new file mode 100644
--- /dev/null
+++ b/mobile/android/tests/browser/robocop/desktopmode_user_agent.sjs
@@ -0,0 +1,11 @@
+function handleRequest(request, response)
+{
+  // avoid confusing cache behaviors
+  response.setHeader("Cache-Control", "no-cache", false);
+  response.setHeader("Content-Type", "text/plain", false);
+  response.setHeader("Access-Control-Allow-Origin", "*", false);
+
+  // used by mobile/desktop user agent tests
+  response.write(request.getHeader("User-Agent"));
+}
+

--- a/mobile/android/tests/browser/robocop/robocop.ini
+++ b/mobile/android/tests/browser/robocop/robocop.ini
@@ -113,16 +113,17 @@ skip-if = android_version == "10" || and
 
 # Using JavascriptTest
 [testAccounts.java]
 [testAndroidLog.java]
 [testBrowserDiscovery.java]
 # disabled on 4.3, bug 1158384
 skip-if = android_version == "18"
 [testDebuggerServer.java]
+[testDesktopUserAgent.java]
 [testDeviceSearchEngine.java]
 [testFilePicker.java]
 [testHistoryService.java]
 # disabled on 4.3, bug 1116036
 skip-if = android_version == "18"
 [testJavaAddons.java]
 [testJNI.java]
 # [testMozPay.java] # see bug 945675

new file mode 100644
--- /dev/null
+++ b/mobile/android/tests/browser/robocop/testDesktopUserAgent.java
@@ -0,0 +1,11 @@
+/* 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/. */
+
+package org.mozilla.gecko.tests;
+
+public class testDesktopUserAgent extends JavascriptTest {
+    public testDesktopUserAgent() {
+        super("testDesktopUserAgent.js");
+    }
+}

new file mode 100644
--- /dev/null
+++ b/mobile/android/tests/browser/robocop/testDesktopUserAgent.js
@@ -0,0 +1,71 @@
+// -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
+
+Cu.import("resource://gre/modules/Services.jsm");
+
+function ok(passed, text) {
+  do_report_result(passed, text, Components.stack.caller, false);
+}
+
+function is(lhs, rhs, text) {
+  do_report_result(lhs === rhs, text, Components.stack.caller, false);
+}
+
+function promiseBrowserEvent(browser, eventType) {
+  return new Promise((resolve) => {
+    function handle(event) {
+      // Since we'll be redirecting, don't make assumptions about the given URL and the loaded URL
+      if (event.target != browser.contentDocument || event.target.location.href == "about:blank") {
+        do_print("Skipping spurious '" + eventType + "' event" + " for " + event.target.location.href);
+        return;
+      }
+      do_print("Received event " + eventType + " from browser");
+      browser.removeEventListener(eventType, handle, true);
+      resolve(event);
+    }
+
+    browser.addEventListener(eventType, handle, true);
+    do_print("Now waiting for " + eventType + " event from browser");
+  });
+}
+
+// Load a custom sjs script that echos our "User-Agent" header back at us
+const TestURI = Services.io.newURI("http://mochi.test:8888/tests/robocop/desktopmode_user_agent.sjs", null, null);
+
+add_task(function* test_desktopmode() {
+  let chromeWin = Services.wm.getMostRecentWindow("navigator:browser");
+  let BrowserApp = chromeWin.BrowserApp;
+
+  // Add a new 'desktop mode' tab with our test page
+  let desktopBrowser = BrowserApp.addTab(TestURI.spec, { selected: true, parentId: BrowserApp.selectedTab.id, desktopMode: true }).browser;
+  yield promiseBrowserEvent(desktopBrowser, "load");
+
+  // Some debugging
+  do_print("desktop: " + desktopBrowser.contentWindow.navigator.userAgent);
+  do_print("desktop: " + desktopBrowser.contentDocument.body.innerHTML);
+
+  // Check the server UA and the navigator UA for 'desktop'
+  ok(desktopBrowser.contentWindow.navigator.userAgent.indexOf("Linux x86_64") != -1, "window.navigator.userAgent has 'Linux' in it");
+  ok(desktopBrowser.contentDocument.body.innerHTML.indexOf("Linux x86_64") != -1, "HTTP header 'User-Agent' has 'Linux' in it");
+
+  // Add a new 'mobile mode' tab with our test page
+  let mobileBrowser = BrowserApp.addTab(TestURI.spec, { selected: true, parentId: BrowserApp.selectedTab.id }).browser;
+  yield promiseBrowserEvent(mobileBrowser, "load");
+
+  // Some debugging
+  do_print("mobile: " + mobileBrowser.contentWindow.navigator.userAgent);
+  do_print("mobile: " + mobileBrowser.contentDocument.body.innerHTML);
+
+  // Check the server UA and the navigator UA for 'mobile'
+  // We only check for 'Android' because we don't know the version or if it's phone or tablet
+  ok(mobileBrowser.contentWindow.navigator.userAgent.indexOf("Android") != -1, "window.navigator.userAgent has 'Android' in it");
+  ok(mobileBrowser.contentDocument.body.innerHTML.indexOf("Android") != -1, "HTTP header 'User-Agent' has 'Android' in it");
+});
+
+run_next_test();