App Version>"
+    let brandBundle = Services.strings.createBundle(URI_BRAND_PROPERTIES);
+    let appName = brandBundle.GetStringFromName("brandShortName");
+    this.name = lazy.gUpdateBundle.formatStringFromName("updateName", [
+      appName,
+      this.displayVersion,
+    ]);
+  }
+}
+Update.prototype = {
+  // nsIUpdate attribute names used to prevent nsIWritablePropertyBag from over
+  // writing nsIUpdate attributes.
+  _attrNames: [
+    "appVersion",
+    "buildID",
+    "channel",
+    "detailsURL",
+    "displayVersion",
+    "elevationFailure",
+    "errorCode",
+    "installDate",
+    "isCompleteUpdate",
+    "name",
+    "previousAppVersion",
+    "promptWaitTime",
+    "serviceURL",
+    "state",
+    "statusText",
+    "type",
+    "unsupported",
+  ],
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  getPatchAt: function Update_getPatchAt(index) {
+    return this._patches[index];
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   *
+   * We use a copy of the state cached on this object in |_state| only when
+   * there is no selected patch, i.e. in the case when we could not load
+   * active updates from the update manager for some reason but still have
+   * the update.status file to work with.
+   */
+  _state: "",
+  get state() {
+    if (this.selectedPatch) {
+      return this.selectedPatch.state;
+    }
+    return this._state;
+  },
+  set state(state) {
+    if (this.selectedPatch) {
+      this.selectedPatch.state = state;
+    }
+    this._state = state;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   *
+   * We use a copy of the errorCode cached on this object in |_errorCode| only
+   * when there is no selected patch, i.e. in the case when we could not load
+   * active updates from the update manager for some reason but still have
+   * the update.status file to work with.
+   */
+  _errorCode: 0,
+  get errorCode() {
+    if (this.selectedPatch) {
+      return this.selectedPatch.errorCode;
+    }
+    return this._errorCode;
+  },
+  set errorCode(errorCode) {
+    if (this.selectedPatch) {
+      this.selectedPatch.errorCode = errorCode;
+    }
+    this._errorCode = errorCode;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get selectedPatch() {
+    for (let i = 0; i < this.patchCount; ++i) {
+      if (this._patches[i].selected) {
+        return this._patches[i];
+      }
+    }
+    return null;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  serialize: function Update_serialize(updates) {
+    // If appVersion isn't defined just return null. This happens when cleaning
+    // up invalid updates (e.g. incorrect channel).
+    if (!this.appVersion) {
+      return null;
+    }
+    let update = updates.createElementNS(URI_UPDATE_NS, "update");
+    update.setAttribute("appVersion", this.appVersion);
+    update.setAttribute("buildID", this.buildID);
+    update.setAttribute("channel", this.channel);
+    update.setAttribute("detailsURL", this.detailsURL);
+    update.setAttribute("displayVersion", this.displayVersion);
+    update.setAttribute("installDate", this.installDate);
+    update.setAttribute("isCompleteUpdate", this.isCompleteUpdate);
+    update.setAttribute("name", this.name);
+    update.setAttribute("previousAppVersion", this.previousAppVersion);
+    update.setAttribute("promptWaitTime", this.promptWaitTime);
+    update.setAttribute("serviceURL", this.serviceURL);
+    update.setAttribute("type", this.type);
+
+    if (this.statusText) {
+      update.setAttribute("statusText", this.statusText);
+    }
+    if (this.unsupported) {
+      update.setAttribute("unsupported", this.unsupported);
+    }
+    if (this.elevationFailure) {
+      update.setAttribute("elevationFailure", this.elevationFailure);
+    }
+
+    for (let [name, value] of Object.entries(this._properties)) {
+      if (value.present && !this._attrNames.includes(name)) {
+        update.setAttribute(name, value.data);
+      }
+    }
+
+    for (let i = 0; i < this.patchCount; ++i) {
+      update.appendChild(this.getPatchAt(i).serialize(updates));
+    }
+
+    updates.documentElement.appendChild(update);
+    return update;
+  },
+
+  /**
+   * See nsIWritablePropertyBag.idl
+   */
+  setProperty: function Update_setProperty(name, value) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdate) " +
+          "when calling method: [nsIWritablePropertyBag::setProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    this._properties[name] = { data: value, present: true };
+  },
+
+  /**
+   * See nsIWritablePropertyBag.idl
+   */
+  deleteProperty: function Update_deleteProperty(name) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdate) " +
+          "when calling method: [nsIWritablePropertyBag::deleteProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    if (name in this._properties) {
+      this._properties[name].present = false;
+    } else {
+      throw Components.Exception("", Cr.NS_ERROR_FAILURE);
+    }
+  },
+
+  /**
+   * See nsIPropertyBag.idl
+   *
+   * Note: this only contains the nsIPropertyBag name value / pairs and not the
+   *       nsIUpdate name / value pairs.
+   */
+  get enumerator() {
+    return this.enumerate();
+  },
+
+  *enumerate() {
+    // An nsISupportsInterfacePointer is used so creating an array using
+    // Array.from will retain the QueryInterface for nsIProperty.
+    let ip = Cc["@mozilla.org/supports-interface-pointer;1"].createInstance(
+      Ci.nsISupportsInterfacePointer
+    );
+    let qi = ChromeUtils.generateQI(["nsIProperty"]);
+    for (let [name, value] of Object.entries(this._properties)) {
+      if (value.present && !this._attrNames.includes(name)) {
+        // The nsIPropertyBag enumerator returns a nsISimpleEnumerator whose
+        // elements are nsIProperty objects. Calling QueryInterface for
+        // nsIProperty on the object doesn't return to the caller an object that
+        // is already queried to nsIProperty but do it just in case it is fixed
+        // at some point.
+        ip.data = { name, value: value.data, QueryInterface: qi };
+        yield ip.data.QueryInterface(Ci.nsIProperty);
+      }
+    }
+  },
+
+  /**
+   * See nsIPropertyBag.idl
+   * Note: returns null instead of throwing when the property doesn't exist to
+   *       simplify code and to silence warnings in debug builds.
+   */
+  getProperty: function Update_getProperty(name) {
+    if (this._attrNames.includes(name)) {
+      throw Components.Exception(
+        "Illegal value '" +
+          name +
+          "' (attribute exists on nsIUpdate) " +
+          "when calling method: [nsIWritablePropertyBag::getProperty]",
+        Cr.NS_ERROR_ILLEGAL_VALUE
+      );
+    }
+    if (name in this._properties && this._properties[name].present) {
+      return this._properties[name].data;
+    }
+    return null;
+  },
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIUpdate",
+    "nsIPropertyBag",
+    "nsIWritablePropertyBag",
+  ]),
+};
+
+/**
+ * UpdateService
+ * A Service for managing the discovery and installation of software updates.
+ * @constructor
+ */
+export function UpdateService() {
+  LOG("Creating UpdateService");
+  // The observor notification to shut down the service must be before
+  // profile-before-change since nsIUpdateManager uses profile-before-change
+  // to shutdown and write the update xml files.
+  Services.obs.addObserver(this, "quit-application");
+  // This one call observes PREF_APP_UPDATE_LOG and PREF_APP_UPDATE_LOG_FILE
+  Services.prefs.addObserver(PREF_APP_UPDATE_LOG, this);
+
+  this._logStatus();
+}
+
+UpdateService.prototype = {
+  /**
+   * The downloader we are using to download updates. There is only ever one of
+   * these.
+   */
+  _downloader: null,
+
+  /**
+   * Whether or not the service registered the "online" observer.
+   */
+  _registeredOnlineObserver: false,
+
+  /**
+   * The current number of consecutive socket errors
+   */
+  _consecutiveSocketErrors: 0,
+
+  /**
+   * A timer used to retry socket errors
+   */
+  _retryTimer: null,
+
+  /**
+   * Whether or not a background update check was initiated by the
+   * application update timer notification.
+   */
+  _isNotify: true,
+
+  /**
+   * Handle Observer Service notifications
+   * @param   subject
+   *          The subject of the notification
+   * @param   topic
+   *          The notification name
+   * @param   data
+   *          Additional data
+   */
+  observe: async function AUS_observe(subject, topic, data) {
+    switch (topic) {
+      case "post-update-processing":
+        // This pref was not cleared out of profiles after it stopped being used
+        // (Bug 1420514), so clear it out on the next update to avoid confusion
+        // regarding its use.
+        Services.prefs.clearUserPref("app.update.enabled");
+        Services.prefs.clearUserPref("app.update.BITS.inTrialGroup");
+
+        // Background tasks do not notify any delayed startup notifications.
+        if (
+          !lazy.gIsBackgroundTaskMode &&
+          Services.appinfo.ID in APPID_TO_TOPIC
+        ) {
+          // Delay post-update processing to ensure that possible update
+          // dialogs are shown in front of the app window, if possible.
+          // See bug 311614.
+          Services.obs.addObserver(this, APPID_TO_TOPIC[Services.appinfo.ID]);
+          break;
+        }
+      // intentional fallthrough
+      case "sessionstore-windows-restored":
+      case "mail-startup-done":
+        // Background tasks do not notify any delayed startup notifications.
+        if (
+          !lazy.gIsBackgroundTaskMode &&
+          Services.appinfo.ID in APPID_TO_TOPIC
+        ) {
+          Services.obs.removeObserver(
+            this,
+            APPID_TO_TOPIC[Services.appinfo.ID]
+          );
+        }
+      // intentional fallthrough
+      case "test-post-update-processing":
+        // Clean up any extant updates
+        await this._postUpdateProcessing();
+        break;
+      case "network:offline-status-changed":
+        await this._offlineStatusChanged(data);
+        break;
+      case "nsPref:changed":
+        if (data == PREF_APP_UPDATE_LOG || data == PREF_APP_UPDATE_LOG_FILE) {
+          lazy.gLogEnabled; // Assigning this before it is lazy-loaded is an error.
+          lazy.gLogEnabled =
+            Services.prefs.getBoolPref(PREF_APP_UPDATE_LOG, false) ||
+            Services.prefs.getBoolPref(PREF_APP_UPDATE_LOG_FILE, false);
+        }
+        if (data == PREF_APP_UPDATE_LOG_FILE) {
+          lazy.gLogfileEnabled; // Assigning this before it is lazy-loaded is an
+          // error.
+          lazy.gLogfileEnabled = Services.prefs.getBoolPref(
+            PREF_APP_UPDATE_LOG_FILE,
+            false
+          );
+          if (lazy.gLogfileEnabled) {
+            this._logStatus();
+          }
+        }
+        break;
+      case "quit-application":
+        Services.obs.removeObserver(this, topic);
+        Services.prefs.removeObserver(PREF_APP_UPDATE_LOG, this);
+
+        if (AppConstants.platform == "win" && gUpdateMutexHandle) {
+          // If we hold the update mutex, let it go!
+          // The OS would clean this up sometime after shutdown,
+          // but that would have no guarantee on timing.
+          closeHandle(gUpdateMutexHandle);
+          gUpdateMutexHandle = null;
+        }
+        if (this._retryTimer) {
+          this._retryTimer.cancel();
+        }
+
+        // When downloading an update with nsIIncrementalDownload the download
+        // is stopped when the quit-application observer notification is
+        // received and networking hasn't started to shutdown. The download will
+        // be resumed the next time the application starts. Downloads using
+        // Windows BITS are not stopped since they don't require Firefox to be
+        // running to perform the download.
+        if (this._downloader) {
+          if (this._downloader.usingBits) {
+            await this._downloader.cleanup();
+          } else {
+            // stopDownload() calls _downloader.cleanup()
+            await this.stopDownload();
+          }
+        }
+        // Prevent leaking the downloader (bug 454964)
+        this._downloader = null;
+        // In case any update checks are in progress.
+        lazy.CheckSvc.stopAllChecks();
+
+        if (gLogfileOutputStream) {
+          gLogfileOutputStream.close();
+        }
+        break;
+      case "test-close-handle-update-mutex":
+        if (Cu.isInAutomation) {
+          if (AppConstants.platform == "win" && gUpdateMutexHandle) {
+            LOG("UpdateService:observe - closing mutex handle for testing");
+            closeHandle(gUpdateMutexHandle);
+            gUpdateMutexHandle = null;
+          }
+        }
+        break;
+    }
+  },
+
+  /**
+   * The following needs to happen during the post-update-processing
+   * notification from nsUpdateServiceStub.js:
+   * 1. post update processing
+   * 2. resume of a download that was in progress during a previous session
+   * 3. start of a complete update download after the failure to apply a partial
+   *    update
+   */
+
+  /**
+   * Perform post-processing on updates lingering in the updates directory
+   * from a previous application session - either report install failures (and
+   * optionally attempt to fetch a different version if appropriate) or
+   * notify the user of install success.
+   */
+  /* eslint-disable-next-line complexity */
+  _postUpdateProcessing: async function AUS__postUpdateProcessing() {
+    if (this.disabled) {
+      // This function is a point when we can potentially enter the update
+      // system, even with update disabled. Make sure that we do not continue
+      // because update code can have side effects that are visible to the user
+      // and give the impression that updates are enabled. For example, if we
+      // can't write to the update directory, we might complain to the user that
+      // update is broken and they should reinstall.
+      return;
+    }
+    if (!this.canCheckForUpdates) {
+      LOG(
+        "UpdateService:_postUpdateProcessing - unable to check for " +
+          "updates... returning early"
+      );
+      return;
+    }
+    let status = readStatusFile(getReadyUpdateDir());
+    LOG(`UpdateService:_postUpdateProcessing - status = "${status}"`);
+
+    if (!this.canApplyUpdates) {
+      LOG(
+        "UpdateService:_postUpdateProcessing - unable to apply " +
+          "updates... returning early"
+      );
+      if (hasUpdateMutex()) {
+        // If the update is present in the update directory somehow,
+        // it would prevent us from notifying the user of further updates.
+        LOG(
+          "UpdateService:_postUpdateProcessing - Cleaning up active updates."
+        );
+        cleanupActiveUpdates();
+      }
+      return;
+    }
+
+    let updates = [];
+    if (lazy.UM.readyUpdate) {
+      updates.push(lazy.UM.readyUpdate);
+    }
+    if (lazy.UM.downloadingUpdate) {
+      updates.push(lazy.UM.downloadingUpdate);
+    }
+
+    if (status == STATE_NONE) {
+      // A status of STATE_NONE in _postUpdateProcessing means that the
+      // update.status file is present but there isn't an update in progress.
+      // This isn't an expected state, so if we find ourselves in it, we want
+      // to just clean things up to go back to a good state.
+      LOG(
+        "UpdateService:_postUpdateProcessing - Cleaning up unexpected state."
+      );
+      if (!updates.length) {
+        updates.push(new Update(null));
+      }
+      for (let update of updates) {
+        update.state = STATE_FAILED;
+        update.errorCode = ERR_UPDATE_STATE_NONE;
+        update.statusText =
+          lazy.gUpdateBundle.GetStringFromName("statusFailed");
+      }
+      let newStatus = STATE_FAILED + ": " + ERR_UPDATE_STATE_NONE;
+      pingStateAndStatusCodes(updates[0], true, newStatus);
+      cleanupActiveUpdates();
+      return;
+    }
+
+    let channelChanged = updates => {
+      for (let update of updates) {
+        if (update.channel != lazy.UpdateUtils.UpdateChannel) {
+          return true;
+        }
+      }
+      return false;
+    };
+    if (channelChanged(updates)) {
+      let channel = lazy.UM.readyUpdate
+        ? lazy.UM.readyUpdate.channel
+        : lazy.UM.downloadingUpdate.channel;
+      LOG(
+        "UpdateService:_postUpdateProcessing - update channel is " +
+          "different than application's channel, removing update. update " +
+          "channel: " +
+          channel +
+          ", expected channel: " +
+          lazy.UpdateUtils.UpdateChannel
+      );
+      // User switched channels, clear out the old active updates and remove
+      // partial downloads
+      for (let update of updates) {
+        update.state = STATE_FAILED;
+        update.errorCode = ERR_CHANNEL_CHANGE;
+        update.statusText =
+          lazy.gUpdateBundle.GetStringFromName("statusFailed");
+      }
+      let newStatus = STATE_FAILED + ": " + ERR_CHANNEL_CHANGE;
+      pingStateAndStatusCodes(updates[0], true, newStatus);
+      cleanupActiveUpdates();
+      return;
+    }
+
+    // Handle the case when the update is the same or older than the current
+    // version and nsUpdateDriver.cpp skipped updating due to the version being
+    // older than the current version. This also handles the general case when
+    // an update is for an older version or the same version and same build ID.
+    if (
+      status == STATE_PENDING ||
+      status == STATE_PENDING_SERVICE ||
+      status == STATE_APPLIED ||
+      status == STATE_APPLIED_SERVICE ||
+      status == STATE_PENDING_ELEVATE ||
+      status == STATE_DOWNLOADING
+    ) {
+      let tooOldUpdate;
+      if (
+        updateIsAtLeastAsOldAs(
+          lazy.UM.readyUpdate,
+          Services.appinfo.version,
+          Services.appinfo.appBuildID
+        )
+      ) {
+        tooOldUpdate = lazy.UM.readyUpdate;
+      } else if (
+        updateIsAtLeastAsOldAs(
+          lazy.UM.downloadingUpdate,
+          Services.appinfo.version,
+          Services.appinfo.appBuildID
+        )
+      ) {
+        tooOldUpdate = lazy.UM.downloadingUpdate;
+      }
+      if (tooOldUpdate) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - removing update for older " +
+            "application version or same application version with same build " +
+            "ID. update application version: " +
+            tooOldUpdate.appVersion +
+            ", " +
+            "application version: " +
+            Services.appinfo.version +
+            ", update " +
+            "build ID: " +
+            tooOldUpdate.buildID +
+            ", application build ID: " +
+            Services.appinfo.appBuildID
+        );
+        tooOldUpdate.state = STATE_FAILED;
+        tooOldUpdate.statusText =
+          lazy.gUpdateBundle.GetStringFromName("statusFailed");
+        tooOldUpdate.errorCode = ERR_OLDER_VERSION_OR_SAME_BUILD;
+        // This could be split out to report telemetry for each case.
+        let newStatus = STATE_FAILED + ": " + ERR_OLDER_VERSION_OR_SAME_BUILD;
+        pingStateAndStatusCodes(tooOldUpdate, true, newStatus);
+        // Cleanup both updates regardless of which one is too old. It's
+        // exceedingly unlikely that a user could end up in a state where one
+        // update is acceptable and the other isn't. And it makes this function
+        // considerably more complex to try to deal with that possibility.
+        cleanupActiveUpdates();
+        return;
+      }
+    }
+
+    pingStateAndStatusCodes(
+      status == STATE_DOWNLOADING
+        ? lazy.UM.downloadingUpdate
+        : lazy.UM.readyUpdate,
+      true,
+      status
+    );
+    if (lazy.UM.downloadingUpdate || status == STATE_DOWNLOADING) {
+      if (status == STATE_SUCCEEDED) {
+        // If we successfully installed an update while we were downloading
+        // another update, the downloading update is now a partial MAR for
+        // a version that is no longer installed. We know that it's a partial
+        // MAR without checking because we currently only download partial MARs
+        // when an update has already been downloaded.
+        LOG(
+          "UpdateService:_postUpdateProcessing - removing downloading patch " +
+            "because we installed a different patch before it finished" +
+            "downloading."
+        );
+        cleanupDownloadingUpdate();
+      } else {
+        // Attempt to resume download
+        if (lazy.UM.downloadingUpdate) {
+          LOG(
+            "UpdateService:_postUpdateProcessing - resuming patch found in " +
+              "downloading state"
+          );
+          let success = await this.downloadUpdate(lazy.UM.downloadingUpdate);
+          if (!success) {
+            LOG(
+              "UpdateService:_postUpdateProcessing - Failed to resume patch. " +
+                "Cleaning up downloading update."
+            );
+            cleanupDownloadingUpdate();
+          }
+        } else {
+          LOG(
+            "UpdateService:_postUpdateProcessing - Warning: found " +
+              "downloading state, but no downloading patch. Cleaning up " +
+              "active updates."
+          );
+          // Put ourselves back in a good state.
+          cleanupActiveUpdates();
+        }
+        if (status == STATE_DOWNLOADING) {
+          // Done dealing with the downloading update, and there is no ready
+          // update, so return early.
