diff --git a/netwerk/protocol/http/UserAgentUpdates.jsm b/netwerk/protocol/http/UserAgentUpdates.jsm
new file mode 100644
index 000000000000..0dd874712872
--- /dev/null
+++ b/netwerk/protocol/http/UserAgentUpdates.jsm
@@ -0,0 +1,203 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["UserAgentUpdates"];
+
+const Ci = Components.interfaces;
+const Cc = Components.classes;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(
+  this, "FileUtils", "resource://gre/modules/FileUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(
+  this, "OS", "resource://gre/modules/osfile.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(
+  this, "UpdateChannel", "resource://gre/modules/UpdateChannel.jsm");
+
+XPCOMUtils.defineLazyServiceGetter(
+  this, "gUpdateTimer", "@mozilla.org/updates/timer-manager;1", "nsIUpdateTimerManager");
+
+XPCOMUtils.defineLazyGetter(this, "gApp",
+  function() Cc["@mozilla.org/xre/app-info;1"]
+    .getService(Ci.nsIXULAppInfo).QueryInterface(Ci.nsIXULRuntime)
+);
+
+XPCOMUtils.defineLazyGetter(this, "gDecoder",
+  function() new TextDecoder()
+);
+
+XPCOMUtils.defineLazyGetter(this, "gEncoder",
+  function() new TextEncoder()
+);
+
+const TIMER_ID = "user-agent-updates-timer";
+
+const PREF_UPDATES = "general.useragent.updates.";
+const PREF_UPDATES_ENABLED = PREF_UPDATES + "enabled";
+const PREF_UPDATES_URL = PREF_UPDATES + "url";
+const PREF_UPDATES_INTERVAL = PREF_UPDATES + "interval";
+const PREF_UPDATES_RETRY = PREF_UPDATES + "retry";
+const PREF_UPDATES_TIMEOUT = PREF_UPDATES + "timeout";
+
+const KEY_PROFILEDIR = "ProfD";
+const KEY_APPDIR = "XCurProcD";
+const FILE_UPDATES = "ua-update.json";
+
+const PREF_APP_DISTRIBUTION = "distribution.id";
+const PREF_APP_DISTRIBUTION_VERSION = "distribution.version";
+
+var gInitialized = false;
+
+this.UserAgentUpdates = {
+  init: function(callback) {
+    if (gInitialized) {
+      return;
+    }
+    gInitialized = true;
+
+    this._callback = callback;
+    this._applySavedUpdate();
+
+    Services.prefs.addObserver(PREF_UPDATES, this, false);
+  },
+
+  uninit: function() {
+    if (!gInitialized) {
+      return;
+    }
+    gInitialized = false;
+    Services.prefs.removeObserver(PREF_UPDATES, this);
+  },
+
+  _applyUpdate: function(update) {
+    this._callback(update);
+  },
+
+  _applySavedUpdate: function() {
+    if (!this._getPref(PREF_UPDATES_ENABLED, false)) {
+      // remove previous overrides
+      this._applyUpdate(null);
+      return;
+    }
+    // try loading from profile dir, then from app dir
+    let dirs = [KEY_PROFILEDIR, KEY_APPDIR];
+    dirs.reduce((prevLoad, dir) => {
+      let file = FileUtils.getFile(dir, [FILE_UPDATES], true).path;
+      // tryNext returns promise to read file under dir and parse it
+      let tryNext = () => OS.File.read(file).then(
+        (bytes) => JSON.parse(gDecoder.decode(bytes))
+      );
+      // try to load next one if the previous load failed
+      return prevLoad ? prevLoad.then(null, tryNext) : tryNext();
+    }, null).then(
+      // apply update if loading was successful
+      (update) => this._applyUpdate(update)
+    );
+    this._scheduleUpdate();
+  },
+
+  _saveToFile: function(update) {
+    let file = OS.Path.join(OS.Constants.Path.profileDir, FILE_UPDATES);
+    let bytes = gEncoder.encode(JSON.stringify(update));
+    OS.File.writeAtomic(file, bytes, {tmpPath: file + ".tmp"}).then(
