new file mode 100644
--- /dev/null
+++ b/toolkit/modules/ServiceRequest.jsm
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { classes: Cc, interfaces: Ci, results: Cr, utils: Cu } = Components;
+
+/**
+  * This module consolidates various code and data update requests, so flags
+  * can be set, Telemetry collected, etc. in a central place.
+  */
+
+Cu.import("resource://gre/modules/Log.jsm");
+Cu.importGlobalProperties(["XMLHttpRequest"]);
+
+this.EXPORTED_SYMBOLS = [ "ServiceRequest" ];
+
+const logger = Log.repository.getLogger("ServiceRequest");
+logger.level = Log.Level.Debug;
+logger.addAppender(new Log.ConsoleAppender(new Log.BasicFormatter()));
+
+/**
+  * ServiceRequest is intended to be a drop-in replacement for current users
+  * of XMLHttpRequest.
+  *
+  * @param {Object} options - Options for underlying XHR, e.g. { mozAnon: bool }
+  */
+class ServiceRequest extends XMLHttpRequest {
+  constructor(options) {
+    super(options);
+  }
+  /**
+    * Opens an XMLHttpRequest, and sets the NSS "beConservative" flag.
+    * Requests are always async.
+    *
+    * @param {String} method - HTTP method to use, e.g. "GET".
+    * @param {String} url - URL to open.
+    * @param {Object} options - Additional options (reserved for future use).
+    */
+  open(method, url, options) {
+    super.open(method, url, true);
+
+    // Disable cutting edge features, like TLS 1.3, where middleboxes might brick us
+    if (super.channel instanceof Ci.nsIHttpChannelInternal) {
+      super.channel.QueryInterface(Ci.nsIHttpChannelInternal).beConservative = true;
+    }
+  }
+}

--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -77,16 +77,17 @@ EXTRA_JS_MODULES += [
     'RemotePageManager.jsm',
     'RemoteSecurityUI.jsm',
     'RemoteWebProgress.jsm',
     'ResetProfile.jsm',
     'secondscreen/RokuApp.jsm',
     'secondscreen/SimpleServiceDiscovery.jsm',
     'SelectContentHelper.jsm',
     'SelectParentHelper.jsm',
+    'ServiceRequest.jsm',
     'Services.jsm',
     'SessionRecorder.jsm',
     'sessionstore/FormData.jsm',
     'sessionstore/ScrollPosition.jsm',
     'sessionstore/XPathGenerator.jsm',
     'ShortcutUtils.jsm',
     'Sntp.jsm',
     'SpatialNavigation.jsm',

new file mode 100644
--- /dev/null
+++ b/toolkit/modules/tests/xpcshell/test_servicerequest_xhr.js
@@ -0,0 +1,25 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var {classes: Cc, interfaces: Ci, utils: Cu} = Components;
+
+Cu.import("resource://gre/modules/ServiceRequest.jsm");
+
+add_task(function* test_tls_conservative() {
+  const request = new ServiceRequest();
+  request.open("GET", "http://example.com", false);
+
+  const sr_channel = request.channel.QueryInterface(Ci.nsIHttpChannelInternal);
+  ok(("beConservative" in sr_channel), "TLS setting is present in SR channel");
+  ok(sr_channel.beConservative, "TLS setting in request channel is set to conservative for SR");
+
+  const xhr = new XMLHttpRequest();
+  xhr.open("GET", "http://example.com", false);
+
+  const xhr_channel = xhr.channel.QueryInterface(Ci.nsIHttpChannelInternal);
+  ok(("beConservative" in xhr_channel), "TLS setting is present in XHR channel");
+  ok(!xhr_channel.beConservative, "TLS setting in request channel is not set to conservative for XHR");
+
+});

--- a/toolkit/modules/tests/xpcshell/xpcshell.ini
+++ b/toolkit/modules/tests/xpcshell/xpcshell.ini
@@ -69,8 +69,9 @@ skip-if = toolkit == 'android'
 skip-if = !updater
 reason = LOCALE is not defined without MOZ_UPDATER
 [test_UpdateUtils_updatechannel.js]
 [test_web_channel.js]
 [test_web_channel_broker.js]
 [test_ZipUtils.js]
 skip-if = toolkit == 'android'
 [test_Log_stackTrace.js]
+[test_servicerequest_xhr.js]