+          return;
+        }
+      }
+    }
+
+    let update = lazy.UM.readyUpdate;
+
+    if (status == STATE_APPLYING) {
+      // This indicates that the background updater service is in either of the
+      // following two states:
+      // 1. It is in the process of applying an update in the background, and
+      //    we just happen to be racing against that.
+      // 2. It has failed to apply an update for some reason, and we hit this
+      //    case because the updater process has set the update status to
+      //    applying, but has never finished.
+      // In order to differentiate between these two states, we look at the
+      // state field of the update object.  If it's "pending", then we know
+      // that this is the first time we're hitting this case, so we switch
+      // that state to "applying" and we just wait and hope for the best.
+      // If it's "applying", we know that we've already been here once, so
+      // we really want to start from a clean state.
+      if (
+        update &&
+        (update.state == STATE_PENDING || update.state == STATE_PENDING_SERVICE)
+      ) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - patch found in applying " +
+            "state for the first time"
+        );
+        update.state = STATE_APPLYING;
+        lazy.UM.saveUpdates();
+        transitionState(Ci.nsIApplicationUpdateService.STATE_STAGING);
+        pollForStagingEnd();
+      } else {
+        // We get here even if we don't have an update object
+        LOG(
+          "UpdateService:_postUpdateProcessing - patch found in applying " +
+            "state for the second time. Cleaning up ready update."
+        );
+        cleanupReadyUpdate();
+      }
+      return;
+    }
+
+    if (!update) {
+      if (status != STATE_SUCCEEDED) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - previous patch failed " +
+            "and no patch available. Cleaning up ready update."
+        );
+        cleanupReadyUpdate();
+        return;
+      }
+      LOG(
+        "UpdateService:_postUpdateProcessing - Update data missing. Creating " +
+          "an empty update object."
+      );
+      update = new Update(null);
+    }
+
+    let parts = status.split(":");
+    update.state = parts[0];
+    LOG(
+      `UpdateService:_postUpdateProcessing - Setting update's state from ` +
+        `the status file (="${update.state}")`
+    );
+    if (update.state == STATE_FAILED && parts[1]) {
+      update.errorCode = parseInt(parts[1]);
+      LOG(
+        `UpdateService:_postUpdateProcessing - Setting update's errorCode ` +
+          `from the status file (="${update.errorCode}")`
+      );
+    }
+
+    if (status != STATE_SUCCEEDED) {
+      // Rotate the update logs so the update log isn't removed. By passing
+      // false the patch directory won't be removed.
+      cleanUpReadyUpdateDir(false);
+    }
+
+    if (status == STATE_SUCCEEDED) {
+      if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CANCELATIONS)) {
+        Services.prefs.clearUserPref(PREF_APP_UPDATE_CANCELATIONS);
+      }
+      update.statusText =
+        lazy.gUpdateBundle.GetStringFromName("installSuccess");
+
+      // The only time that update is not a reference to readyUpdate is when
+      // readyUpdate is null.
+      if (!lazy.UM.readyUpdate) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - Assigning successful update " +
+            "readyUpdate before cleaning it up."
+        );
+        lazy.UM.readyUpdate = update;
+      }
+
+      // Done with this update. Clean it up.
+      LOG(
+        "UpdateService:_postUpdateProcessing - Cleaning up successful ready " +
+          "update."
+      );
+      cleanupReadyUpdate();
+
+      Services.prefs.setIntPref(PREF_APP_UPDATE_ELEVATE_ATTEMPTS, 0);
+    } else if (status == STATE_PENDING_ELEVATE) {
+      // In case the active-update.xml file is deleted.
+      if (!update) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - status is pending-elevate " +
+            "but there isn't a ready update, removing update"
+        );
+        cleanupReadyUpdate();
+      } else {
+        transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+        if (Services.startup.wasSilentlyStarted) {
+          // This check _should_ be unnecessary since we should not silently
+          // restart if state == pending-elevate. But the update elevation
+          // dialog is a way that we could potentially show UI on startup, even
+          // with no windows open. Which we really do not want to do on a silent
+          // restart.
+          // So this is defense in depth.
+          LOG(
+            "UpdateService:_postUpdateProcessing - status is " +
+              "pending-elevate, but this is a silent startup, so the " +
+              "elevation window has been suppressed."
+          );
+        } else {
+          LOG(
+            "UpdateService:_postUpdateProcessing - status is " +
+              "pending-elevate. Showing Update elevation dialog."
+          );
+          let uri = "chrome://mozapps/content/update/updateElevation.xhtml";
+          let features =
+            "chrome,centerscreen,resizable=no,titlebar,toolbar=no,dialog=no";
+          Services.ww.openWindow(null, uri, "Update:Elevation", features, null);
+        }
+      }
+    } else {
+      // If there was an I/O error it is assumed that the patch is not invalid
+      // and it is set to pending so an attempt to apply it again will happen
+      // when the application is restarted.
+      if (update.state == STATE_FAILED && update.errorCode) {
+        LOG(
+          "UpdateService:_postUpdateProcessing - Attempting handleUpdateFailure"
+        );
+        if (handleUpdateFailure(update)) {
+          LOG(
+            "UpdateService:_postUpdateProcessing - handleUpdateFailure success."
+          );
+          return;
+        }
+      }
+
+      LOG(
+        "UpdateService:_postUpdateProcessing - Attempting to fall back to a " +
+          "complete update."
+      );
+      // Something went wrong with the patch application process.
+      await handleFallbackToCompleteUpdate();
+    }
+  },
+
+  /**
+   * Register an observer when the network comes online, so we can short-circuit
+   * the app.update.interval when there isn't connectivity
+   */
+  _registerOnlineObserver: function AUS__registerOnlineObserver() {
+    if (this._registeredOnlineObserver) {
+      LOG(
+        "UpdateService:_registerOnlineObserver - observer already registered"
+      );
+      return;
+    }
+
+    LOG(
+      "UpdateService:_registerOnlineObserver - waiting for the network to " +
+        "be online, then forcing another check"
+    );
+
+    Services.obs.addObserver(this, "network:offline-status-changed");
+    this._registeredOnlineObserver = true;
+  },
+
+  /**
+   * Called from the network:offline-status-changed observer.
+   */
+  _offlineStatusChanged: async function AUS__offlineStatusChanged(status) {
+    if (status !== "online") {
+      return;
+    }
+
+    Services.obs.removeObserver(this, "network:offline-status-changed");
+    this._registeredOnlineObserver = false;
+
+    LOG(
+      "UpdateService:_offlineStatusChanged - network is online, forcing " +
+        "another background check"
+    );
+
+    // the background checker is contained in notify
+    await this._attemptResume();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  onCheckComplete: async function AUS_onCheckComplete(result) {
+    if (result.succeeded) {
+      await this._selectAndInstallUpdate(result.updates);
+      return;
+    }
+
+    if (!result.checksAllowed) {
+      LOG("UpdateService:onCheckComplete - checks not allowed");
+      return;
+    }
+
+    // On failure, result.updates is guaranteed to have exactly one update
+    // containing error information.
+    let update = result.updates[0];
+
+    LOG(
+      "UpdateService:onCheckComplete - error during background update. error " +
+        "code: " +
+        update.errorCode +
+        ", status text: " +
+        update.statusText
+    );
+
+    if (update.errorCode == NETWORK_ERROR_OFFLINE) {
+      // Register an online observer to try again
+      this._registerOnlineObserver();
+      if (this._pingSuffix) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_OFFLINE);
+      }
+      return;
+    }
+
+    // Send the error code to telemetry
+    AUSTLMY.pingCheckExError(this._pingSuffix, update.errorCode);
+    update.errorCode = BACKGROUNDCHECK_MULTIPLE_FAILURES;
+    let errCount = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_BACKGROUNDERRORS,
+      0
+    );
+
+    // If we already have an update ready, we don't want to worry the user over
+    // update check failures. As far as the user knows, the update status is
+    // the status of the ready update. We don't want to confuse them by saying
+    // that an update check failed.
+    if (lazy.UM.readyUpdate) {
+      LOG(
+        "UpdateService:onCheckComplete - Ignoring error because another " +
+          "update is ready."
+      );
+      return;
+    }
+
+    errCount++;
+    Services.prefs.setIntPref(PREF_APP_UPDATE_BACKGROUNDERRORS, errCount);
+    // Don't allow the preference to set a value greater than 20 for max errors.
+    let maxErrors = Math.min(
+      Services.prefs.getIntPref(PREF_APP_UPDATE_BACKGROUNDMAXERRORS, 10),
+      20
+    );
+
+    if (errCount >= maxErrors) {
+      LOG(
+        "UpdateService:onCheckComplete - notifying observers of error. " +
+          "topic: update-error, status: check-attempts-exceeded"
+      );
+      Services.obs.notifyObservers(
+        update,
+        "update-error",
+        "check-attempts-exceeded"
+      );
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_GENERAL_ERROR_PROMPT);
+    } else {
+      LOG(
+        "UpdateService:onCheckComplete - notifying observers of error. " +
+          "topic: update-error, status: check-attempt-failed"
+      );
+      Services.obs.notifyObservers(
+        update,
+        "update-error",
+        "check-attempt-failed"
+      );
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_GENERAL_ERROR_SILENT);
+    }
+  },
+
+  /**
+   * Called when a connection should be resumed
+   */
+  _attemptResume: async function AUS_attemptResume() {
+    LOG("UpdateService:_attemptResume");
+    // If a download is in progress and we aren't already downloading it, then
+    // resume it.
+    if (this.isDownloading) {
+      // There is nothing to resume. We are already downloading.
+      LOG("UpdateService:_attemptResume - already downloading.");
+      return;
+    }
+    if (
+      this._downloader &&
+      this._downloader._patch &&
+      this._downloader._patch.state == STATE_DOWNLOADING &&
+      this._downloader._update
+    ) {
+      LOG(
+        "UpdateService:_attemptResume - _patch.state: " +
+          this._downloader._patch.state
+      );
+      let success = await this.downloadUpdate(this._downloader._update);
+      LOG("UpdateService:_attemptResume - downloadUpdate success: " + success);
+      if (!success) {
+        LOG(
+          "UpdateService:_attemptResume - Resuming download failed. Cleaning " +
+            "up downloading update."
+        );
+        cleanupDownloadingUpdate();
+      }
+      return;
+    }
+
+    // Kick off an update check
+    (async () => {
+      let check = lazy.CheckSvc.checkForUpdates(lazy.CheckSvc.BACKGROUND_CHECK);
+      await this.onCheckComplete(await check.result);
+    })();
+  },
+
+  /**
+   * Notified when a timer fires
+   * @param   timer
+   *          The timer that fired
+   */
+  notify: function AUS_notify(timer) {
+    this._checkForBackgroundUpdates(true);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  checkForBackgroundUpdates: function AUS_checkForBackgroundUpdates() {
+    return this._checkForBackgroundUpdates(false);
+  },
+
+  // The suffix used for background update check telemetry histogram ID's.
+  get _pingSuffix() {
+    if (lazy.UM.readyUpdate) {
+      // Once an update has been downloaded, all later updates will be reported
+      // to telemetry as subsequent updates. We move the first update into
+      // readyUpdate as soon as the download is complete, so any update checks
+      // after readyUpdate is no longer null are subsequent update checks.
+      return AUSTLMY.SUBSEQUENT;
+    }
+    return this._isNotify ? AUSTLMY.NOTIFY : AUSTLMY.EXTERNAL;
+  },
+
+  /**
+   * Checks for updates in the background.
+   * @param   isNotify
+   *          Whether or not a background update check was initiated by the
+   *          application update timer notification.
+   */
+  _checkForBackgroundUpdates: function AUS__checkForBackgroundUpdates(
+    isNotify
+  ) {
+    if (!this.disabled && AppConstants.NIGHTLY_BUILD) {
+      // Scalar ID: update.suppress_prompts
+      AUSTLMY.pingSuppressPrompts();
+    }
+    if (this.disabled || this.manualUpdateOnly) {
+      // Return immediately if we are disabled by policy. Otherwise, just the
+      // telemetry we try to collect below can potentially trigger a restart
+      // prompt if the update directory isn't writable. And we shouldn't be
+      // telling the user about update failures if update is disabled.
+      // See Bug 1599590.
+      // Note that we exit unconditionally here if we are only doing manual
+      // update checks, because manual update checking uses a completely
+      // different code path (AppUpdater.jsm creates its own nsIUpdateChecker),
+      // bypassing this function completely.
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_DISABLED_BY_POLICY);
+      return false;
+    }
+
+    this._isNotify = isNotify;
+
+    // Histogram IDs:
+    // UPDATE_PING_COUNT_EXTERNAL
+    // UPDATE_PING_COUNT_NOTIFY
+    // UPDATE_PING_COUNT_SUBSEQUENT
+    AUSTLMY.pingGeneric("UPDATE_PING_COUNT_" + this._pingSuffix, true, false);
+
+    // Histogram IDs:
+    // UPDATE_UNABLE_TO_APPLY_EXTERNAL
+    // UPDATE_UNABLE_TO_APPLY_NOTIFY
+    // UPDATE_UNABLE_TO_APPLY_SUBSEQUENT
+    AUSTLMY.pingGeneric(
+      "UPDATE_UNABLE_TO_APPLY_" + this._pingSuffix,
+      getCanApplyUpdates(),
+      true
+    );
+    // Histogram IDs:
+    // UPDATE_CANNOT_STAGE_EXTERNAL
+    // UPDATE_CANNOT_STAGE_NOTIFY
+    // UPDATE_CANNOT_STAGE_SUBSEQUENT
+    AUSTLMY.pingGeneric(
+      "UPDATE_CANNOT_STAGE_" + this._pingSuffix,
+      getCanStageUpdates(),
+      true
+    );
+    if (AppConstants.platform == "win") {
+      // Histogram IDs:
+      // UPDATE_CAN_USE_BITS_EXTERNAL
+      // UPDATE_CAN_USE_BITS_NOTIFY
+      // UPDATE_CAN_USE_BITS_SUBSEQUENT
+      AUSTLMY.pingGeneric(
+        "UPDATE_CAN_USE_BITS_" + this._pingSuffix,
+        getCanUseBits()
+      );
+    }
+    // Histogram IDs:
+    // UPDATE_INVALID_LASTUPDATETIME_EXTERNAL
+    // UPDATE_INVALID_LASTUPDATETIME_NOTIFY
+    // UPDATE_INVALID_LASTUPDATETIME_SUBSEQUENT
+    // UPDATE_LAST_NOTIFY_INTERVAL_DAYS_EXTERNAL
+    // UPDATE_LAST_NOTIFY_INTERVAL_DAYS_NOTIFY
+    // UPDATE_LAST_NOTIFY_INTERVAL_DAYS_SUBSEQUENT
+    AUSTLMY.pingLastUpdateTime(this._pingSuffix);
+    // Histogram IDs:
+    // UPDATE_NOT_PREF_UPDATE_AUTO_EXTERNAL
+    // UPDATE_NOT_PREF_UPDATE_AUTO_NOTIFY
+    // UPDATE_NOT_PREF_UPDATE_AUTO_SUBSEQUENT
+    lazy.UpdateUtils.getAppUpdateAutoEnabled().then(enabled => {
+      AUSTLMY.pingGeneric(
+        "UPDATE_NOT_PREF_UPDATE_AUTO_" + this._pingSuffix,
+        enabled,
+        true
+      );
+    });
+    // Histogram IDs:
+    // UPDATE_NOT_PREF_UPDATE_STAGING_ENABLED_EXTERNAL
+    // UPDATE_NOT_PREF_UPDATE_STAGING_ENABLED_NOTIFY
+    // UPDATE_NOT_PREF_UPDATE_STAGING_ENABLED_SUBSEQUENT
+    AUSTLMY.pingBoolPref(
+      "UPDATE_NOT_PREF_UPDATE_STAGING_ENABLED_" + this._pingSuffix,
+      PREF_APP_UPDATE_STAGING_ENABLED,
+      true,
+      true
+    );
+    if (AppConstants.platform == "win" || AppConstants.platform == "macosx") {
+      // Histogram IDs:
+      // UPDATE_PREF_UPDATE_CANCELATIONS_EXTERNAL
+      // UPDATE_PREF_UPDATE_CANCELATIONS_NOTIFY
+      // UPDATE_PREF_UPDATE_CANCELATIONS_SUBSEQUENT
+      AUSTLMY.pingIntPref(
+        "UPDATE_PREF_UPDATE_CANCELATIONS_" + this._pingSuffix,
+        PREF_APP_UPDATE_CANCELATIONS,
+        0,
+        0
+      );
+    }
+    if (AppConstants.MOZ_MAINTENANCE_SERVICE) {
+      // Histogram IDs:
+      // UPDATE_NOT_PREF_UPDATE_SERVICE_ENABLED_EXTERNAL
+      // UPDATE_NOT_PREF_UPDATE_SERVICE_ENABLED_NOTIFY
+      // UPDATE_NOT_PREF_UPDATE_SERVICE_ENABLED_SUBSEQUENT
+      AUSTLMY.pingBoolPref(
+        "UPDATE_NOT_PREF_UPDATE_SERVICE_ENABLED_" + this._pingSuffix,
+        PREF_APP_UPDATE_SERVICE_ENABLED,
+        true
+      );
+      // Histogram IDs:
+      // UPDATE_PREF_SERVICE_ERRORS_EXTERNAL
+      // UPDATE_PREF_SERVICE_ERRORS_NOTIFY
+      // UPDATE_PREF_SERVICE_ERRORS_SUBSEQUENT
+      AUSTLMY.pingIntPref(
+        "UPDATE_PREF_SERVICE_ERRORS_" + this._pingSuffix,
+        PREF_APP_UPDATE_SERVICE_ERRORS,
+        0,
+        0
+      );
+      if (AppConstants.platform == "win") {
+        // Histogram IDs:
+        // UPDATE_SERVICE_INSTALLED_EXTERNAL
+        // UPDATE_SERVICE_INSTALLED_NOTIFY
+        // UPDATE_SERVICE_INSTALLED_SUBSEQUENT
+        // UPDATE_SERVICE_MANUALLY_UNINSTALLED_EXTERNAL
+        // UPDATE_SERVICE_MANUALLY_UNINSTALLED_NOTIFY
+        // UPDATE_SERVICE_MANUALLY_UNINSTALLED_SUBSEQUENT
+        AUSTLMY.pingServiceInstallStatus(
+          this._pingSuffix,
+          isServiceInstalled()
+        );
+      }
+    }
+
+    // If a download is in progress or the patch has been staged do nothing.
+    if (this.isDownloading) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_IS_DOWNLOADING);
+      return false;
+    }
+
+    // Once we have downloaded a complete update, do not download further
+    // updates until the complete update is installed. This is important,
+    // because if we fall back from a partial update to a complete update,
+    // it might be because of changes to the patch directory (which would cause
+    // a failure to apply any partial MAR). So we really don't want to replace
+    // a downloaded complete update with a downloaded partial update. And we
+    // do not currently download complete updates if there is already a
+    // readyUpdate available.
+    if (
+      lazy.UM.readyUpdate &&
+      lazy.UM.readyUpdate.selectedPatch &&
+      lazy.UM.readyUpdate.selectedPatch.type == "complete"
+    ) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_IS_DOWNLOADED);
+      return false;
+    }
+
+    // If we start downloading an update while the readyUpdate is staging, we
+    // run the risk of eventually wanting to overwrite readyUpdate with the
+    // downloadingUpdate while the readyUpdate is still staging. Then we would
+    // have to have a weird intermediate state where the downloadingUpdate has
+    // finished downloading, but can't be moved yet. It's simpler to just not
+    // start a new update if the old one is still staging.
+    if (this.currentState == Ci.nsIApplicationUpdateService.STATE_STAGING) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_IS_DOWNLOADED);
+      return false;
+    }
+
+    // Asynchronously kick off update checking
+    (async () => {
+      let validUpdateURL = true;
+      try {
+        await lazy.CheckSvc.getUpdateURL(lazy.CheckSvc.BACKGROUND_CHECK);
+      } catch (e) {
+        validUpdateURL = false;
+      }
+
+      // The following checks are done here so they can be differentiated from
+      // foreground checks.
