diff -r 000000000000 -r 6474c204b198 browser/devtools/responsivedesign/test/browser_responsive_devicewidth.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/responsivedesign/test/browser_responsive_devicewidth.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  let instance;
+  let mgr = ResponsiveUI.ResponsiveUIManager;
+
+  waitForExplicitFinish();
+
+  gBrowser.selectedTab = gBrowser.addTab();
+  gBrowser.selectedBrowser.addEventListener("load", function onload() {
+    gBrowser.selectedBrowser.removeEventListener("load", onload, true);
+    waitForFocus(startTest, content);
+  }, true);
+
+  content.location = "data:text/html,mop";
+
+  function startTest() {
+    mgr.once("on", function() {executeSoon(onUIOpen)});
+    document.getElementById("Tools:ResponsiveUI").doCommand();
+  }
+
+  function onUIOpen() {
+    instance = gBrowser.selectedTab.__responsiveUI;
+    instance.stack.setAttribute("notransition", "true");
+    ok(instance, "instance of the module is attached to the tab.");
+
+    instance.setSize(110, 500);
+    ok(content.innerWidth, 110, "initial width is valid");
+
+    let mql = content.matchMedia("(max-device-width:100px)")
+
+    ok(!mql.matches, "media query doesn't match.");
+
+    mql.addListener(onMediaChange);
+    instance.setSize(90, 500);
+  }
+
+  function onMediaChange(mql) {
+    mql.removeListener(onMediaChange);
+    ok(mql.matches, "media query matches.");
+    ok(window.screen.width != content.screen.width, "screen.width is not the size of the screen.");
+    is(content.screen.width, 90, "screen.width is the width of the page.");
+    is(content.screen.height, 500, "screen.height is the height of the page.");
+
+
+    let docShell = content.QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIWebNavigation)
+                          .QueryInterface(Ci.nsIDocShell);
+
+    mql.addListener(onMediaChange2);
+    docShell.deviceSizeIsPageSize = false;
+  }
+
+  function onMediaChange2(mql) {
+    mql.removeListener(onMediaChange);
+    ok(!mql.matches, "media query has been re-evaluated.");
+    ok(window.screen.width == content.screen.width, "screen.width is not the size of the screen.");
+    instance.stack.removeAttribute("notransition");
+    document.getElementById("Tools:ResponsiveUI").doCommand();
+    gBrowser.removeCurrentTab();
+    finish();
+  }
+}