+      null, Cu.reportError
+    );
+  },
+
+  _getPref: function(name, def) {
+    try {
+      switch (typeof def) {
+        case "number": return Services.prefs.getIntPref(name);
+        case "boolean": return Services.prefs.getBoolPref(name);
+      }
+      return Services.prefs.getCharPref(name);
+    } catch (e) {
+      return def;
+    }
+  },
+
+  _getParameters: function() ({
+    "%DATE%": function() Date.now().toString(),
+    "%PRODUCT%": function() gApp.name,
+    "%APP_ID%": function() gApp.ID,
+    "%APP_VERSION%": function() gApp.version,
+    "%BUILD_ID%": function() gApp.appBuildID,
+    "%OS%": function() gApp.OS,
+    "%CHANNEL%": function() UpdateChannel.get(),
+    "%DISTRIBUTION%": function() this._getPref(PREF_APP_DISTRIBUTION, ""),
+    "%DISTRIBUTION_VERSION%": function() this._getPref(PREF_APP_DISTRIBUTION_VERSION, ""),
+  }),
+
+  _getUpdateURL: function() {
+    let url = this._getPref(PREF_UPDATES_URL, "");
+    let params = this._getParameters();
+    return url.replace(/%[A-Z_]+%/g, function(match) {
+      let param = params[match];
+      // preserve the %FOO% string (e.g. as an encoding) if it's not a valid parameter
+      return param ? encodeURIComponent(param()) : match;
+    });
+  },
+
+  _fetchUpdate: function(url, success, error) {
+    let request = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"]
+                    .createInstance(Ci.nsIXMLHttpRequest);
+    request.mozBackgroundRequest = true;
+    request.timeout = this._getPref(PREF_UPDATES_TIMEOUT, 60000);
+    request.open("GET", url, true);
+    request.overrideMimeType("application/json");
+    request.responseType = "json";
+
+    request.addEventListener("load", function() success(request.response));
+    request.addEventListener("error", error);
+    request.send();
+  },
+
+  _update: function() {
+    let url = this._getUpdateURL();
+    url && this._fetchUpdate(url,
+      (function(response) { // success
+        // apply update and save overrides to profile
+        this._applyUpdate(response);
+        this._saveToFile(response);
+        this._scheduleUpdate(); // cancel any retries
+      }).bind(this),
+      (function(response) { // error
+        this._scheduleUpdate(true /* retry */);
+      }).bind(this));
+  },
+
+  _scheduleUpdate: function(retry) {
+    let interval = this._getPref(PREF_UPDATES_INTERVAL, 604800 /* 1 week */);
+    if (retry) {
+      interval = this._getPref(PREF_UPDATES_RETRY, interval);
+    }
+    gUpdateTimer.registerTimer(TIMER_ID, this, Math.max(1, interval));
+  },
+
+  notify: function(timer) {
+    // timer notification
+    if (this._getPref(PREF_UPDATES_ENABLED, false)) {
+      this._update();
+    }
+  },
+
+  observe: function(subject, topic, data) {
+    switch (topic) {
+      case "nsPref:changed":
+        if (data === PREF_UPDATES_ENABLED) {
+          this._applySavedUpdate();
+        } else if (data === PREF_UPDATES_INTERVAL) {
+          this._scheduleUpdate();
+        }
+        break;
+    }
+  },
+};
diff --git a/netwerk/protocol/http/moz.build b/netwerk/protocol/http/moz.build
index d95d6b1bf317..c8b7fa412861 100644
--- a/netwerk/protocol/http/moz.build
+++ b/netwerk/protocol/http/moz.build
@@ -78,6 +78,7 @@ IPDL_SOURCES += [
 ]
 EXTRA_JS_MODULES += [
     'UserAgentOverrides.jsm',
+    'UserAgentUpdates.jsm',
 ]
 
 FAIL_ON_WARNINGS = True