+      if (!lazy.UpdateUtils.OSVersion) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_NO_OS_VERSION);
+      } else if (!lazy.UpdateUtils.ABI) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_NO_OS_ABI);
+      } else if (!validUpdateURL) {
+        AUSTLMY.pingCheckCode(
+          this._pingSuffix,
+          AUSTLMY.CHK_INVALID_DEFAULT_URL
+        );
+      } else if (!hasUpdateMutex()) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_NO_MUTEX);
+      } else if (isOtherInstanceRunning()) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_OTHER_INSTANCE);
+      } else if (!this.canCheckForUpdates) {
+        AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_UNABLE_TO_CHECK);
+      }
+
+      let check = lazy.CheckSvc.checkForUpdates(lazy.CheckSvc.BACKGROUND_CHECK);
+      await this.onCheckComplete(await check.result);
+    })();
+
+    return true;
+  },
+
+  /**
+   * Determine the update from the specified updates that should be offered.
+   * If both valid major and minor updates are available the minor update will
+   * be offered.
+   * @param   updates
+   *          An array of available nsIUpdate items
+   * @return  The nsIUpdate to offer.
+   */
+  selectUpdate: function AUS_selectUpdate(updates) {
+    if (!updates.length) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_NO_UPDATE_FOUND);
+      return null;
+    }
+
+    // The ping for unsupported is sent after the call to showPrompt.
+    if (updates.length == 1 && updates[0].unsupported) {
+      return updates[0];
+    }
+
+    // Choose the newest of the available minor and major updates.
+    var majorUpdate = null;
+    var minorUpdate = null;
+    var vc = Services.vc;
+    let lastCheckCode = AUSTLMY.CHK_NO_COMPAT_UPDATE_FOUND;
+
+    updates.forEach(function (aUpdate) {
+      // Ignore updates for older versions of the application and updates for
+      // the same version of the application with the same build ID.
+      if (updateIsAtLeastAsOldAsCurrentVersion(aUpdate)) {
+        LOG(
+          "UpdateService:selectUpdate - skipping update because the " +
+            "update's application version is not greater than the current " +
+            "application version"
+        );
+        lastCheckCode = AUSTLMY.CHK_UPDATE_PREVIOUS_VERSION;
+        return;
+      }
+
+      if (updateIsAtLeastAsOldAsReadyUpdate(aUpdate)) {
+        LOG(
+          "UpdateService:selectUpdate - skipping update because the " +
+            "update's application version is not greater than that of the " +
+            "currently downloaded update"
+        );
+        lastCheckCode = AUSTLMY.CHK_UPDATE_PREVIOUS_VERSION;
+        return;
+      }
+
+      if (lazy.UM.readyUpdate && !getPatchOfType(aUpdate, "partial")) {
+        LOG(
+          "UpdateService:selectUpdate - skipping update because no partial " +
+            "patch is available and an update has already been downloaded."
+        );
+        lastCheckCode = AUSTLMY.CHK_NO_PARTIAL_PATCH;
+        return;
+      }
+
+      switch (aUpdate.type) {
+        case "major":
+          if (!majorUpdate) {
+            majorUpdate = aUpdate;
+          } else if (
+            vc.compare(majorUpdate.appVersion, aUpdate.appVersion) <= 0
+          ) {
+            majorUpdate = aUpdate;
+          }
+          break;
+        case "minor":
+          if (!minorUpdate) {
+            minorUpdate = aUpdate;
+          } else if (
+            vc.compare(minorUpdate.appVersion, aUpdate.appVersion) <= 0
+          ) {
+            minorUpdate = aUpdate;
+          }
+          break;
+        default:
+          LOG(
+            "UpdateService:selectUpdate - skipping unknown update type: " +
+              aUpdate.type
+          );
+          lastCheckCode = AUSTLMY.CHK_UPDATE_INVALID_TYPE;
+          break;
+      }
+    });
+
+    let update = minorUpdate || majorUpdate;
+    if (AppConstants.platform == "macosx" && update) {
+      if (getElevationRequired()) {
+        let installAttemptVersion = Services.prefs.getCharPref(
+          PREF_APP_UPDATE_ELEVATE_VERSION,
+          null
+        );
+        if (vc.compare(installAttemptVersion, update.appVersion) != 0) {
+          Services.prefs.setCharPref(
+            PREF_APP_UPDATE_ELEVATE_VERSION,
+            update.appVersion
+          );
+          if (
+            Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CANCELATIONS_OSX)
+          ) {
+            Services.prefs.clearUserPref(PREF_APP_UPDATE_CANCELATIONS_OSX);
+          }
+          if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_ELEVATE_NEVER)) {
+            Services.prefs.clearUserPref(PREF_APP_UPDATE_ELEVATE_NEVER);
+          }
+        } else {
+          let numCancels = Services.prefs.getIntPref(
+            PREF_APP_UPDATE_CANCELATIONS_OSX,
+            0
+          );
+          let rejectedVersion = Services.prefs.getCharPref(
+            PREF_APP_UPDATE_ELEVATE_NEVER,
+            ""
+          );
+          let maxCancels = Services.prefs.getIntPref(
+            PREF_APP_UPDATE_CANCELATIONS_OSX_MAX,
+            DEFAULT_CANCELATIONS_OSX_MAX
+          );
+          if (numCancels >= maxCancels) {
+            LOG(
+              "UpdateService:selectUpdate - the user requires elevation to " +
+                "install this update, but the user has exceeded the max " +
+                "number of elevation attempts."
+            );
+            update.elevationFailure = true;
+            AUSTLMY.pingCheckCode(
+              this._pingSuffix,
+              AUSTLMY.CHK_ELEVATION_DISABLED_FOR_VERSION
+            );
+          } else if (vc.compare(rejectedVersion, update.appVersion) == 0) {
+            LOG(
+              "UpdateService:selectUpdate - the user requires elevation to " +
+                "install this update, but elevation is disabled for this " +
+                "version."
+            );
+            update.elevationFailure = true;
+            AUSTLMY.pingCheckCode(
+              this._pingSuffix,
+              AUSTLMY.CHK_ELEVATION_OPTOUT_FOR_VERSION
+            );
+          } else {
+            LOG(
+              "UpdateService:selectUpdate - the user requires elevation to " +
+                "install the update."
+            );
+          }
+        }
+      } else {
+        // Clear elevation-related prefs since they no longer apply (the user
+        // may have gained write access to the Firefox directory or an update
+        // was executed with a different profile).
+        if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_ELEVATE_VERSION)) {
+          Services.prefs.clearUserPref(PREF_APP_UPDATE_ELEVATE_VERSION);
+        }
+        if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CANCELATIONS_OSX)) {
+          Services.prefs.clearUserPref(PREF_APP_UPDATE_CANCELATIONS_OSX);
+        }
+        if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_ELEVATE_NEVER)) {
+          Services.prefs.clearUserPref(PREF_APP_UPDATE_ELEVATE_NEVER);
+        }
+      }
+    } else if (!update) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, lastCheckCode);
+    }
+
+    return update;
+  },
+
+  /**
+   * Determine which of the specified updates should be installed and begin the
+   * download/installation process or notify the user about the update.
+   * @param   updates
+   *          An array of available updates
+   */
+  _selectAndInstallUpdate: async function AUS__selectAndInstallUpdate(updates) {
+    // Return early if there's an active update. The user is already aware and
+    // is downloading or performed some user action to prevent notification.
+    if (lazy.UM.downloadingUpdate) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_HAS_ACTIVEUPDATE);
+      return;
+    }
+
+    if (this.disabled) {
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_DISABLED_BY_POLICY);
+      LOG(
+        "UpdateService:_selectAndInstallUpdate - not prompting because " +
+          "update is disabled"
+      );
+      return;
+    }
+
+    var update = this.selectUpdate(updates);
+    if (!update || update.elevationFailure) {
+      return;
+    }
+
+    if (update.unsupported) {
+      LOG(
+        "UpdateService:_selectAndInstallUpdate - update not supported for " +
+          "this system. Notifying observers. topic: update-available, " +
+          "status: unsupported"
+      );
+      Services.obs.notifyObservers(update, "update-available", "unsupported");
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_UNSUPPORTED);
+      return;
+    }
+
+    if (!getCanApplyUpdates()) {
+      LOG(
+        "UpdateService:_selectAndInstallUpdate - the user is unable to " +
+          "apply updates... prompting. Notifying observers. " +
+          "topic: update-available, status: cant-apply"
+      );
+      Services.obs.notifyObservers(null, "update-available", "cant-apply");
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_UNABLE_TO_APPLY);
+      return;
+    }
+
+    /**
+     * From this point on there are two possible outcomes:
+     * 1. download and install the update automatically
+     * 2. notify the user about the availability of an update
+     *
+     * Notes:
+     * a) if the app.update.auto preference is false then automatic download and
+     *    install is disabled and the user will be notified.
+     *
+     * If the update when it is first read does not have an appVersion attribute
+     * the following deprecated behavior will occur:
+     * Update Type   Outcome
+     * Major         Notify
+     * Minor         Auto Install
+     */
+    let updateAuto = await lazy.UpdateUtils.getAppUpdateAutoEnabled();
+    if (!updateAuto) {
+      LOG(
+        "UpdateService:_selectAndInstallUpdate - prompting because silent " +
+          "install is disabled. Notifying observers. topic: update-available, " +
+          "status: show-prompt"
+      );
+      AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_SHOWPROMPT_PREF);
+      Services.obs.notifyObservers(update, "update-available", "show-prompt");
+      return;
+    }
+
+    LOG("UpdateService:_selectAndInstallUpdate - download the update");
+    let success = await this.downloadUpdate(update);
+    if (!success && !this.isDownloading) {
+      LOG(
+        "UpdateService:_selectAndInstallUpdate - Failed to start downloading " +
+          "update. Cleaning up downloading update."
+      );
+      cleanupDownloadingUpdate();
+    }
+    AUSTLMY.pingCheckCode(this._pingSuffix, AUSTLMY.CHK_DOWNLOAD_UPDATE);
+  },
+
+  get disabledForTesting() {
+    return (
+      (Cu.isInAutomation ||
+        lazy.Marionette.running ||
+        lazy.RemoteAgent.running) &&
+      Services.prefs.getBoolPref(PREF_APP_UPDATE_DISABLEDFORTESTING, false)
+    );
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get disabled() {
+    return (
+      (Services.policies && !Services.policies.isAllowed("appUpdate")) ||
+      this.disabledForTesting ||
+      Services.sysinfo.getProperty("isPackagedApp")
+    );
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get manualUpdateOnly() {
+    return (
+      Services.policies && !Services.policies.isAllowed("autoAppUpdateChecking")
+    );
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canUsuallyCheckForUpdates() {
+    if (this.disabled) {
+      LOG(
+        "UpdateService.canUsuallyCheckForUpdates - unable to automatically check " +
+          "for updates, the option has been disabled by the administrator."
+      );
+      return false;
+    }
+
+    // If we don't know the binary platform we're updating, we can't update.
+    if (!lazy.UpdateUtils.ABI) {
+      LOG(
+        "UpdateService.canUsuallyCheckForUpdates - unable to check for updates, " +
+          "unknown ABI"
+      );
+      return false;
+    }
+
+    // If we don't know the OS version we're updating, we can't update.
+    if (!lazy.UpdateUtils.OSVersion) {
+      LOG(
+        "UpdateService.canUsuallyCheckForUpdates - unable to check for updates, " +
+          "unknown OS version"
+      );
+      return false;
+    }
+
+    LOG("UpdateService.canUsuallyCheckForUpdates - able to check for updates");
+    return true;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canCheckForUpdates() {
+    if (!this.canUsuallyCheckForUpdates) {
+      return false;
+    }
+
+    if (!hasUpdateMutex()) {
+      LOG(
+        "UpdateService.canCheckForUpdates - unable to check for updates, " +
+          "unable to acquire update mutex"
+      );
+      return false;
+    }
+
+    if (isOtherInstanceRunning()) {
+      // This doesn't block update checks, but we will have to wait until either
+      // the other instance is gone or we time out waiting for it.
+      LOG(
+        "UpdateService.canCheckForUpdates - another instance is holding the " +
+          "lock, will need to wait for it prior to checking for updates"
+      );
+    }
+
+    LOG("UpdateService.canCheckForUpdates - able to check for updates");
+    return true;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get elevationRequired() {
+    return getElevationRequired();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canUsuallyApplyUpdates() {
+    return getCanApplyUpdates();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canApplyUpdates() {
+    return (
+      this.canUsuallyApplyUpdates &&
+      hasUpdateMutex() &&
+      !isOtherInstanceRunning()
+    );
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canUsuallyStageUpdates() {
+    return getCanStageUpdates(false);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canStageUpdates() {
+    return getCanStageUpdates();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canUsuallyUseBits() {
+    return getCanUseBits(false) == "CanUseBits";
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get canUseBits() {
+    return getCanUseBits() == "CanUseBits";
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get isOtherInstanceHandlingUpdates() {
+    return !hasUpdateMutex() || isOtherInstanceRunning();
+  },
+
+  /**
+   * A set of download listeners to be notified by this._downloader when it
+   * receives nsIRequestObserver or nsIProgressEventSink method calls.
+   *
+   * These are stored on the UpdateService rather than on the Downloader,
+   * because they ought to persist across multiple Downloader instances.
+   */
+  _downloadListeners: new Set(),
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  addDownloadListener: function AUS_addDownloadListener(listener) {
+    let oldSize = this._downloadListeners.size;
+    this._downloadListeners.add(listener);
+
+    if (this._downloadListeners.size == oldSize) {
+      LOG(
+        "UpdateService:addDownloadListener - Warning: Didn't add duplicate " +
+          "listener"
+      );
+      return;
+    }
+
+    if (this._downloader) {
+      this._downloader.onDownloadListenerAdded();
+    }
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  removeDownloadListener: function AUS_removeDownloadListener(listener) {
+    let elementRemoved = this._downloadListeners.delete(listener);
+
+    if (!elementRemoved) {
+      LOG(
+        "UpdateService:removeDownloadListener - Warning: Didn't remove " +
+          "non-existent listener"
+      );
+      return;
+    }
+
+    if (this._downloader) {
+      this._downloader.onDownloadListenerRemoved();
+    }
+  },
+
+  /**
+   * Returns a boolean indicating whether there are any download listeners
+   */
+  get hasDownloadListeners() {
+    return !!this._downloadListeners.length;
+  },
+
+  /*
+   * Calls the provided function once with each download listener that is
+   * currently registered.
+   */
+  forEachDownloadListener: function AUS_forEachDownloadListener(fn) {
+    // Make a shallow copy in case listeners remove themselves.
+    let listeners = new Set(this._downloadListeners);
+    listeners.forEach(fn);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  downloadUpdate: async function AUS_downloadUpdate(update) {
+    if (!update) {
+      throw Components.Exception("", Cr.NS_ERROR_NULL_POINTER);
+    }
+
+    // Don't download the update if the update's version is less than the
+    // current application's version or the update's version is the same as the
+    // application's version and the build ID is the same as the application's
+    // build ID. If we already have an update ready, we want to apply those
+    // same checks against the version of the ready update, so that we don't
+    // download an update that isn't newer than the one we already have.
+    if (updateIsAtLeastAsOldAsCurrentVersion(update)) {
+      LOG(
+        "UpdateService:downloadUpdate - Skipping download of update since " +
+          "it is for an earlier or same application version and build ID.\n" +
+          "current application version: " +
+          Services.appinfo.version +
+          "\n" +
+          "update application version : " +
+          update.appVersion +
+          "\n" +
+          "current build ID: " +
+          Services.appinfo.appBuildID +
+          "\n" +
+          "update build ID : " +
+          update.buildID
+      );
+      return false;
+    }
+    if (updateIsAtLeastAsOldAsReadyUpdate(update)) {
+      LOG(
+        "UpdateService:downloadUpdate - not downloading update because the " +
+          "update that's already been downloaded is the same version or " +
+          "newer.\n" +
+          "currently downloaded update application version: " +
+          lazy.UM.readyUpdate.appVersion +
+          "\n" +
+          "available update application version : " +
+          update.appVersion +
+          "\n" +
+          "currently downloaded update build ID: " +
+          lazy.UM.readyUpdate.buildID +
+          "\n" +
+          "available update build ID : " +
+          update.buildID
+      );
+      return false;
+    }
+
+    // If a download request is in progress vs. a download ready to resume
+    if (this.isDownloading) {
+      if (update.isCompleteUpdate == this._downloader.isCompleteUpdate) {
+        LOG(
+          "UpdateService:downloadUpdate - no support for downloading more " +
+            "than one update at a time"
+        );
+        return true;
+      }
+      this._downloader.cancel();
+    }
+    this._downloader = new Downloader(this);
+    return this._downloader.downloadUpdate(update);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  stopDownload: async function AUS_stopDownload() {
+    if (this.isDownloading) {
+      await this._downloader.cancel();
+    } else if (this._retryTimer) {
+      // Download status is still considered as 'downloading' during retry.
+      // We need to cancel both retry and download at this stage.
+      this._retryTimer.cancel();
+      this._retryTimer = null;
+      if (this._downloader) {
+        await this._downloader.cancel();
+      }
+    }
+    if (this._downloader) {
+      await this._downloader.cleanup();
+    }
+    this._downloader = null;
+  },
+
+  /**
+   * Note that this is different from checking if `currentState` is
+   * `STATE_DOWNLOADING` because if we are downloading a second update, this
+   * will be `true` while `currentState` will be `STATE_PENDING`.
+   */
+  get isDownloading() {
+    return this._downloader && this._downloader.isBusy;
+  },
+
+  _logStatus: function AUS__logStatus() {
+    if (!lazy.gLogEnabled) {
+      return;
+    }
+    if (this.disabled) {
+      LOG("Current UpdateService status: disabled");
+      // Return early if UpdateService is disabled by policy. Otherwise some of
+      // the getters we call to display status information may discover that the
+      // update directory is not writable, which automatically results in the
+      // permissions being fixed. Which we shouldn't really be doing if update
+      // is disabled by policy.
+      return;
+    }
+    LOG("Logging current UpdateService status:");
+    // These getters print their own logging
+    this.canCheckForUpdates;
+    this.canApplyUpdates;
+    this.canStageUpdates;
+    LOG("Elevation required: " + this.elevationRequired);
+    LOG(
+      "Other instance of the application currently running: " +
+        this.isOtherInstanceHandlingUpdates
+    );
+    LOG("Downloading: " + !!this.isDownloading);
+    if (this._downloader && this._downloader.isBusy) {
+      LOG("Downloading complete update: " + this._downloader.isCompleteUpdate);
+      LOG("Downloader using BITS: " + this._downloader.usingBits);
+      if (this._downloader._patch) {
+        // This will print its own logging
+        this._downloader._canUseBits(this._downloader._patch);
+
+        // Downloader calls QueryInterface(Ci.nsIWritablePropertyBag) on
+        // its _patch member as soon as it is assigned, so no need to do so
+        // again here.
+        let bitsResult = this._downloader._patch.getProperty("bitsResult");
+        if (bitsResult != null) {
+          LOG("Patch BITS result: " + bitsResult);
+        }
+        let internalResult =
+          this._downloader._patch.getProperty("internalResult");
+        if (internalResult != null) {
+          LOG("Patch nsIIncrementalDownload result: " + internalResult);
+        }
+      }
+    }
+    LOG("End of UpdateService status");
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get onlyDownloadUpdatesThisSession() {
+    return gOnlyDownloadUpdatesThisSession;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  set onlyDownloadUpdatesThisSession(newValue) {
+    gOnlyDownloadUpdatesThisSession = newValue;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  getStateName(state) {
+    switch (state) {
+      case Ci.nsIApplicationUpdateService.STATE_IDLE:
+        return "STATE_IDLE";
+      case Ci.nsIApplicationUpdateService.STATE_DOWNLOADING:
+        return "STATE_DOWNLOADING";
+      case Ci.nsIApplicationUpdateService.STATE_STAGING:
+        return "STATE_STAGING";
+      case Ci.nsIApplicationUpdateService.STATE_PENDING:
+        return "STATE_PENDING";
+      case Ci.nsIApplicationUpdateService.STATE_SWAP:
+        return "STATE_SWAP";
+    }
+    return `[unknown update state: ${state}]`;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get currentState() {
+    return gUpdateState;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get stateTransition() {
+    return gStateTransitionPromise.promise;
+  },
+
+  classID: UPDATESERVICE_CID,
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIApplicationUpdateService",
+    "nsITimerCallback",
+    "nsIObserver",
+  ]),
+};
+
+/**
+ * A service to manage active and past updates.
+ * @constructor
+ */
+export function UpdateManager() {
+  // Load the active-update.xml file to see if there is an active update.
+  let activeUpdates = this._loadXMLFileIntoArray(FILE_ACTIVE_UPDATE_XML);
+  if (activeUpdates.length) {
+    // Set the active update directly on the var used to cache the value.
+    this._readyUpdate = activeUpdates[0];
+    if (activeUpdates.length >= 2) {
+      this._downloadingUpdate = activeUpdates[1];
+    }
+    let status = readStatusFile(getReadyUpdateDir());
+    LOG(`UpdateManager:UpdateManager - status = "${status}"`);
+    // This check is performed here since UpdateService:_postUpdateProcessing
+    // won't be called when there isn't an update.status file.
+    if (status == STATE_NONE) {
+      // Under some edgecases such as Windows system restore the
+      // active-update.xml will contain a pending update without the status
+      // file. To recover from this situation clean the updates dir and move
+      // the active update to the update history.
+      LOG(
+        "UpdateManager:UpdateManager - Found update data with no status " +
+          "file. Cleaning up..."
+      );
+      this._readyUpdate.state = STATE_FAILED;
+      this._readyUpdate.errorCode = ERR_UPDATE_STATE_NONE;
+      this._readyUpdate.statusText =
+        lazy.gUpdateBundle.GetStringFromName("statusFailed");
+      let newStatus = STATE_FAILED + ": " + ERR_UPDATE_STATE_NONE;
+      pingStateAndStatusCodes(this._readyUpdate, true, newStatus);
+      this.addUpdateToHistory(this._readyUpdate);
+      this._readyUpdate = null;
+      this.saveUpdates();
+      cleanUpReadyUpdateDir();
+      cleanUpDownloadingUpdateDir();
+    } else if (status == STATE_DOWNLOADING) {
+      // The first update we read out of activeUpdates may not be the ready
+      // update, it may be the downloading update.
+      if (this._downloadingUpdate) {
+        // If the first update we read is a downloading update, it's
+        // unexpected to have read another active update. That would seem to
+        // indicate that we were downloading two updates at once, which we don't
+        // do.
+        LOG(
+          "UpdateManager:UpdateManager - Warning: Found and discarded a " +
+            "second downloading update."
+        );
+      }
+      this._downloadingUpdate = this._readyUpdate;
+      this._readyUpdate = null;
+    }
+  }
+
+  LOG(
+    "UpdateManager:UpdateManager - Initialized downloadingUpdate to " +
+      this._downloadingUpdate
+  );
+  if (this._downloadingUpdate) {
+    LOG(
+      "UpdateManager:UpdateManager - Initialized downloadingUpdate state to " +
+        this._downloadingUpdate.state
+    );
+  }
+  LOG(
+    "UpdateManager:UpdateManager - Initialized readyUpdate to " +
+      this._readyUpdate
+  );
+  if (this._readyUpdate) {
+    LOG(
+      "UpdateManager:UpdateManager - Initialized readyUpdate state to " +
+        this._readyUpdate.state
+    );
+  }
+}
+
+UpdateManager.prototype = {
+  /**
+   * The nsIUpdate object for the update that has been downloaded.
+   */
+  _readyUpdate: null,
+
+  /**
+   * The nsIUpdate object for the update currently being downloaded.
+   */
+  _downloadingUpdate: null,
+
+  /**
+   * Whether the update history stored in _updates has changed since it was
+   * loaded.
+   */
+  _updatesDirty: false,
+
+  /**
+   * See nsIObserver.idl
+   */
+  observe: function UM_observe(subject, topic, data) {
+    // Hack to be able to run and cleanup tests by reloading the update data.
+    if (topic == "um-reload-update-data") {
+      if (!Cu.isInAutomation) {
+        return;
+      }
+      LOG("UpdateManager:observe - Reloading update data.");
+      if (this._updatesXMLSaver) {
+        this._updatesXMLSaver.disarm();
+      }
+
+      let updates = [];
+      this._updatesDirty = true;
+      this._readyUpdate = null;
+      this._downloadingUpdate = null;
+      transitionState(Ci.nsIApplicationUpdateService.STATE_IDLE);
+      if (data != "skip-files") {
+        let activeUpdates = this._loadXMLFileIntoArray(FILE_ACTIVE_UPDATE_XML);
+        if (activeUpdates.length) {
+          this._readyUpdate = activeUpdates[0];
+          if (activeUpdates.length >= 2) {
+            this._downloadingUpdate = activeUpdates[1];
+          }
+          let status = readStatusFile(getReadyUpdateDir());
+          LOG(`UpdateManager:observe - Got status = ${status}`);
+          if (status == STATE_DOWNLOADING) {
+            this._downloadingUpdate = this._readyUpdate;
+            this._readyUpdate = null;
+            transitionState(Ci.nsIApplicationUpdateService.STATE_DOWNLOADING);
+          } else if (
+            [
+              STATE_PENDING,
+              STATE_PENDING_SERVICE,
+              STATE_PENDING_ELEVATE,
+              STATE_APPLIED,
+              STATE_APPLIED_SERVICE,
+            ].includes(status)
+          ) {
+            transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+          }
+        }
+        updates = this._loadXMLFileIntoArray(FILE_UPDATES_XML);
+      }
+      this._updatesCache = updates;
+
+      LOG(
+        "UpdateManager:observe - Reloaded downloadingUpdate as " +
+          this._downloadingUpdate
+      );
+      if (this._downloadingUpdate) {
+        LOG(
+          "UpdateManager:observe - Reloaded downloadingUpdate state as " +
+            this._downloadingUpdate.state
+        );
+      }
+      LOG(
+        "UpdateManager:observe - Reloaded readyUpdate as " + this._readyUpdate
+      );
+      if (this._readyUpdate) {
+        LOG(
+          "UpdateManager:observe - Reloaded readyUpdate state as " +
+            this._readyUpdate.state
+        );
+      }
+    }
+  },
+
+  /**
+   * Loads an updates.xml formatted file into an array of nsIUpdate items.
+   * @param   fileName
+   *          The file name in the updates directory to load.
+   * @return  The array of nsIUpdate items held in the file.
+   */
+  _loadXMLFileIntoArray: function UM__loadXMLFileIntoArray(fileName) {
+    let updates = [];
+    let file = getUpdateFile([fileName]);
+    if (!file.exists()) {
+      LOG(
+        "UpdateManager:_loadXMLFileIntoArray - XML file does not exist. " +
+          "path: " +
+          file.path
+      );
+      return updates;
+    }
+
+    // Open the active-update.xml file with both read and write access so
+    // opening it will fail if it isn't possible to also write to the file. When
+    // opening it fails it means that it isn't possible to update and the code
+    // below will return early without loading the active-update.xml. This will
+    // also make it so notifications to update manually will still be shown.
+    let mode =
+      fileName == FILE_ACTIVE_UPDATE_XML
+        ? FileUtils.MODE_RDWR
+        : FileUtils.MODE_RDONLY;
+    let fileStream = Cc[
+      "@mozilla.org/network/file-input-stream;1"
+    ].createInstance(Ci.nsIFileInputStream);
+    try {
+      fileStream.init(file, mode, FileUtils.PERMS_FILE, 0);
+    } catch (e) {
+      LOG(
+        "UpdateManager:_loadXMLFileIntoArray - error initializing file " +
+          "stream. Exception: " +
+          e
+      );
+      return updates;
+    }
+    try {
+      var parser = new DOMParser();
+      var doc = parser.parseFromStream(
+        fileStream,
+        "UTF-8",
+        fileStream.available(),
+        "text/xml"
+      );
+
+      var updateCount = doc.documentElement.childNodes.length;
+      for (var i = 0; i < updateCount; ++i) {
+        var updateElement = doc.documentElement.childNodes.item(i);
+        if (
+          updateElement.nodeType != updateElement.ELEMENT_NODE ||
+          updateElement.localName != "update"
+        ) {
+          continue;
+        }
+
+        let update;
+        try {
+          update = new Update(updateElement);
+        } catch (e) {
+          LOG("UpdateManager:_loadXMLFileIntoArray - invalid update");
+          continue;
+        }
+        updates.push(update);
+      }
+    } catch (ex) {
+      LOG(
+        "UpdateManager:_loadXMLFileIntoArray - error constructing update " +
+          "list. Exception: " +
+          ex
+      );
+    }
+    fileStream.close();
+    if (!updates.length) {
+      LOG(
+        "UpdateManager:_loadXMLFileIntoArray - update xml file " +
+          fileName +
+          " exists but doesn't contain any updates"
+      );
+      // The file exists but doesn't contain any updates so remove it.
+      try {
+        file.remove(false);
+      } catch (e) {
+        LOG(
+          "UpdateManager:_loadXMLFileIntoArray - error removing " +
+            fileName +
+            " file. Exception: " +
+            e
+        );
+      }
+    }
+    return updates;
+  },
+
+  /**
+   * Loads the update history from the updates.xml file into a cache.
+   */
+  _getUpdates() {
+    if (!this._updatesCache) {
+      this._updatesCache = this._loadXMLFileIntoArray(FILE_UPDATES_XML);
+    }
+    return this._updatesCache;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  getUpdateAt: function UM_getUpdateAt(aIndex) {
+    return this._getUpdates()[aIndex];
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  getUpdateCount() {
+    return this._getUpdates().length;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get readyUpdate() {
+    return this._readyUpdate;
+  },
+  set readyUpdate(aUpdate) {
+    this._readyUpdate = aUpdate;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  get downloadingUpdate() {
+    return this._downloadingUpdate;
+  },
+  set downloadingUpdate(aUpdate) {
+    this._downloadingUpdate = aUpdate;
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  addUpdateToHistory(aUpdate) {
+    this._updatesDirty = true;
+    let updates = this._getUpdates();
+    updates.unshift(aUpdate);
+    // Limit the update history to 10 updates.
+    updates.splice(10);
+  },
+
+  /**
+   * Serializes an array of updates to an XML file or removes the file if the
+   * array length is 0.
+   * @param   updates
+   *          An array of nsIUpdate objects
+   * @param   fileName
+   *          The file name in the updates directory to write to.
+   * @return  true on success, false on error
+   */
+  _writeUpdatesToXMLFile: async function UM__writeUpdatesToXMLFile(
+    updates,
+    fileName
+  ) {
+    let file;
+    try {
+      file = getUpdateFile([fileName]);
+    } catch (e) {
+      LOG(
+        "UpdateManager:_writeUpdatesToXMLFile - Unable to get XML file - " +
+          "Exception: " +
+          e
+      );
+      return false;
+    }
+    if (!updates.length) {
+      LOG(
+        "UpdateManager:_writeUpdatesToXMLFile - no updates to write. " +
+          "removing file: " +
+          file.path
+      );
+      try {
+        await IOUtils.remove(file.path);
+      } catch (e) {
+        LOG(
+          "UpdateManager:_writeUpdatesToXMLFile - Delete file exception: " + e
+        );
+        return false;
+      }
+      return true;
+    }
+
+    const EMPTY_UPDATES_DOCUMENT_OPEN =
+      '<?xml version="1.0"?><updates xmlns="' + URI_UPDATE_NS + '">';
+    const EMPTY_UPDATES_DOCUMENT_CLOSE = "</updates>";
+    try {
+      var parser = new DOMParser();
+      var doc = parser.parseFromString(
+        EMPTY_UPDATES_DOCUMENT_OPEN + EMPTY_UPDATES_DOCUMENT_CLOSE,
+        "text/xml"
+      );
+
+      for (var i = 0; i < updates.length; ++i) {
+        doc.documentElement.appendChild(updates[i].serialize(doc));
+      }
+
+      var xml =
+        EMPTY_UPDATES_DOCUMENT_OPEN +
+        doc.documentElement.innerHTML +
+        EMPTY_UPDATES_DOCUMENT_CLOSE;
+      // If the destination file existed and is removed while the following is
+      // being performed the copy of the tmp file to the destination file will
+      // fail.
+      await IOUtils.writeUTF8(file.path, xml, {
+        tmpPath: file.path + ".tmp",
+      });
+      await IOUtils.setPermissions(file.path, FileUtils.PERMS_FILE);
+    } catch (e) {
+      LOG("UpdateManager:_writeUpdatesToXMLFile - Exception: " + e);
+      return false;
+    }
+    return true;
+  },
+
+  _updatesXMLSaver: null,
+  _updatesXMLSaverCallback: null,
+  /**
+   * See nsIUpdateService.idl
+   */
+  saveUpdates: function UM_saveUpdates() {
+    if (!this._updatesXMLSaver) {
+      this._updatesXMLSaverCallback = () => this._updatesXMLSaver.finalize();
+
+      this._updatesXMLSaver = new lazy.DeferredTask(
+        () => this._saveUpdatesXML(),
+        XML_SAVER_INTERVAL_MS
+      );
+      lazy.AsyncShutdown.profileBeforeChange.addBlocker(
+        "UpdateManager: writing update xml data",
+        this._updatesXMLSaverCallback
+      );
+    } else {
+      this._updatesXMLSaver.disarm();
+    }
+
+    this._updatesXMLSaver.arm();
+  },
+
+  /**
+   * Saves the active-updates.xml and updates.xml when the updates history has
+   * been modified files.
+   */
+  _saveUpdatesXML: function UM__saveUpdatesXML() {
+    // This mechanism for how we store the updates might seem a bit odd, since,
+    // if only one update is stored, we don't know if it's the ready update or
+    // the downloading update. However, we can determine which it is by reading
+    // update.status. If we read STATE_DOWNLOADING, it must be a downloading
+    // update and otherwise it's a ready update. This method has the additional
+    // advantage of requiring no migration from when we used to only store a
+    // single active update.
+    let updates = [];
+    if (this._readyUpdate) {
+      updates.push(this._readyUpdate);
+    }
+    if (this._downloadingUpdate) {
+      updates.push(this._downloadingUpdate);
+    }
+
+    // The active update stored in the active-update.xml file will change during
+    // the lifetime of an active update and the file should always be updated
+    // when saveUpdates is called.
+    let promises = [];
+    promises[0] = this._writeUpdatesToXMLFile(updates, FILE_ACTIVE_UPDATE_XML);
+    // The update history stored in the updates.xml file should only need to be
+    // updated when an active update has been added to it in which case
+    // |_updatesDirty| will be true.
+    if (this._updatesDirty) {
+      this._updatesDirty = false;
+      promises[1] = this._writeUpdatesToXMLFile(
+        this._getUpdates(),
+        FILE_UPDATES_XML
+      );
+    }
+    return Promise.all(promises);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  refreshUpdateStatus: async function UM_refreshUpdateStatus() {
+    try {
+      LOG("UpdateManager:refreshUpdateStatus - Staging done.");
+
+      var update = this._readyUpdate;
+      if (!update) {
+        LOG("UpdateManager:refreshUpdateStatus - Missing ready update?");
+        return;
+      }
+
+      var status = readStatusFile(getReadyUpdateDir());
+      pingStateAndStatusCodes(update, false, status);
+      LOG(`UpdateManager:refreshUpdateStatus - status = ${status}`);
+
+      let parts = status.split(":");
+      update.state = parts[0];
+      if (update.state == STATE_APPLYING) {
+        LOG(
+          "UpdateManager:refreshUpdateStatus - Staging appears to have crashed."
+        );
+        update.state = STATE_FAILED;
+        update.errorCode = ERR_UPDATER_CRASHED;
+      } else if (update.state == STATE_FAILED) {
+        LOG("UpdateManager:refreshUpdateStatus - Staging failed.");
+        if (parts[1]) {
+          update.errorCode = parseInt(parts[1]) || INVALID_UPDATER_STATUS_CODE;
+        } else {
+          update.errorCode = INVALID_UPDATER_STATUS_CODE;
+        }
+      }
+
+      // Rotate the update logs so the update log isn't removed if a complete
+      // update is downloaded. By passing false the patch directory won't be
+      // removed.
+      cleanUpReadyUpdateDir(false);
+
+      if (update.state == STATE_FAILED) {
+        let isMemError = isMemoryAllocationErrorCode(update.errorCode);
+        if (
+          update.errorCode == DELETE_ERROR_STAGING_LOCK_FILE ||
+          update.errorCode == UNEXPECTED_STAGING_ERROR ||
+          isMemError
+        ) {
+          update.state = getBestPendingState();
+          writeStatusFile(getReadyUpdateDir(), update.state);
+          if (isMemError) {
+            LOG(
+              `UpdateManager:refreshUpdateStatus - Updater failed to ` +
+                `allocate enough memory to successfully stage. Setting ` +
+                `status to "${update.state}"`
+            );
+          } else {
+            LOG(
+              `UpdateManager:refreshUpdateStatus - Unexpected staging error. ` +
+                `Setting status to "${update.state}"`
+            );
+          }
+        } else if (isServiceSpecificErrorCode(update.errorCode)) {
+          // Sometimes when staging, we might encounter an error that is
+          // specific to the Maintenance Service. If this happens, we should try
+          // to update without the Service.
+          LOG(
+            `UpdateManager:refreshUpdateStatus - Encountered service ` +
+              `specific error code: ${update.errorCode}. Will try installing ` +
+              `update without the Maintenance Service. Setting state to pending`
+          );
+          update.state = STATE_PENDING;
+          writeStatusFile(getReadyUpdateDir(), update.state);
+        } else {
+          LOG(
+            "UpdateManager:refreshUpdateStatus - Attempting handleUpdateFailure"
+          );
+          if (!handleUpdateFailure(update)) {
+            LOG(
+              "UpdateManager:refreshUpdateStatus - handleUpdateFailure " +
+                "failed. Attempting to fall back to complete update."
+            );
+            await handleFallbackToCompleteUpdate();
+          }
+        }
+      }
+      if (update.state == STATE_APPLIED && shouldUseService()) {
+        LOG(
+          `UpdateManager:refreshUpdateStatus - Staging successful. ` +
+            `Setting status to "${STATE_APPLIED_SERVICE}"`
+        );
+        writeStatusFile(
+          getReadyUpdateDir(),
+          (update.state = STATE_APPLIED_SERVICE)
+        );
+      }
+
+      // Now that the active update's properties have been updated write the
+      // active-update.xml to disk. Since there have been no changes to the
+      // update history the updates.xml will not be written to disk.
+      this.saveUpdates();
+
+      // Send an observer notification which the app update doorhanger uses to
+      // display a restart notification after any langpacks have staged.
+      await promiseLangPacksUpdated(update);
+
+      if (
+        update.state == STATE_APPLIED ||
+        update.state == STATE_APPLIED_SERVICE ||
+        update.state == STATE_PENDING ||
+        update.state == STATE_PENDING_SERVICE ||
+        update.state == STATE_PENDING_ELEVATE
+      ) {
+        LOG("UpdateManager:refreshUpdateStatus - Setting state STATE_PENDING");
+        transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+      }
+
+      LOG(
+        "UpdateManager:refreshUpdateStatus - Notifying observers that " +
+          "the update was staged. topic: update-staged, status: " +
+          update.state
+      );
+      Services.obs.notifyObservers(update, "update-staged", update.state);
+    } finally {
+      // This function being called is the one thing that tells us that staging
+      // is done so be very sure that we don't exit it leaving the current
+      // state at STATE_STAGING.
+      // The only cases where we haven't already done a state transition are
+      // error cases, so if another state isn't set, assume that we hit an error
+      // and aborted the update.
+      if (
+        lazy.AUS.currentState == Ci.nsIApplicationUpdateService.STATE_STAGING
+      ) {
+        LOG("UpdateManager:refreshUpdateStatus - Setting state STATE_IDLE");
+        transitionState(Ci.nsIApplicationUpdateService.STATE_IDLE);
+      }
+    }
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  elevationOptedIn: function UM_elevationOptedIn() {
+    // The user has been been made aware that the update requires elevation.
+    let update = this._readyUpdate;
+    if (!update) {
+      return;
+    }
+    let status = readStatusFile(getReadyUpdateDir());
+    let parts = status.split(":");
+    update.state = parts[0];
+    if (update.state == STATE_PENDING_ELEVATE) {
+      LOG("UpdateManager:elevationOptedIn - Setting state to pending.");
+      // Proceed with the pending update.
+      // Note: STATE_PENDING_ELEVATE stands for "pending user's approval to
+      // proceed with an elevated update". As long as we see this state, we will
+      // notify the user of the availability of an update that requires
+      // elevation. |elevationOptedIn| (this function) is called when the user
+      // gives us approval to proceed, so we want to switch to STATE_PENDING.
+      // The updater then detects whether or not elevation is required and
+      // displays the elevation prompt if necessary. This last step does not
+      // depend on the state in the status file.
+      writeStatusFile(getReadyUpdateDir(), STATE_PENDING);
+    } else {
+      LOG("UpdateManager:elevationOptedIn - Not in pending-elevate state.");
+    }
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  cleanupDownloadingUpdate: function UM_cleanupDownloadingUpdate() {
+    LOG(
+      "UpdateManager:cleanupDownloadingUpdate - cleaning up downloading update."
+    );
+    cleanupDownloadingUpdate();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  cleanupReadyUpdate: function UM_cleanupReadyUpdate() {
+    LOG("UpdateManager:cleanupReadyUpdate - cleaning up ready update.");
+    cleanupReadyUpdate();
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  doInstallCleanup: async function UM_doInstallCleanup(isUninstall) {
+    LOG("UpdateManager:doInstallCleanup - cleaning up");
+    let completionPromises = [];
+
+    const delete_or_log = path =>
+      IOUtils.remove(path).catch(ex =>
+        console.error(`Failed to delete ${path}`, ex)
+      );
+
+    for (const key of [KEY_OLD_UPDROOT, KEY_UPDROOT]) {
+      const root = Services.dirsvc.get(key, Ci.nsIFile);
+
+      const activeUpdateXml = root.clone();
+      activeUpdateXml.append(FILE_ACTIVE_UPDATE_XML);
+      completionPromises.push(delete_or_log(activeUpdateXml.path));
+
+      const downloadingMar = root.clone();
+      downloadingMar.append(DIR_UPDATES);
+      downloadingMar.append(DIR_UPDATE_DOWNLOADING);
+      downloadingMar.append(FILE_UPDATE_MAR);
+      completionPromises.push(delete_or_log(downloadingMar.path));
+
+      const readyDir = root.clone();
+      readyDir.append(DIR_UPDATES);
+      readyDir.append(DIR_UPDATE_READY);
+      const readyMar = readyDir.clone();
+      readyMar.append(FILE_UPDATE_MAR);
+      completionPromises.push(delete_or_log(readyMar.path));
+      const readyStatus = readyDir.clone();
+      readyStatus.append(FILE_UPDATE_STATUS);
+      completionPromises.push(delete_or_log(readyStatus.path));
+      const versionFile = readyDir.clone();
+      versionFile.append(FILE_UPDATE_VERSION);
+      completionPromises.push(delete_or_log(versionFile.path));
+    }
+
+    return Promise.allSettled(completionPromises);
+  },
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  doUninstallCleanup: async function UM_doUninstallCleanup(isUninstall) {
+    LOG("UpdateManager:doUninstallCleanup - cleaning up.");
+    let completionPromises = [];
+
+    completionPromises.push(
+      IOUtils.remove(Services.dirsvc.get(KEY_UPDROOT, Ci.nsIFile).path, {
+        recursive: true,
+      }).catch(ex => console.error("Failed to remove update directory", ex))
+    );
+    completionPromises.push(
+      IOUtils.remove(Services.dirsvc.get(KEY_OLD_UPDROOT, Ci.nsIFile).path, {
+        recursive: true,
+      }).catch(ex => console.error("Failed to remove old update directory", ex))
+    );
+
+    return Promise.allSettled(completionPromises);
+  },
+
+  classID: Components.ID("{093C2356-4843-4C65-8709-D7DBCBBE7DFB}"),
+  QueryInterface: ChromeUtils.generateQI(["nsIUpdateManager", "nsIObserver"]),
+};
+
+/**
+ * CheckerService
+ * Provides an interface for checking for new updates. When more checks are
+ * made while an equivalent check is already in-progress, they will be coalesced
+ * into a single update check request.
+ */
+export class CheckerService {
+  #nextUpdateCheckId = 1;
+
+  // Most of the update checking data is looked up via a "request key". This
+  // allows us to lookup the request key for a particular check id, since
+  // multiple checks can correspond to a single request.
+  // When a check is cancelled or completed, it will be removed from this
+  // object.
+  #requestKeyByCheckId = {};
+
+  // This object will relate request keys to update check data objects. The
+  // format of the update check data objects is defined by
+  // #makeUpdateCheckDataObject, below.
+  // When an update request is cancelled (by all of the corresponding update
+  // checks being cancelled) or completed, its key will be removed from this
+  // object.
+  #updateCheckData = {};
+
+  #makeUpdateCheckDataObject(type, promise) {
+    return { type, promise, request: null };
+  }
+
+  /**
+   * Indicates whether the passed parameter is one of the valid enumerated
+   * values that indicates a type of update check.
+   */
+  #validUpdateCheckType(checkType) {
+    return [
+      Ci.nsIUpdateChecker.BACKGROUND_CHECK,
+      Ci.nsIUpdateChecker.FOREGROUND_CHECK,
+    ].includes(checkType);
+  }
+
+  #getCanMigrate() {
+    if (AppConstants.platform != "win") {
+      return false;
+    }
+
+    // The first element of the array is whether the build target is 32 or 64
+    // bit and the third element of the array is whether the client's Windows OS
+    // system processor is 32 or 64 bit.
+    let aryABI = lazy.UpdateUtils.ABI.split("-");
+    if (aryABI[0] != "x86" || aryABI[2] != "x64") {
+      return false;
+    }
+
+    let wrk = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+      Ci.nsIWindowsRegKey
+    );
+
+    let regPath =
+      "SOFTWARE\\Mozilla\\" + Services.appinfo.name + "\\32to64DidMigrate";
+    let regValHKCU = lazy.WindowsRegistry.readRegKey(
+      wrk.ROOT_KEY_CURRENT_USER,
+      regPath,
+      "Never",
+      wrk.WOW64_32
+    );
+    let regValHKLM = lazy.WindowsRegistry.readRegKey(
+      wrk.ROOT_KEY_LOCAL_MACHINE,
+      regPath,
+      "Never",
+      wrk.WOW64_32
+    );
+    // The Never registry key value allows configuring a system to never migrate
+    // any of the installations.
+    if (regValHKCU === 1 || regValHKLM === 1) {
+      LOG(
+        "CheckerService:#getCanMigrate - all installations should not be " +
+          "migrated"
+      );
+      return false;
+    }
+
+    let appBaseDirPath = getAppBaseDir().path;
+    regValHKCU = lazy.WindowsRegistry.readRegKey(
+      wrk.ROOT_KEY_CURRENT_USER,
+      regPath,
+      appBaseDirPath,
+      wrk.WOW64_32
+    );
+    regValHKLM = lazy.WindowsRegistry.readRegKey(
+      wrk.ROOT_KEY_LOCAL_MACHINE,
+      regPath,
+      appBaseDirPath,
+      wrk.WOW64_32
+    );
+    // When the registry value is 1 for the installation directory path value
+    // name then the installation has already been migrated once or the system
+    // was configured to not migrate that installation.
+    if (regValHKCU === 1 || regValHKLM === 1) {
+      LOG(
+        "CheckerService:#getCanMigrate - this installation should not be " +
+          "migrated"
+      );
+      return false;
+    }
+
+    // When the registry value is 0 for the installation directory path value
+    // name then the installation has updated to Firefox 56 and can be migrated.
+    if (regValHKCU === 0 || regValHKLM === 0) {
+      LOG("CheckerService:#getCanMigrate - this installation can be migrated");
+      return true;
+    }
+
+    LOG(
+      "CheckerService:#getCanMigrate - no registry entries for this " +
+        "installation"
+    );
+    return false;
+  }
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  async getUpdateURL(checkType) {
+    LOG("CheckerService:getUpdateURL - checkType: " + checkType);
+    if (!this.#validUpdateCheckType(checkType)) {
+      LOG("CheckerService:getUpdateURL - Invalid checkType");
+      throw Components.Exception("", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let url = Services.appinfo.updateURL;
+    let updatePin;
+
+    if (Services.policies) {
+      let policies = Services.policies.getActivePolicies();
+      if (policies) {
+        if ("AppUpdateURL" in policies) {
+          url = policies.AppUpdateURL.toString();
+        }
+        if ("AppUpdatePin" in policies) {
+          updatePin = policies.AppUpdatePin;
+
+          // Scalar ID: update.version_pin
+          AUSTLMY.pingPinPolicy(updatePin);
+        }
+      }
+    }
+
+    if (!url) {
+      LOG("CheckerService:getUpdateURL - update URL not defined");
+      return null;
+    }
+
+    url = await lazy.UpdateUtils.formatUpdateURL(url);
+
+    if (checkType == Ci.nsIUpdateChecker.FOREGROUND_CHECK) {
+      url += (url.includes("?") ? "&" : "?") + "force=1";
+    }
+
+    if (this.#getCanMigrate()) {
+      url += (url.includes("?") ? "&" : "?") + "mig64=1";
+    }
+
+    if (updatePin) {
+      url +=
+        (url.includes("?") ? "&" : "?") +
+        "pin=" +
+        encodeURIComponent(updatePin);
+    }
+
+    LOG("CheckerService:getUpdateURL - update URL: " + url);
+    return url;
+  }
+
+  /**
+   * See nsIUpdateService.idl
+   */
+
+  checkForUpdates(checkType) {
+    LOG("CheckerService:checkForUpdates - checkType: " + checkType);
+    if (!this.#validUpdateCheckType(checkType)) {
+      LOG("CheckerService:checkForUpdates - Invalid checkType");
+      throw Components.Exception("", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let checkId = this.#nextUpdateCheckId;
+    this.#nextUpdateCheckId += 1;
+
+    // `checkType == FOREGROUND_CHECK`` can override `canCheckForUpdates`. But
+    // nothing should override enterprise policies.
+    if (lazy.AUS.disabled) {
+      LOG("CheckerService:checkForUpdates - disabled by policy");
+      return this.#getChecksNotAllowedObject(checkId);
+    }
+    if (
+      checkType == Ci.nsIUpdateChecker.BACKGROUND_CHECK &&
+      !lazy.AUS.canCheckForUpdates
+    ) {
+      LOG("CheckerService:checkForUpdates - !canCheckForUpdates");
+      return this.#getChecksNotAllowedObject(checkId);
+    }
+
+    // We want to combine simultaneous requests, but only ones that are
+    // equivalent. If, say, one of them uses the force parameter and one
+    // doesn't, we want those two requests to remain separate. This key will
+    // allow us to map equivalent requests together. It is also the key that we
+    // use to lookup the update check data in this.#updateCheckData.
+    let requestKey = checkType;
+
+    if (requestKey in this.#updateCheckData) {
+      LOG(
+        `CheckerService:checkForUpdates - Connecting check id ${checkId} to ` +
+          `existing check request.`
+      );
+    } else {
+      LOG(
+        `CheckerService:checkForUpdates - Making new check request for check ` +
+          `id ${checkId}.`
+      );
+      this.#updateCheckData[requestKey] = this.#makeUpdateCheckDataObject(
+        checkType,
+        this.#updateCheck(checkType, requestKey)
+      );
+    }
+
+    this.#requestKeyByCheckId[checkId] = requestKey;
+
+    return {
+      id: checkId,
+      result: this.#updateCheckData[requestKey].promise,
+      QueryInterface: ChromeUtils.generateQI(["nsIUpdateCheck"]),
+    };
+  }
+
+  #getChecksNotAllowedObject(checkId) {
+    return {
+      id: checkId,
+      result: Promise.resolve(
+        Object.freeze({
+          checksAllowed: false,
+          succeeded: false,
+          request: null,
+          updates: [],
+          QueryInterface: ChromeUtils.generateQI(["nsIUpdateCheckResult"]),
+        })
+      ),
+      QueryInterface: ChromeUtils.generateQI(["nsIUpdateCheck"]),
+    };
+  }
+
+  async #updateCheck(checkType, requestKey) {
+    await waitForOtherInstances();
+
+    let url;
+    try {
+      url = await this.getUpdateURL(checkType);
+    } catch (ex) {}
+
+    if (!url) {
+      LOG("CheckerService:#updateCheck - !url");
+      return this.#getCheckFailedObject("update_url_not_available");
+    }
+
+    let request = new XMLHttpRequest();
+    request.open("GET", url, true);
+    request.channel.notificationCallbacks = new lazy.CertUtils.BadCertHandler(
+      false
+    );
+    // Prevent the request from reading from the cache.
+    request.channel.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
+    // Prevent the request from writing to the cache.
+    request.channel.loadFlags |= Ci.nsIRequest.INHIBIT_CACHING;
+    // Disable cutting edge features, like TLS 1.3, where middleboxes might
+    // brick us
+    request.channel.QueryInterface(
+      Ci.nsIHttpChannelInternal
+    ).beConservative = true;
+
+    request.overrideMimeType("text/xml");
+    // The Cache-Control header is only interpreted by proxies and the
+    // final destination. It does not help if a resource is already
+    // cached locally.
+    request.setRequestHeader("Cache-Control", "no-cache");
+    // HTTP/1.0 servers might not implement Cache-Control and
+    // might only implement Pragma: no-cache
+    request.setRequestHeader("Pragma", "no-cache");
+
+    const UPDATE_CHECK_LOAD_SUCCESS = 1;
+    const UPDATE_CHECK_LOAD_ERROR = 2;
+    const UPDATE_CHECK_CANCELLED = 3;
+
+    let result = await new Promise(resolve => {
+      // It's important that nothing potentially asynchronous happens between
+      // checking if the request has been cancelled and starting the request.
+      // If an update check cancellation happens before dispatching the request
+      // and we end up dispatching it anyways, we will never call cancel on the
+      // request later and the cancellation effectively won't happen.
+      if (!(requestKey in this.#updateCheckData)) {
+        LOG(
+          "CheckerService:#updateCheck - check was cancelled before request " +
+            "was able to start"
+        );
+        resolve(UPDATE_CHECK_CANCELLED);
+        return;
+      }
+
+      let onLoad = event => {
+        request.removeEventListener("load", onLoad);
+        LOG("CheckerService:#updateCheck - request got 'load' event");
+        resolve(UPDATE_CHECK_LOAD_SUCCESS);
+      };
+      request.addEventListener("load", onLoad);
+      let onError = event => {
+        request.removeEventListener("error", onLoad);
+        LOG("CheckerService:#updateCheck - request got 'error' event");
+        resolve(UPDATE_CHECK_LOAD_ERROR);
+      };
+      request.addEventListener("error", onError);
+
+      LOG("CheckerService:#updateCheck - sending request to: " + url);
+      request.send(null);
+      this.#updateCheckData[requestKey].request = request;
+    });
+
+    // Remove all entries for this request key. This marks the request and the
+    // associated check ids as no longer in-progress.
+    delete this.#updateCheckData[requestKey];
+    for (const checkId of Object.keys(this.#requestKeyByCheckId)) {
+      if (this.#requestKeyByCheckId[checkId] == requestKey) {
+        delete this.#requestKeyByCheckId[checkId];
+      }
+    }
+
+    if (result == UPDATE_CHECK_CANCELLED) {
+      return this.#getCheckFailedObject(Cr.NS_BINDING_ABORTED);
+    }
+
+    if (result == UPDATE_CHECK_LOAD_ERROR) {
+      let status = this.#getChannelStatus(request);
+      LOG("CheckerService:#updateCheck - Failed. request.status: " + status);
+
+      // Set MitM pref.
+      try {
+        let secInfo = request.channel.securityInfo;
+        if (secInfo.serverCert && secInfo.serverCert.issuerName) {
+          Services.prefs.setStringPref(
+            "security.pki.mitm_canary_issuer",
+            secInfo.serverCert.issuerName
+          );
+        }
+      } catch (e) {
+        LOG("CheckerService:#updateCheck - Getting secInfo failed.");
+      }
+
+      return this.#getCheckFailedObject(status, 404, request);
+    }
+
+    LOG("CheckerService:#updateCheck - request completed downloading document");
+    Services.prefs.clearUserPref("security.pki.mitm_canary_issuer");
+    // Check whether there is a mitm, i.e. check whether the root cert is
+    // built-in or not.
+    try {
+      let sslStatus = request.channel.securityInfo;
+      if (sslStatus && sslStatus.succeededCertChain) {
+        let rootCert = null;
+        // The root cert is the last cert in the chain.
+        if (sslStatus.succeededCertChain.length) {
+          rootCert =
+            sslStatus.succeededCertChain[
+              sslStatus.succeededCertChain.length - 1
+            ];
+        }
+        if (rootCert) {
+          Services.prefs.setBoolPref(
+            "security.pki.mitm_detected",
+            !rootCert.isBuiltInRoot
+          );
+        }
+      }
+    } catch (e) {
+      LOG("CheckerService:#updateCheck - Getting sslStatus failed.");
+    }
+
+    let updates;
+    try {
+      // Analyze the resulting DOM and determine the set of updates.
+      updates = this.#parseUpdates(request);
+    } catch (e) {
+      LOG(
+        "CheckerService:#updateCheck - there was a problem checking for " +
+          "updates. Exception: " +
+          e
+      );
+      let status = this.#getChannelStatus(request);
+      // If we can't find an error string specific to this status code,
+      // just use the 200 message from above, which means everything
+      // "looks" fine but there was probably an XML error or a bogus file.
+      return this.#getCheckFailedObject(status, 200, request);
+    }
+
+    LOG(
+      "CheckerService:#updateCheck - number of updates available: " +
+        updates.length
+    );
+
+    if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_BACKGROUNDERRORS)) {
+      Services.prefs.clearUserPref(PREF_APP_UPDATE_BACKGROUNDERRORS);
+    }
+
+    return Object.freeze({
+      checksAllowed: true,
+      succeeded: true,
+      request,
+      updates,
+      QueryInterface: ChromeUtils.generateQI(["nsIUpdateCheckResult"]),
+    });
+  }
+
+  /**
+   * @param   errorCode
+   *          The error code to include in the return value. If possible, we
+   *          will get the update status text based on this error code.
+   * @param   defaultCode
+   *          Optional. The error code to use to get the status text if there
+   *          isn't status text available for `errorCode`.
+   * @param   request
+   *          The XMLHttpRequest used to check for updates. Or null, if one was
+   *          never constructed.
+   * @returns An nsIUpdateCheckResult object indicating an error, using the
+   *          error data passed to this function.
+   */
+  #getCheckFailedObject(
+    errorCode,
+    defaultCode = Cr.NS_BINDING_FAILED,
+    request = null
+  ) {
+    let update = new Update(null);
+    update.errorCode = errorCode;
+    update.statusText = getStatusTextFromCode(errorCode, defaultCode);
+
+    if (errorCode == Cr.NS_ERROR_OFFLINE) {
+      // We use a separate constant here because nsIUpdate.errorCode is signed
+      update.errorCode = NETWORK_ERROR_OFFLINE;
+    } else if (this.#isHttpStatusCode(errorCode)) {
+      update.errorCode = HTTP_ERROR_OFFSET + errorCode;
+    }
+
+    return Object.freeze({
+      checksAllowed: true,
+      succeeded: false,
+      request,
+      updates: [update],
+      QueryInterface: ChromeUtils.generateQI(["nsIUpdateCheckResult"]),
+    });
+  }
+
+  /**
+   * Returns the status code for the XMLHttpRequest
+   */
+  #getChannelStatus(request) {
+    var status = 0;
+    try {
+      status = request.status;
+    } catch (e) {}
+
+    if (status == 0) {
+      status = request.channel.QueryInterface(Ci.nsIRequest).status;
+    }
+    return status;
+  }
+
+  #isHttpStatusCode(status) {
+    return status >= 100 && status <= 599;
+  }
+
+  /**
+   * @param   request
+   *          The XMLHttpRequest that successfully loaded the update XML.
+   * @returns An array of 0 or more nsIUpdate objects describing the available
+   *          updates.
+   * @throws  If the XML document element node name is not updates.
+   */
+  #parseUpdates(request) {
+    let updatesElement = request.responseXML.documentElement;
+    if (!updatesElement) {
+      LOG("CheckerService:#parseUpdates - empty updates document?!");
+      return [];
+    }
+
+    if (updatesElement.nodeName != "updates") {
+      LOG("CheckerService:#parseUpdates - unexpected node name!");
+      throw new Error(
+        "Unexpected node name, expected: updates, got: " +
+          updatesElement.nodeName
+      );
+    }
+
+    let updates = [];
+    for (const updateElement of updatesElement.childNodes) {
+      if (
+        updateElement.nodeType != updateElement.ELEMENT_NODE ||
+        updateElement.localName != "update"
+      ) {
+        continue;
+      }
+
+      let update;
+      try {
+        update = new Update(updateElement);
+      } catch (e) {
+        LOG("CheckerService:#parseUpdates - invalid <update/>, ignoring...");
+        continue;
+      }
+      update.serviceURL = request.responseURL;
+      update.channel = lazy.UpdateUtils.UpdateChannel;
+      updates.push(update);
+    }
+
+    return updates;
+  }
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  stopCheck(checkId) {
+    if (!(checkId in this.#requestKeyByCheckId)) {
+      LOG(`CheckerService:stopCheck - Non-existent check id ${checkId}`);
+      return;
+    }
+    LOG(`CheckerService:stopCheck - Cancelling check id ${checkId}`);
+    let requestKey = this.#requestKeyByCheckId[checkId];
+    delete this.#requestKeyByCheckId[checkId];
+    if (Object.values(this.#requestKeyByCheckId).includes(requestKey)) {
+      LOG(
+        `CheckerService:stopCheck - Not actually cancelling request because ` +
+          `other check id's depend on it.`
+      );
+    } else {
+      LOG(
+        `CheckerService:stopCheck - This is the last check using this ` +
+          `request. Cancelling the request now.`
+      );
+      let request = this.#updateCheckData[requestKey].request;
+      delete this.#updateCheckData[requestKey];
+      if (request) {
+        LOG(`CheckerService:stopCheck - Aborting XMLHttpRequest`);
+        request.abort();
+      } else {
+        LOG(
+          `CheckerService:stopCheck - Not aborting XMLHttpRequest. It ` +
+            `doesn't appear to have started yet.`
+        );
+      }
+    }
+  }
+
+  /**
+   * See nsIUpdateService.idl
+   */
+  stopAllChecks() {
+    LOG("CheckerService:stopAllChecks - stopping all checks.");
+    for (const checkId of Object.keys(this.#requestKeyByCheckId)) {
+      this.stopCheck(checkId);
+    }
+  }
+
+  classID = Components.ID("{898CDC9B-E43F-422F-9CC4-2F6291B415A3}");
+  QueryInterface = ChromeUtils.generateQI(["nsIUpdateChecker"]);
+}
+
+/**
+ * Manages the download of updates
+ * @param   background
+ *          Whether or not this downloader is operating in background
+ *          update mode.
+ * @param   updateService
+ *          The update service that created this downloader.
+ * @constructor
+ */
+function Downloader(updateService) {
+  LOG("Creating Downloader");
+  this.updateService = updateService;
+}
+Downloader.prototype = {
+  /**
+   * The nsIUpdatePatch that we are downloading
+   */
+  _patch: null,
+
+  /**
+   * The nsIUpdate that we are downloading
+   */
+  _update: null,
+
+  /**
+   * The nsIRequest object handling the download.
+   */
+  _request: null,
+
+  /**
+   * Whether or not the update being downloaded is a complete replacement of
+   * the user's existing installation or a patch representing the difference
+   * between the new version and the previous version.
+   */
+  isCompleteUpdate: null,
+
+  /**
+   * We get the nsIRequest from nsIBITS asynchronously. When downloadUpdate has
+   * been called, but this._request is not yet valid, _pendingRequest will be
+   * a promise that will resolve when this._request has been set.
+   */
+  _pendingRequest: null,
+
+  /**
+   * When using BITS, cancel actions happen asynchronously. This variable
+   * keeps track of any cancel action that is in-progress.
+   * If the cancel action fails, this will be set back to null so that the
+   * action can be attempted again. But if the cancel action succeeds, the
+   * resolved promise will remain stored in this variable to prevent cancel
+   * from being called twice (which, for BITS, is an error).
+   */
+  _cancelPromise: null,
+
+  /**
+   * BITS receives progress notifications slowly, unless a user is watching.
+   * This tracks what frequency notifications are happening at.
+   *
+   * This is needed because BITS downloads are started asynchronously.
+   * Specifically, this is needed to prevent a situation where the download is
+   * still starting (Downloader._pendingRequest has not resolved) when the first
+   * observer registers itself. Without this variable, there is no way of
+   * knowing whether the download was started as Active or Idle and, therefore,
+   * we don't know if we need to start Active mode when _pendingRequest
+   * resolves.
+   */
+  _bitsActiveNotifications: false,
+
+  /**
+   * This is a function that when called will stop the update process from
+   * waiting for language pack updates. This is for safety to ensure that a
+   * problem in the add-ons manager doesn't delay updates by much.
+   */
+  _langPackTimeout: null,
+
+  /**
+   * If gOnlyDownloadUpdatesThisSession is true, we prevent the update process
+   * from progressing past the downloading stage. If the download finishes,
+   * pretend that it hasn't in order to keep the current update in the
+   * "downloading" state.
+   */
+  _pretendingDownloadIsNotDone: false,
+
+  /**
+   * Cancels the active download.
+   *
+   * For a BITS download, this will cancel and remove the download job. For
+   * an nsIIncrementalDownload, this will stop the download, but leaves the
+   * data around to allow the transfer to be resumed later.
+   */
+  cancel: async function Downloader_cancel(cancelError) {
+    LOG("Downloader: cancel");
+    if (cancelError === undefined) {
+      cancelError = Cr.NS_BINDING_ABORTED;
+    }
+    if (this.usingBits) {
+      // If a cancel action is already in progress, just return when that
+      // promise resolved. Trying to cancel the same request twice is an error.
+      if (this._cancelPromise) {
+        await this._cancelPromise;
+        return;
+      }
+
+      if (this._pendingRequest) {
+        await this._pendingRequest;
+      }
+      if (this._patch.getProperty("bitsId") != null) {
+        // Make sure that we don't try to resume this download after it was
+        // cancelled.
+        this._patch.deleteProperty("bitsId");
+      }
+      try {
+        this._cancelPromise = this._request.cancelAsync(cancelError);
+        await this._cancelPromise;
+      } catch (e) {
+        // On success, we will not set the cancel promise to null because
+        // we want to prevent two cancellations of the same request. But
+        // retrying after a failed cancel is not an error, so we will set the
+        // cancel promise to null in the failure case.
+        this._cancelPromise = null;
+        throw e;
+      }
+    } else if (this._request && this._request instanceof Ci.nsIRequest) {
+      // Normally, cancelling an nsIIncrementalDownload results in it stopping
+      // the download but leaving the downloaded data so that we can resume the
+      // download later. If we've already finished the download, there is no
+      // transfer to stop.
+      // Note that this differs from the BITS case. Cancelling a BITS job, even
+      // when the transfer has completed, results in all data being deleted.
+      // Therefore, even if the transfer has completed, cancelling a BITS job
+      // has effects that we must not skip.
+      if (this._pretendingDownloadIsNotDone) {
+        LOG(
+          "Downloader: cancel - Ignoring cancel request of finished download"
+        );
+      } else {
+        this._request.cancel(cancelError);
+      }
+    }
+  },
+
+  /**
+   * Verify the downloaded file.  We assume that the download is complete at
+   * this point.
+   */
+  _verifyDownload: function Downloader__verifyDownload() {
+    LOG("Downloader:_verifyDownload called");
+    if (!this._request) {
+      AUSTLMY.pingDownloadCode(
+        this.isCompleteUpdate,
+        AUSTLMY.DWNLD_ERR_VERIFY_NO_REQUEST
+      );
+      return false;
+    }
+
+    let destination = getDownloadingUpdateDir();
+    destination.append(FILE_UPDATE_MAR);
+
+    // Ensure that the file size matches the expected file size.
+    if (destination.fileSize != this._patch.size) {
+      LOG("Downloader:_verifyDownload downloaded size != expected size.");
+      AUSTLMY.pingDownloadCode(
+        this.isCompleteUpdate,
+        AUSTLMY.DWNLD_ERR_VERIFY_PATCH_SIZE_NOT_EQUAL
+      );
+      return false;
+    }
+
+    LOG("Downloader:_verifyDownload downloaded size == expected size.");
+    return true;
+  },
+
+  /**
+   * Select the patch to use given the current state of updateDir and the given
+   * set of update patches.
+   * @param   update
+   *          A nsIUpdate object to select a patch from
+   * @param   updateDir
+   *          A nsIFile representing the update directory
+   * @return  A nsIUpdatePatch object to download
+   */
+  _selectPatch: function Downloader__selectPatch(update, updateDir) {
+    // Given an update to download, we will always try to download the patch
+    // for a partial update over the patch for a full update.
+
+    // Look to see if any of the patches in the Update object has been
+    // pre-selected for download, otherwise we must figure out which one
+    // to select ourselves.
+    var selectedPatch = update.selectedPatch;
+
+    var state = selectedPatch ? selectedPatch.state : STATE_NONE;
+
+    // If this is a patch that we know about, then select it.  If it is a patch
+    // that we do not know about, then remove it and use our default logic.
+    var useComplete = false;
+    if (selectedPatch) {
+      LOG(
+        "Downloader:_selectPatch - found existing patch with state: " + state
+      );
+      if (state == STATE_DOWNLOADING) {
+        LOG("Downloader:_selectPatch - resuming download");
+        return selectedPatch;
+      }
+      if (
+        state == STATE_PENDING ||
+        state == STATE_PENDING_SERVICE ||
+        state == STATE_PENDING_ELEVATE ||
+        state == STATE_APPLIED ||
+        state == STATE_APPLIED_SERVICE
+      ) {
+        LOG("Downloader:_selectPatch - already downloaded");
+        return null;
+      }
+
+      // When downloading the patch failed using BITS, there hasn't been an
+      // attempt to download the patch using the internal application download
+      // mechanism, and an attempt to stage or apply the patch hasn't failed
+      // which indicates that a different patch should be downloaded since
+      // re-downloading the same patch with the internal application download
+      // mechanism will likely also fail when trying to stage or apply it then
+      // try to download the same patch using the internal application download
+      // mechanism.
+      selectedPatch.QueryInterface(Ci.nsIWritablePropertyBag);
+      if (
+        selectedPatch.getProperty("bitsResult") != null &&
+        selectedPatch.getProperty("internalResult") == null &&
+        !selectedPatch.errorCode
+      ) {
+        LOG(
+          "Downloader:_selectPatch - Falling back to non-BITS download " +
+            "mechanism for the same patch due to existing BITS result: " +
+            selectedPatch.getProperty("bitsResult")
+        );
+        return selectedPatch;
+      }
+
+      if (update && selectedPatch.type == "complete") {
+        // This is a pretty fatal error.  Just bail.
+        LOG("Downloader:_selectPatch - failed to apply complete patch!");
+        cleanupDownloadingUpdate();
+        return null;
+      }
+
+      // Something went wrong when we tried to apply the previous patch.
+      // Try the complete patch next time.
+      useComplete = true;
+      selectedPatch = null;
+    }
+
+    // If we were not able to discover an update from a previous download, we
+    // select the best patch from the given set.
+    var partialPatch = getPatchOfType(update, "partial");
+    if (!useComplete) {
+      selectedPatch = partialPatch;
+    }
+    if (!selectedPatch) {
+      if (lazy.UM.readyUpdate) {
+        // If we already have a ready update, we download partials only.
+        LOG(
+          "Downloader:_selectPatch - not selecting a complete patch because " +
+            "this is not the first download of the session"
+        );
+        return null;
+      }
+
+      if (partialPatch) {
+        partialPatch.selected = false;
+      }
+      selectedPatch = getPatchOfType(update, "complete");
+    }
+
+    // if update only contains a partial patch, selectedPatch == null here if
+    // the partial patch has been attempted and fails and we're trying to get a
+    // complete patch
+    if (selectedPatch) {
+      selectedPatch.selected = true;
+      update.isCompleteUpdate = selectedPatch.type == "complete";
+    }
+
+    LOG(
+      "Downloader:_selectPatch - Patch selected. Assigning update to " +
+        "downloadingUpdate."
+    );
+    lazy.UM.downloadingUpdate = update;
+
+    return selectedPatch;
+  },
+
+  /**
+   * Whether or not the user wants to be notified that an update is being
+   * downloaded.
+   */
+  get _notifyDuringDownload() {
+    return Services.prefs.getBoolPref(
+      PREF_APP_UPDATE_NOTIFYDURINGDOWNLOAD,
+      false
+    );
+  },
+
+  _notifyDownloadStatusObservers:
+    function Downloader_notifyDownloadStatusObservers() {
+      if (this._notifyDuringDownload) {
+        let status = this.updateService.isDownloading ? "downloading" : "idle";
+        Services.obs.notifyObservers(
+          this._update,
+          "update-downloading",
+          status
+        );
+      }
+    },
+
+  /**
+   * Whether or not we are currently downloading something.
+   */
+  get isBusy() {
+    return this._request != null || this._pendingRequest != null;
+  },
+
+  get usingBits() {
+    return this._pendingRequest != null || this._request instanceof BitsRequest;
+  },
+
+  /**
+   * Returns true if the specified patch can be downloaded with BITS.
+   */
+  _canUseBits: function Downloader__canUseBits(patch) {
+    if (getCanUseBits() != "CanUseBits") {
+      // This will have printed its own logging. No need to print more.
+      return false;
+    }
+    // Regardless of success or failure, don't download the same patch with BITS
+    // twice.
+    if (patch.getProperty("bitsResult") != null) {
+      LOG(
+        "Downloader:_canUseBits - Not using BITS because it was already tried"
+      );
+      return false;
+    }
+    LOG("Downloader:_canUseBits - Patch is able to use BITS download");
+    return true;
+  },
+
+  /**
+   * Instruct the add-ons manager to start downloading language pack updates in
+   * preparation for the current update.
+   */
+  _startLangPackUpdates: function Downloader__startLangPackUpdates() {
+    if (!Services.prefs.getBoolPref(PREF_APP_UPDATE_LANGPACK_ENABLED, false)) {
+      return;
+    }
+
+    // A promise that we can resolve at some point to time out the language pack
+    // update process.
+    let timeoutPromise = new Promise(resolve => {
+      this._langPackTimeout = resolve;
+    });
+
+    let update = unwrap(this._update);
+
+    let existing = LangPackUpdates.get(update);
+    if (existing) {
+      // We have already started staging lang packs for this update, no need to
+      // do it again.
+      return;
+    }
+
+    // Note that we don't care about success or failure here, either way we will
+    // continue with the update process.
+    let langPackPromise = lazy.AddonManager.stageLangpacksForAppUpdate(
+      update.appVersion,
+      update.appVersion
+    )
+      .catch(error => {
+        LOG(
+          `Add-ons manager threw exception while updating language packs: ${error}`
+        );
+      })
+      .finally(() => {
+        this._langPackTimeout = null;
+
+        if (TelemetryStopwatch.running("UPDATE_LANGPACK_OVERTIME", update)) {
+          TelemetryStopwatch.finish("UPDATE_LANGPACK_OVERTIME", update);
+        }
+      });
+
+    LangPackUpdates.set(
+      update,
+      Promise.race([langPackPromise, timeoutPromise])
+    );
+  },
+
+  /**
+   * Download and stage the given update.
+   * @param   update
+   *          A nsIUpdate object to download a patch for. Cannot be null.
+   */
+  downloadUpdate: async function Downloader_downloadUpdate(update) {
+    LOG("UpdateService:downloadUpdate");
+    if (!update) {
+      AUSTLMY.pingDownloadCode(undefined, AUSTLMY.DWNLD_ERR_NO_UPDATE);
+      throw Components.Exception("", Cr.NS_ERROR_NULL_POINTER);
+    }
+
+    var updateDir = getDownloadingUpdateDir();
+
+    this._update = update;
+
+    // This function may return null, which indicates that there are no patches
+    // to download.
+    this._patch = this._selectPatch(update, updateDir);
+    if (!this._patch) {
+      LOG("Downloader:downloadUpdate - no patch to download");
+      AUSTLMY.pingDownloadCode(undefined, AUSTLMY.DWNLD_ERR_NO_UPDATE_PATCH);
+      return false;
+    }
+    // The update and the patch implement nsIWritablePropertyBag. Expose that
+    // interface immediately after a patch is assigned so that
+    // this.(_patch|_update).(get|set)Property can always safely be called.
+    this._update.QueryInterface(Ci.nsIWritablePropertyBag);
+    this._patch.QueryInterface(Ci.nsIWritablePropertyBag);
+
+    if (
+      this._update.getProperty("disableBackgroundUpdates") != null &&
+      lazy.gIsBackgroundTaskMode
+    ) {
+      LOG(
+        "Downloader:downloadUpdate - Background update disabled by update " +
+          "advertisement"
+      );
+      return false;
+    }
+
+    this.isCompleteUpdate = this._patch.type == "complete";
+
+    let canUseBits = false;
+    // Allow the advertised update to disable BITS.
+    if (this._update.getProperty("disableBITS") != null) {
+      LOG(
+        "Downloader:downloadUpdate - BITS downloads disabled by update " +
+          "advertisement"
+      );
+    } else {
+      canUseBits = this._canUseBits(this._patch);
+    }
+
+    if (!canUseBits) {
+      this._pendingRequest = null;
+
+      let patchFile = updateDir.clone();
+      patchFile.append(FILE_UPDATE_MAR);
+
+      if (lazy.gIsBackgroundTaskMode) {
+        // We don't normally run a background update if we can't use BITS, but
+        // this branch is possible because we do fall back from BITS failures by
+        // attempting an internal download.
+        // If this happens, we are just going to need to wait for interactive
+        // Firefox to download the update. We don't, however, want to be in the
+        // "downloading" state when interactive Firefox runs because we want to
+        // download the newest update available which, at that point, may not be
+        // the one that we are currently trying to download.
+        // However, we can't just unconditionally clobber the current update
+        // because interactive Firefox might already be part way through an
+        // internal update download, and we definitely don't want to interrupt
+        // that.
+        let readyUpdateDir = getReadyUpdateDir();
+        let status = readStatusFile(readyUpdateDir);
+        // nsIIncrementalDownload doesn't use an intermediate download location
+        // for partially downloaded files. If we have started an update
+        // download with it, it will be available at its ultimate location.
+        if (!(status == STATE_DOWNLOADING && patchFile.exists())) {
+          LOG(
+            "Downloader:downloadUpdate - Can't download with internal " +
+              "downloader from a background task. Cleaning up downloading " +
+              "update."
+          );
+          cleanupDownloadingUpdate();
+        }
+        return false;
+      }
+
+      // The interval is 0 since there is no need to throttle downloads.
+      let interval = 0;
+
+      LOG(
+        "Downloader:downloadUpdate - Starting nsIIncrementalDownload with " +
+          "url: " +
+          this._patch.URL +
+          ", path: " +
+          patchFile.path +
+          ", interval: " +
+          interval
+      );
+      let uri = Services.io.newURI(this._patch.URL);
+
+      this._request = Cc[
+        "@mozilla.org/network/incremental-download;1"
+      ].createInstance(Ci.nsIIncrementalDownload);
+      this._request.init(uri, patchFile, DOWNLOAD_CHUNK_SIZE, interval);
+      this._request.start(this, null);
+    } else {
+      let noProgressTimeout = BITS_IDLE_NO_PROGRESS_TIMEOUT_SECS;
+      let monitorInterval = BITS_IDLE_POLL_RATE_MS;
+      this._bitsActiveNotifications = false;
+      // The monitor's timeout should be much greater than the longest monitor
+      // poll interval. If the timeout is too short, delay in the pipe to the
+      // update agent might cause BITS to falsely report an error, causing an
+      // unnecessary fallback to nsIIncrementalDownload.
+      let monitorTimeout = Math.max(10 * monitorInterval, 10 * 60 * 1000);
+      if (this.hasDownloadListeners) {
+        noProgressTimeout = BITS_ACTIVE_NO_PROGRESS_TIMEOUT_SECS;
+        monitorInterval = BITS_ACTIVE_POLL_RATE_MS;
+        this._bitsActiveNotifications = true;
+      }
+
+      let updateRootDir = FileUtils.getDir(KEY_UPDROOT, [], true);
+      let jobName = "MozillaUpdate " + updateRootDir.leafName;
+      let updatePath = updateDir.path;
+      if (!Bits.initialized) {
+        Bits.init(jobName, updatePath, monitorTimeout);
+      }
+
+      this._cancelPromise = null;
+
+      let bitsId = this._patch.getProperty("bitsId");
+      if (bitsId) {
+        LOG(
+          "Downloader:downloadUpdate - Connecting to in-progress download. " +
+            "BITS ID: " +
+            bitsId
+        );
+
+        this._pendingRequest = Bits.monitorDownload(
+          bitsId,
+          monitorInterval,
+          this,
+          null
+        );
+      } else {
+        LOG(
+          "Downloader:downloadUpdate - Starting BITS download with url: " +
+            this._patch.URL +
+            ", updateDir: " +
+            updatePath +
+            ", filename: " +
+            FILE_UPDATE_MAR
+        );
+
+        this._pendingRequest = Bits.startDownload(
+          this._patch.URL,
+          FILE_UPDATE_MAR,
+          Ci.nsIBits.PROXY_PRECONFIG,
+          noProgressTimeout,
+          monitorInterval,
+          this,
+          null
+        );
+      }
+      let request;
+      try {
+        request = await this._pendingRequest;
+      } catch (error) {
+        if (
+          (error.type == Ci.nsIBits.ERROR_TYPE_FAILED_TO_GET_BITS_JOB ||
+            error.type == Ci.nsIBits.ERROR_TYPE_FAILED_TO_CONNECT_TO_BCM) &&
+          error.action == Ci.nsIBits.ERROR_ACTION_MONITOR_DOWNLOAD &&
+          error.stage == Ci.nsIBits.ERROR_STAGE_BITS_CLIENT &&
+          error.codeType == Ci.nsIBits.ERROR_CODE_TYPE_HRESULT &&
+          error.code == HRESULT_E_ACCESSDENIED
+        ) {
+          LOG(
+            "Downloader:downloadUpdate - Failed to connect to existing " +
+              "BITS job. It is likely owned by another user."
+          );
+          // This isn't really a failure code since the BITS job may be working
+          // just fine on another account, so convert this to a code that
+          // indicates that. This will make it easier to identify in telemetry.
+          error.type = Ci.nsIBits.ERROR_TYPE_ACCESS_DENIED_EXPECTED;
+          error.codeType = Ci.nsIBits.ERROR_CODE_TYPE_NONE;
+          error.code = null;
+          // When we detect this situation, disable BITS until Firefox shuts
+          // down. There are a couple of reasons for this. First, without any
+          // kind of flag, we enter an infinite loop here where we keep trying
+          // BITS over and over again (normally setting bitsResult prevents
+          // this, but we don't know the result of the BITS job, so we don't
+          // want to set that). Second, since we are trying to update, this
+          // process must have the update mutex. We don't ever give up the
+          // update mutex, so even if the other user starts Firefox, they will
+          // not complete the BITS job while this Firefox instance is around.
+          gBITSInUseByAnotherUser = true;
+        } else {
+          this._patch.setProperty("bitsResult", Cr.NS_ERROR_FAILURE);
+          lazy.UM.saveUpdates();
+
+          LOG(
+            "Downloader:downloadUpdate - Failed to start to BITS job. " +
+              "Error: " +
+              error
+          );
+        }
+
+        this._pendingRequest = null;
+
+        AUSTLMY.pingBitsError(this.isCompleteUpdate, error);
+
+        // Try download again with nsIIncrementalDownload
+        return this.downloadUpdate(this._update);
+      }
+
+      this._request = request;
+      this._patch.setProperty("bitsId", request.bitsId);
+
+      LOG(
+        "Downloader:downloadUpdate - BITS download running. BITS ID: " +
+          request.bitsId
+      );
+
+      if (this.hasDownloadListeners) {
+        this._maybeStartActiveNotifications();
+      } else {
+        this._maybeStopActiveNotifications();
+      }
+
+      lazy.UM.saveUpdates();
+      this._pendingRequest = null;
+    }
+
+    if (!lazy.UM.readyUpdate) {
+      LOG("Downloader:downloadUpdate - Setting status to downloading");
+      writeStatusFile(getReadyUpdateDir(), STATE_DOWNLOADING);
+    }
+    if (this._patch.state != STATE_DOWNLOADING) {
+      LOG("Downloader:downloadUpdate - Setting state to downloading");
+      this._patch.state = STATE_DOWNLOADING;
+      lazy.UM.saveUpdates();
+    }
+
+    // If we are downloading a second update, we don't change the state until
+    // STATE_SWAP.
+    if (lazy.AUS.currentState == Ci.nsIApplicationUpdateService.STATE_PENDING) {
+      LOG(
+        "Downloader:downloadUpdate - not setting state because download is " +
+          "already pending."
+      );
+    } else {
+      LOG(
+        "Downloader:downloadUpdate - setting currentState to STATE_DOWNLOADING"
+      );
+      transitionState(Ci.nsIApplicationUpdateService.STATE_DOWNLOADING);
+    }
+
+    this._startLangPackUpdates();
+
+    this._notifyDownloadStatusObservers();
+
+    return true;
+  },
+
+  /**
+   * This is run when a download listener is added.
+   */
+  onDownloadListenerAdded: function Downloader_onDownloadListenerAdded() {
+    // Increase the status update frequency when someone starts listening
+    this._maybeStartActiveNotifications();
+  },
+
+  /**
+   * This is run when a download listener is removed.
+   */
+  onDownloadListenerRemoved: function Downloader_onDownloadListenerRemoved() {
+    // Decrease the status update frequency when no one is listening
+    if (!this.hasDownloadListeners) {
+      this._maybeStopActiveNotifications();
+    }
+  },
+
+  get hasDownloadListeners() {
+    return this.updateService.hasDownloadListeners;
+  },
+
+  /**
+   * This speeds up BITS progress notifications in response to a user watching
+   * the notifications.
+   */
+  _maybeStartActiveNotifications:
+    async function Downloader__maybeStartActiveNotifications() {
+      if (
+        this.usingBits &&
+        !this._bitsActiveNotifications &&
+        this.hasDownloadListeners &&
+        this._request
+      ) {
+        LOG(
+          "Downloader:_maybeStartActiveNotifications - Starting active " +
+            "notifications"
+        );
+        this._bitsActiveNotifications = true;
+        await Promise.all([
+          this._request
+            .setNoProgressTimeout(BITS_ACTIVE_NO_PROGRESS_TIMEOUT_SECS)
+            .catch(error => {
+              LOG(
+                "Downloader:_maybeStartActiveNotifications - Failed to set " +
+                  "no progress timeout. Error: " +
+                  error
+              );
+            }),
+          this._request
+            .changeMonitorInterval(BITS_ACTIVE_POLL_RATE_MS)
+            .catch(error => {
+              LOG(
+                "Downloader:_maybeStartActiveNotifications - Failed to increase " +
+                  "status update frequency. Error: " +
+                  error
+              );
+            }),
+        ]);
+      }
+    },
+
+  /**
+   * This slows down BITS progress notifications in response to a user no longer
+   * watching the notifications.
+   */
+  _maybeStopActiveNotifications:
+    async function Downloader__maybeStopActiveNotifications() {
+      if (
+        this.usingBits &&
+        this._bitsActiveNotifications &&
+        !this.hasDownloadListeners &&
+        this._request
+      ) {
+        LOG(
+          "Downloader:_maybeStopActiveNotifications - Stopping active " +
+            "notifications"
+        );
+        this._bitsActiveNotifications = false;
+        await Promise.all([
+          this._request
+            .setNoProgressTimeout(BITS_IDLE_NO_PROGRESS_TIMEOUT_SECS)
+            .catch(error => {
+              LOG(
+                "Downloader:_maybeStopActiveNotifications - Failed to set " +
+                  "no progress timeout: " +
+                  error
+              );
+            }),
+          this._request
+            .changeMonitorInterval(BITS_IDLE_POLL_RATE_MS)
+            .catch(error => {
+              LOG(
+                "Downloader:_maybeStopActiveNotifications - Failed to decrease " +
+                  "status update frequency: " +
+                  error
+              );
+            }),
+        ]);
+      }
+    },
+
+  /**
+   * When the async request begins
+   * @param   request
+   *          The nsIRequest object for the transfer
+   */
+  onStartRequest: function Downloader_onStartRequest(request) {
+    if (this.usingBits) {
+      LOG("Downloader:onStartRequest");
+    } else {
+      LOG(
+        "Downloader:onStartRequest - original URI spec: " +
+          request.URI.spec +
+          ", final URI spec: " +
+          request.finalURI.spec
+      );
+      // Set finalURL in onStartRequest if it is different.
+      if (this._patch.finalURL != request.finalURI.spec) {
+        this._patch.finalURL = request.finalURI.spec;
+        lazy.UM.saveUpdates();
+      }
+    }
+
+    this.updateService.forEachDownloadListener(listener => {
+      listener.onStartRequest(request);
+    });
+  },
+
+  /**
+   * When new data has been downloaded
+   * @param   request
+   *          The nsIRequest object for the transfer
+   * @param   progress
+   *          The current number of bytes transferred
+   * @param   maxProgress
+   *          The total number of bytes that must be transferred
+   */
+  onProgress: function Downloader_onProgress(request, progress, maxProgress) {
+    LOG("Downloader:onProgress - progress: " + progress + "/" + maxProgress);
+
+    if (progress > this._patch.size) {
+      LOG(
+        "Downloader:onProgress - progress: " +
+          progress +
+          " is higher than patch size: " +
+          this._patch.size
+      );
+      AUSTLMY.pingDownloadCode(
+        this.isCompleteUpdate,
+        AUSTLMY.DWNLD_ERR_PATCH_SIZE_LARGER
+      );
+      this.cancel(Cr.NS_ERROR_UNEXPECTED);
+      return;
+    }
+
+    // Wait until the transfer has started (progress > 0) to verify maxProgress
+    // so that we don't check it before it is available (in which case, -1 would
+    // have been passed).
+    if (progress > 0 && maxProgress != this._patch.size) {
+      LOG(
+        "Downloader:onProgress - maxProgress: " +
+          maxProgress +
+          " is not equal to expected patch size: " +
+          this._patch.size
+      );
+      AUSTLMY.pingDownloadCode(
+        this.isCompleteUpdate,
+        AUSTLMY.DWNLD_ERR_PATCH_SIZE_NOT_EQUAL
+      );
+      this.cancel(Cr.NS_ERROR_UNEXPECTED);
+      return;
+    }
+
+    this.updateService.forEachDownloadListener(listener => {
+      if (listener instanceof Ci.nsIProgressEventSink) {
+        listener.onProgress(request, progress, maxProgress);
+      }
+    });
+    this.updateService._consecutiveSocketErrors = 0;
+  },
+
+  /**
+   * When we have new status text
+   * @param   request
+   *          The nsIRequest object for the transfer
+   * @param   status
+   *          A status code
+   * @param   statusText
+   *          Human readable version of |status|
+   */
+  onStatus: function Downloader_onStatus(request, status, statusText) {
+    LOG(
+      "Downloader:onStatus - status: " + status + ", statusText: " + statusText
+    );
+
+    this.updateService.forEachDownloadListener(listener => {
+      if (listener instanceof Ci.nsIProgressEventSink) {
+        listener.onStatus(request, status, statusText);
+      }
+    });
+  },
+
+  /**
+   * When data transfer ceases
+   * @param   request
+   *          The nsIRequest object for the transfer
+   * @param   status
+   *          Status code containing the reason for the cessation.
+   */
+  /* eslint-disable-next-line complexity */
+  onStopRequest: async function Downloader_onStopRequest(request, status) {
+    if (gOnlyDownloadUpdatesThisSession) {
+      LOG(
+        "Downloader:onStopRequest - End of update download detected and " +
+          "ignored because we are restricted to update downloads this " +
+          "session. We will continue with this update next session."
+      );
+      // In order to keep the update from progressing past the downloading
+      // stage, we will pretend that the download is still going.
+      // A lot of this work is done for us by just not setting this._request to
+      // null, which usually signals that the transfer has completed.
+      this._pretendingDownloadIsNotDone = true;
+      // This notification is currently used only for testing.
+      Services.obs.notifyObservers(null, "update-download-restriction-hit");
+      return;
+    }
+
+    if (!this.usingBits) {
+      LOG(
+        "Downloader:onStopRequest - downloader: nsIIncrementalDownload, " +
+          "original URI spec: " +
+          request.URI.spec +
+          ", final URI spec: " +
+          request.finalURI.spec +
+          ", status: " +
+          status
+      );
+    } else {
+      LOG("Downloader:onStopRequest - downloader: BITS, status: " + status);
+    }
+
+    let bitsCompletionError;
+    if (this.usingBits) {
+      if (Components.isSuccessCode(status)) {
+        try {
+          await request.complete();
+        } catch (e) {
+          LOG(
+            "Downloader:onStopRequest - Unable to complete BITS download: " + e
+          );
+          status = Cr.NS_ERROR_FAILURE;
+          bitsCompletionError = e;
+        }
+      } else {
+        // BITS jobs that failed to complete should still have cancel called on
+        // them to remove the job.
+        try {
+          await this.cancel();
+        } catch (e) {
+          // This will fail if the job stopped because it was cancelled.
+          // Even if this is a "real" error, there isn't really anything to do
+          // about it, and it's not really a big problem. It just means that the
+          // BITS job will stay around until it is removed automatically
+          // (default of 90 days).
+        }
+      }
+    }
+
+    var state = this._patch.state;
+    var shouldShowPrompt = false;
+    var shouldRegisterOnlineObserver = false;
+    var shouldRetrySoon = false;
+    var deleteActiveUpdate = false;
+    let migratedToReadyUpdate = false;
+    let nonDownloadFailure = false;
+    var retryTimeout = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_SOCKET_RETRYTIMEOUT,
+      DEFAULT_SOCKET_RETRYTIMEOUT
+    );
+    // Prevent the preference from setting a value greater than 10000.
+    retryTimeout = Math.min(retryTimeout, 10000);
+    var maxFail = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_SOCKET_MAXERRORS,
+      DEFAULT_SOCKET_MAX_ERRORS
+    );
+    // Prevent the preference from setting a value greater than 20.
+    maxFail = Math.min(maxFail, 20);
+    LOG(
+      "Downloader:onStopRequest - status: " +
+        status +
+        ", " +
+        "current fail: " +
+        this.updateService._consecutiveSocketErrors +
+        ", " +
+        "max fail: " +
+        maxFail +
+        ", " +
+        "retryTimeout: " +
+        retryTimeout
+    );
+    if (Components.isSuccessCode(status)) {
+      if (this._verifyDownload()) {
+        AUSTLMY.pingDownloadCode(this.isCompleteUpdate, AUSTLMY.DWNLD_SUCCESS);
+
+        LOG(
+          "Downloader:onStopRequest - Clearing readyUpdate in preparation of " +
+            "moving downloadingUpdate into readyUpdate."
+        );
+
+        // Clear out any old update before we notify anyone about the new one.
+        // It will be invalid in a moment anyways when we call
+        // `cleanUpReadyUpdateDir()`.
+        lazy.UM.readyUpdate = null;
+
+        // We're about to clobber the ready update so we can replace it with the
+        // downloading update that just finished. We need to let observers know
+        // about this.
+        if (
+          lazy.AUS.currentState == Ci.nsIApplicationUpdateService.STATE_PENDING
+        ) {
+          transitionState(Ci.nsIApplicationUpdateService.STATE_SWAP);
+        }
+        Services.obs.notifyObservers(this._update, "update-swap");
+
+        // Swap the downloading update into the ready update directory.
+        cleanUpReadyUpdateDir();
+        let downloadedMar = getDownloadingUpdateDir();
+        downloadedMar.append(FILE_UPDATE_MAR);
+        let readyDir = getReadyUpdateDir();
+        try {
+          downloadedMar.moveTo(readyDir, FILE_UPDATE_MAR);
+          migratedToReadyUpdate = true;
+        } catch (e) {
+          migratedToReadyUpdate = false;
+        }
+
+        if (migratedToReadyUpdate) {
+          AUSTLMY.pingMoveResult(AUSTLMY.MOVE_RESULT_SUCCESS);
+          state = getBestPendingState();
+          shouldShowPrompt = !getCanStageUpdates();
+
+          // Tell the updater.exe we're ready to apply.
+          LOG(
+            `Downloader:onStopRequest - Ready to apply. Setting state to ` +
+              `"${state}".`
+          );
+          writeStatusFile(getReadyUpdateDir(), state);
+          writeVersionFile(getReadyUpdateDir(), this._update.appVersion);
+          this._update.installDate = new Date().getTime();
+          this._update.statusText =
+            lazy.gUpdateBundle.GetStringFromName("installPending");
+          Services.prefs.setIntPref(PREF_APP_UPDATE_DOWNLOAD_ATTEMPTS, 0);
+        } else {
+          LOG(
+            "Downloader:onStopRequest - failed to move the downloading " +
+              "update to the ready update directory."
+          );
+          AUSTLMY.pingMoveResult(AUSTLMY.MOVE_RESULT_UNKNOWN_FAILURE);
+
+          state = STATE_DOWNLOAD_FAILED;
+          status = Cr.NS_ERROR_FILE_COPY_OR_MOVE_FAILED;
+
+          const mfCode = "move_failed";
+          let message = getStatusTextFromCode(mfCode, mfCode);
+          this._update.statusText = message;
+
+          nonDownloadFailure = true;
+          deleteActiveUpdate = true;
+
+          cleanUpDownloadingUpdateDir();
+        }
+      } else {
+        LOG("Downloader:onStopRequest - download verification failed");
+        state = STATE_DOWNLOAD_FAILED;
+        status = Cr.NS_ERROR_CORRUPTED_CONTENT;
+
+        // Yes, this code is a string.
+        const vfCode = "verification_failed";
+        var message = getStatusTextFromCode(vfCode, vfCode);
+        this._update.statusText = message;
+
+        if (this._update.isCompleteUpdate || this._update.patchCount != 2) {
+          LOG("Downloader:onStopRequest - No alternative patch to try");
+          deleteActiveUpdate = true;
+        }
+
+        // Destroy the updates directory, since we're done with it.
+        cleanUpDownloadingUpdateDir();
+      }
+    } else if (status == Cr.NS_ERROR_OFFLINE) {
+      // Register an online observer to try again.
+      // The online observer will continue the incremental download by
+      // calling downloadUpdate on the active update which continues
+      // downloading the file from where it was.
+      LOG("Downloader:onStopRequest - offline, register online observer: true");
+      AUSTLMY.pingDownloadCode(
+        this.isCompleteUpdate,
+        AUSTLMY.DWNLD_RETRY_OFFLINE
+      );
+      shouldRegisterOnlineObserver = true;
+      deleteActiveUpdate = false;
+
+      // Each of NS_ERROR_NET_TIMEOUT, ERROR_CONNECTION_REFUSED,
+      // NS_ERROR_NET_RESET and NS_ERROR_DOCUMENT_NOT_CACHED can be returned
+      // when disconnecting the internet while a download of a MAR is in
+      // progress.  There may be others but I have not encountered them during
+      // testing.
+    } else if (
+      (status == Cr.NS_ERROR_NET_TIMEOUT ||
+        status == Cr.NS_ERROR_CONNECTION_REFUSED ||
+        status == Cr.NS_ERROR_NET_RESET ||
+        status == Cr.NS_ERROR_DOCUMENT_NOT_CACHED) &&
+      this.updateService._consecutiveSocketErrors < maxFail
+    ) {
+      LOG("Downloader:onStopRequest - socket error, shouldRetrySoon: true");
+      let dwnldCode = AUSTLMY.DWNLD_RETRY_CONNECTION_REFUSED;
+      if (status == Cr.NS_ERROR_NET_TIMEOUT) {
+        dwnldCode = AUSTLMY.DWNLD_RETRY_NET_TIMEOUT;
+      } else if (status == Cr.NS_ERROR_NET_RESET) {
+        dwnldCode = AUSTLMY.DWNLD_RETRY_NET_RESET;
+      } else if (status == Cr.NS_ERROR_DOCUMENT_NOT_CACHED) {
+        dwnldCode = AUSTLMY.DWNLD_ERR_DOCUMENT_NOT_CACHED;
+      }
+      AUSTLMY.pingDownloadCode(this.isCompleteUpdate, dwnldCode);
+      shouldRetrySoon = true;
+      deleteActiveUpdate = false;
+    } else if (status != Cr.NS_BINDING_ABORTED && status != Cr.NS_ERROR_ABORT) {
+      if (
+        status == Cr.NS_ERROR_FILE_ACCESS_DENIED ||
+        status == Cr.NS_ERROR_FILE_READ_ONLY
+      ) {
+        LOG("Downloader:onStopRequest - permission error");
+        nonDownloadFailure = true;
+      } else {
+        LOG("Downloader:onStopRequest - non-verification failure");
+      }
+
+      let dwnldCode = AUSTLMY.DWNLD_ERR_BINDING_ABORTED;
+      if (status == Cr.NS_ERROR_ABORT) {
+        dwnldCode = AUSTLMY.DWNLD_ERR_ABORT;
+      }
+      AUSTLMY.pingDownloadCode(this.isCompleteUpdate, dwnldCode);
+
+      // Some sort of other failure, log this in the |statusText| property
+      state = STATE_DOWNLOAD_FAILED;
+
+      // XXXben - if |request| (The Incremental Download) provided a means
+      // for accessing the http channel we could do more here.
+
+      this._update.statusText = getStatusTextFromCode(
+        status,
+        Cr.NS_BINDING_FAILED
+      );
+
+      // Destroy the updates directory, since we're done with it.
+      cleanUpDownloadingUpdateDir();
+
+      deleteActiveUpdate = true;
+    }
+    if (!this.usingBits) {
+      LOG(`Downloader:onStopRequest - Setting internalResult to ${status}`);
+      this._patch.setProperty("internalResult", status);
+    } else {
+      LOG(`Downloader:onStopRequest - Setting bitsResult to ${status}`);
+      this._patch.setProperty("bitsResult", status);
+
+      // If we failed when using BITS, we want to override the retry decision
+      // since we need to retry with nsIncrementalDownload before we give up.
+      // However, if the download was cancelled, don't retry. If the transfer
+      // was cancelled, we don't want it to restart on its own.
+      if (
+        !Components.isSuccessCode(status) &&
+        status != Cr.NS_BINDING_ABORTED &&
+        status != Cr.NS_ERROR_ABORT
+      ) {
+        deleteActiveUpdate = false;
+        shouldRetrySoon = true;
+      }
+
+      // Send BITS Telemetry
+      if (Components.isSuccessCode(status)) {
+        AUSTLMY.pingBitsSuccess(this.isCompleteUpdate);
+      } else {
+        let error;
+        if (bitsCompletionError) {
+          error = bitsCompletionError;
+        } else if (status == Cr.NS_ERROR_CORRUPTED_CONTENT) {
+          error = new BitsVerificationError();
+        } else {
+          error = request.transferError;
+          if (!error) {
+            error = new BitsUnknownError();
+          }
+        }
+        AUSTLMY.pingBitsError(this.isCompleteUpdate, error);
+      }
+    }
+
+    LOG("Downloader:onStopRequest - setting state to: " + state);
+    if (this._patch.state != state) {
+      this._patch.state = state;
+    }
+    if (deleteActiveUpdate) {
+      LOG("Downloader:onStopRequest - Clearing downloadingUpdate.");
+      this._update.installDate = new Date().getTime();
+      lazy.UM.addUpdateToHistory(lazy.UM.downloadingUpdate);
+      lazy.UM.downloadingUpdate = null;
+    } else if (
+      lazy.UM.downloadingUpdate &&
+      lazy.UM.downloadingUpdate.state != state
+    ) {
+      lazy.UM.downloadingUpdate.state = state;
+    }
+    if (migratedToReadyUpdate) {
+      LOG(
+        "Downloader:onStopRequest - Moving downloadingUpdate into readyUpdate"
+      );
+      lazy.UM.readyUpdate = lazy.UM.downloadingUpdate;
+      lazy.UM.downloadingUpdate = null;
+    }
+    lazy.UM.saveUpdates();
+
+    // Only notify listeners about the stopped state if we
+    // aren't handling an internal retry.
+    if (!shouldRetrySoon && !shouldRegisterOnlineObserver) {
+      this.updateService.forEachDownloadListener(listener => {
+        listener.onStopRequest(request, status);
+      });
+    }
+
+    this._request = null;
+
+    // This notification must happen after _request is set to null so that
+    // the correct this.updateService.isDownloading value is available in
+    // _notifyDownloadStatusObservers().
+    this._notifyDownloadStatusObservers();
+
+    if (state == STATE_DOWNLOAD_FAILED) {
+      var allFailed = true;
+      // Don't bother retrying the download if we got an error that isn't
+      // download related.
+      if (!nonDownloadFailure) {
+        // If we haven't already, attempt to download without BITS
+        if (request instanceof BitsRequest) {
+          LOG(
+            "Downloader:onStopRequest - BITS download failed. Falling back " +
+              "to nsIIncrementalDownload"
+          );
+          let success = await this.downloadUpdate(this._update);
+          if (!success) {
+            LOG(
+              "Downloader:onStopRequest - Failed to fall back to " +
+                "nsIIncrementalDownload. Cleaning up downloading update."
+            );
+            cleanupDownloadingUpdate();
+          } else {
+            allFailed = false;
+          }
+        }
+
+        // Check if there is a complete update patch that can be downloaded.
+        if (
+          allFailed &&
+          !this._update.isCompleteUpdate &&
+          this._update.patchCount == 2
+        ) {
+          LOG(
+            "Downloader:onStopRequest - verification of patch failed, " +
+              "downloading complete update patch"
+          );
+          this._update.isCompleteUpdate = true;
+          let success = await this.downloadUpdate(this._update);
+
+          if (!success) {
+            LOG(
+              "Downloader:onStopRequest - Failed to fall back to complete " +
+                "patch. Cleaning up downloading update."
+            );
+            cleanupDownloadingUpdate();
+          } else {
+            allFailed = false;
+          }
+        }
+      }
+
+      if (allFailed) {
+        let downloadAttempts = Services.prefs.getIntPref(
+          PREF_APP_UPDATE_DOWNLOAD_ATTEMPTS,
+          0
+        );
+        downloadAttempts++;
+        Services.prefs.setIntPref(
+          PREF_APP_UPDATE_DOWNLOAD_ATTEMPTS,
+          downloadAttempts
+        );
+        let maxAttempts = Math.min(
+          Services.prefs.getIntPref(PREF_APP_UPDATE_DOWNLOAD_MAXATTEMPTS, 2),
+          10
+        );
+
+        transitionState(Ci.nsIApplicationUpdateService.STATE_IDLE);
+
+        if (downloadAttempts > maxAttempts) {
+          LOG(
+            "Downloader:onStopRequest - notifying observers of error. " +
+              "topic: update-error, status: download-attempts-exceeded, " +
+              "downloadAttempts: " +
+              downloadAttempts +
+              " " +
+              "maxAttempts: " +
+              maxAttempts
+          );
+          Services.obs.notifyObservers(
+            this._update,
+            "update-error",
+            "download-attempts-exceeded"
+          );
+        } else {
+          this._update.selectedPatch.selected = false;
+          LOG(
+            "Downloader:onStopRequest - notifying observers of error. " +
+              "topic: update-error, status: download-attempt-failed"
+          );
+          Services.obs.notifyObservers(
+            this._update,
+            "update-error",
+            "download-attempt-failed"
+          );
+        }
+        // We don't care about language pack updates now.
+        this._langPackTimeout = null;
+        LangPackUpdates.delete(unwrap(this._update));
+
+        // Prevent leaking the update object (bug 454964).
+        this._update = null;
+
+        // allFailed indicates that we didn't (successfully) call downloadUpdate
+        // to try to download a different MAR. In this case, this Downloader
+        // is no longer being used.
+        this.updateService._downloader = null;
+      }
+      // A complete download has been initiated or the failure was handled.
+      return;
+    }
+
+    // If the download has succeeded or failed, we are done with this Downloader
+    // object. However, in some cases (ex: network disconnection), we will
+    // attempt to resume using this same Downloader.
+    if (state != STATE_DOWNLOADING) {
+      this.updateService._downloader = null;
+    }
+
+    if (
+      state == STATE_PENDING ||
+      state == STATE_PENDING_SERVICE ||
+      state == STATE_PENDING_ELEVATE
+    ) {
+      if (getCanStageUpdates()) {
+        LOG(
+          "Downloader:onStopRequest - attempting to stage update: " +
+            this._update.name
+        );
+        // Stage the update
+        let stagingStarted = true;
+        try {
+          Cc["@mozilla.org/updates/update-processor;1"]
+            .createInstance(Ci.nsIUpdateProcessor)
+            .processUpdate();
+        } catch (e) {
+          LOG(
+            "Downloader:onStopRequest - failed to stage update. Exception: " + e
+          );
+          stagingStarted = false;
+        }
+        if (stagingStarted) {
+          transitionState(Ci.nsIApplicationUpdateService.STATE_STAGING);
+        } else {
+          // Fail gracefully in case the application does not support the update
+          // processor service.
+          shouldShowPrompt = true;
+        }
+      }
+    }
+
+    // If we're still waiting on language pack updates then run a timer to time
+    // out the attempt after an appropriate amount of time.
+    if (this._langPackTimeout) {
+      // Start a timer to measure how much longer it takes for the language
+      // packs to stage.
+      TelemetryStopwatch.start(
+        "UPDATE_LANGPACK_OVERTIME",
+        unwrap(this._update),
+        { inSeconds: true }
+      );
+
+      lazy.setTimeout(
+        this._langPackTimeout,
+        Services.prefs.getIntPref(
+          PREF_APP_UPDATE_LANGPACK_TIMEOUT,
+          LANGPACK_UPDATE_DEFAULT_TIMEOUT
+        )
+      );
+    }
+
+    // Do this after *everything* else, since it will likely cause the app
+    // to shut down.
+    if (shouldShowPrompt) {
+      // Wait for language packs to stage before showing any prompt to restart.
+      let update = this._update;
+      promiseLangPacksUpdated(update).then(() => {
+        LOG(
+          "Downloader:onStopRequest - Notifying observers that " +
+            "an update was downloaded. topic: update-downloaded, status: " +
+            update.state
+        );
+        transitionState(Ci.nsIApplicationUpdateService.STATE_PENDING);
+        Services.obs.notifyObservers(update, "update-downloaded", update.state);
+      });
+    }
+
+    if (shouldRegisterOnlineObserver) {
+      LOG("Downloader:onStopRequest - Registering online observer");
+      this.updateService._registerOnlineObserver();
+    } else if (shouldRetrySoon) {
+      LOG("Downloader:onStopRequest - Retrying soon");
+      this.updateService._consecutiveSocketErrors++;
+      if (this.updateService._retryTimer) {
+        this.updateService._retryTimer.cancel();
+      }
+      this.updateService._retryTimer = Cc[
+        "@mozilla.org/timer;1"
+      ].createInstance(Ci.nsITimer);
+      this.updateService._retryTimer.initWithCallback(
+        async () => {
+          await this.updateService._attemptResume();
+        },
+        retryTimeout,
+        Ci.nsITimer.TYPE_ONE_SHOT
+      );
+    } else {
+      // Prevent leaking the update object (bug 454964)
+      this._update = null;
+    }
+  },
+
+  /**
+   * This function should be called when shutting down so that resources get
+   * freed properly.
+   */
+  cleanup: async function Downloader_cleanup() {
+    if (this.usingBits) {
+      if (this._pendingRequest) {
+        await this._pendingRequest;
+      }
+      this._request.shutdown();
+    }
+  },
+
+  /**
+   * See nsIInterfaceRequestor.idl
+   */
+  getInterface: function Downloader_getInterface(iid) {
+    // The network request may require proxy authentication, so provide the
+    // default nsIAuthPrompt if requested.
+    if (iid.equals(Ci.nsIAuthPrompt)) {
+      var prompt =
+        Cc["@mozilla.org/network/default-auth-prompt;1"].createInstance();
+      return prompt.QueryInterface(iid);
+    }
+    throw Components.Exception("", Cr.NS_NOINTERFACE);
+  },
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIRequestObserver",
+    "nsIProgressEventSink",
+    "nsIInterfaceRequestor",
+  ]),
+};
+
+// On macOS, all browser windows can be closed without Firefox exiting. If it
+// is left in this state for a while and an update is pending, we should restart
+// Firefox on our own to apply the update. This class will do that
+// automatically.
+class RestartOnLastWindowClosed {
+  #enabled = false;
+  #hasShutdown = false;
+
+  #restartTimer = null;
+  #restartTimerExpired = false;
+
+  constructor() {
+    this.#maybeEnableOrDisable();
+
+    Services.prefs.addObserver(
+      PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_ENABLED,
+      this
+    );
+    Services.obs.addObserver(this, "quit-application");
+  }
+
+  shutdown() {
+    LOG("RestartOnLastWindowClosed.shutdown - Shutting down");
+    this.#hasShutdown = true;
+
+    Services.prefs.removeObserver(
+      PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_ENABLED,
+      this
+    );
+    Services.obs.removeObserver(this, "quit-application");
+
+    this.#maybeEnableOrDisable();
+  }
+
+  get shouldEnable() {
+    if (AppConstants.platform != "macosx") {
+      return false;
+    }
+    if (this.#hasShutdown) {
+      return false;
+    }
+    return Services.prefs.getBoolPref(
+      PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_ENABLED,
+      false
+    );
+  }
+
+  get enabled() {
+    return this.#enabled;
+  }
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "nsPref:changed":
+        if (data == PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_ENABLED) {
+          this.#maybeEnableOrDisable();
+        }
+        break;
+      case "quit-application":
+        this.shutdown();
+        break;
+      case "domwindowclosed":
+        this.#onWindowClose();
+        break;
+      case "domwindowopened":
+        this.#onWindowOpen();
+        break;
+      case "update-downloaded":
+      case "update-staged":
+        this.#onUpdateReady(data);
+        break;
+    }
+  }
+
+  // Returns true if any windows are open. Otherwise, false.
+  #windowsAreOpen() {
+    // eslint-disable-next-line no-unused-vars
+    for (const win of Services.wm.getEnumerator(null)) {
+      return true;
+    }
+    return false;
+  }
+
+  // Enables or disables this class's functionality based on the value of
+  // this.shouldEnable. Does nothing if the class is already in the right state
+  // (i.e. if the class should be enabled and already is, or should be disabled
+  // and already is).
+  #maybeEnableOrDisable() {
+    if (this.shouldEnable) {
+      if (this.#enabled) {
+        return;
+      }
+      LOG("RestartOnLastWindowClosed.#maybeEnableOrDisable - Enabling");
+
+      Services.obs.addObserver(this, "domwindowclosed");
+      Services.obs.addObserver(this, "domwindowopened");
+      Services.obs.addObserver(this, "update-downloaded");
+      Services.obs.addObserver(this, "update-staged");
+
+      this.#restartTimer = null;
+      this.#restartTimerExpired = false;
+
+      this.#enabled = true;
+
+      // Synchronize with external state.
+      this.#onWindowClose();
+    } else {
+      if (!this.#enabled) {
+        return;
+      }
+      LOG("RestartOnLastWindowClosed.#maybeEnableOrDisable - Disabling");
+
+      Services.obs.removeObserver(this, "domwindowclosed");
+      Services.obs.removeObserver(this, "domwindowopened");
+      Services.obs.removeObserver(this, "update-downloaded");
+      Services.obs.removeObserver(this, "update-staged");
+
+      this.#enabled = false;
+
+      if (this.#restartTimer) {
+        this.#restartTimer.cancel();
+      }
+      this.#restartTimer = null;
+    }
+  }
+
+  // Note: Since we keep track of the update state even when this class is
+  //       disabled, this function will run even in that case.
+  #onUpdateReady(updateState) {
+    // Note that we do not count pending-elevate as a ready state, because we
+    // cannot silently restart in that state.
+    if (
+      [
+        STATE_APPLIED,
+        STATE_PENDING,
+        STATE_APPLIED_SERVICE,
+        STATE_PENDING_SERVICE,
+      ].includes(updateState)
+    ) {
+      if (this.#enabled) {
+        LOG("RestartOnLastWindowClosed.#onUpdateReady - update ready");
+        this.#maybeRestartBrowser();
+      }
+    } else if (this.#enabled) {
+      LOG(
+        `RestartOnLastWindowClosed.#onUpdateReady - Not counting update as ` +
+          `ready because the state is ${updateState}`
+      );
+    }
+  }
+
+  #onWindowClose() {
+    if (!this.#windowsAreOpen()) {
+      this.#onLastWindowClose();
+    }
+  }
+
+  #onLastWindowClose() {
+    if (this.#restartTimer || this.#restartTimerExpired) {
+      LOG(
+        "RestartOnLastWindowClosed.#onLastWindowClose - Restart timer is " +
+          "either already running or has already expired"
+      );
+      return;
+    }
+
+    let timeout = Services.prefs.getIntPref(
+      PREF_APP_UPDATE_NO_WINDOW_AUTO_RESTART_DELAY_MS,
+      5 * 60 * 1000
+    );
+
+    LOG(
+      "RestartOnLastWindowClosed.#onLastWindowClose - Last window closed. " +
+        "Starting restart timer"
+    );
+    this.#restartTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    this.#restartTimer.initWithCallback(
+      () => this.#onRestartTimerExpire(),
+      timeout,
+      Ci.nsITimer.TYPE_ONE_SHOT
+    );
+  }
+
+  #onWindowOpen() {
+    if (this.#restartTimer) {
+      LOG(
+        "RestartOnLastWindowClosed.#onWindowOpen - Window opened. Cancelling " +
+          "restart timer."
+      );
+      this.#restartTimer.cancel();
+    }
+    this.#restartTimer = null;
+    this.#restartTimerExpired = false;
+  }
+
+  #onRestartTimerExpire() {
+    LOG("RestartOnLastWindowClosed.#onRestartTimerExpire - Timer Expired");
+
+    this.#restartTimer = null;
+    this.#restartTimerExpired = true;
+    this.#maybeRestartBrowser();
+  }
+
+  #maybeRestartBrowser() {
+    if (!this.#restartTimerExpired) {
+      LOG(
+        "RestartOnLastWindowClosed.#maybeRestartBrowser - Still waiting for " +
+          "all windows to be closed and restartTimer to expire. " +
+          "(not restarting)"
+      );
+      return;
+    }
+
+    if (lazy.AUS.currentState != Ci.nsIApplicationUpdateService.STATE_PENDING) {
+      LOG(
+        "RestartOnLastWindowClosed.#maybeRestartBrowser - No update ready. " +
+          "(not restarting)"
+      );
+      return;
+    }
+
+    if (getElevationRequired()) {
+      // We check for STATE_PENDING_ELEVATE elsewhere, but this is actually
+      // different from that because it is technically possible that the user
+      // gave permission to elevate, but we haven't actually elevated yet.
+      // This is a bit of a corner case. We only call elevationOptedIn() right
+      // before we restart to apply the update immediately. But it is possible
+      // that something could stop the browser from shutting down.
+      LOG(
+        "RestartOnLastWindowClosed.#maybeRestartBrowser - This update will " +
+          "require user elevation (not restarting)"
+      );
+      return;
+    }
+
+    if (this.#windowsAreOpen()) {
+      LOG(
+        "RestartOnLastWindowClosed.#maybeRestartBrowser - Window " +
+          "unexpectedly still open! (not restarting)"
+      );
+      return;
+    }
+
+    if (!this.shouldEnable) {
+      LOG(
+        "RestartOnLastWindowClosed.#maybeRestartBrowser - Unexpectedly " +
+          "attempted to restart when RestartOnLastWindowClosed ought to be " +
+          "disabled! (not restarting)"
+      );
+      return;
+    }
+
+    LOG("RestartOnLastWindowClosed.#maybeRestartBrowser - Restarting now");
+    Services.telemetry.scalarAdd("update.no_window_auto_restarts", 1);
+    Services.startup.quit(
+      Ci.nsIAppStartup.eAttemptQuit |
+        Ci.nsIAppStartup.eRestart |
+        Ci.nsIAppStartup.eSilently
+    );
+  }
+}
+// Nothing actually uses this variable at the moment, but let's make sure that
+// we hold the reference to the RestartOnLastWindowClosed instance somewhere.
+// eslint-disable-next-line no-unused-vars
+let restartOnLastWindowClosed = new RestartOnLastWindowClosed();