--- a/testing/mochitest/bootstrap.js
+++ b/testing/mochitest/bootstrap.js
@@ -2,77 +2,52 @@
  * 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/. */
 
 ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
 ChromeUtils.import("resource://gre/modules/NetUtil.jsm");
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
 
+function loadChromeScripts(win) {
+  Services.scriptloader.loadSubScript("chrome://mochikit/content/chrome-harness.js", win);
+  Services.scriptloader.loadSubScript("chrome://mochikit/content/mochitest-e10s-utils.js", win);
+  Services.scriptloader.loadSubScript("chrome://mochikit/content/browser-test.js", win);
+}
+
 /////// Android ///////
 
 Cu.importGlobalProperties(["TextDecoder"]);
 
-class DefaultMap extends Map {
-  constructor(defaultConstructor = undefined, init = undefined) {
-    super(init);
-    if (defaultConstructor) {
-      this.defaultConstructor = defaultConstructor;
-    }
-  }
-
-  get(key) {
-    let value = super.get(key);
-    if (value === undefined && !this.has(key)) {
-      value = this.defaultConstructor(key);
-      this.set(key, value);
-    }
-    return value;
-  }
-}
-
 const windowTracker = {
   init() {
     Services.obs.addObserver(this, "chrome-document-global-created");
   },
 
-  overlays: new DefaultMap(() => new Set()),
-
   async observe(window, topic, data) {
     if (topic === "chrome-document-global-created") {
       await new Promise(resolve =>
         window.addEventListener("DOMContentLoaded", resolve, {once: true}));
 
       let {document} = window;
       let {documentURI} = document;
 
-      if (this.overlays.has(documentURI)) {
-        for (let overlay of this.overlays.get(documentURI)) {
-          document.loadOverlay(overlay, null);
-        }
+      if (documentURI !== "chrome://browser/content/browser.xul") {
+        return;
       }
+      loadChromeScripts(window);
     }
   },
 };
 
-function readSync(uri) {
-  let channel = NetUtil.newChannel({uri, loadUsingSystemPrincipal: true});
-  let buffer = NetUtil.readInputStream(channel.open2());
-  return new TextDecoder().decode(buffer);
-}
-
 function androidStartup(data, reason) {
-  windowTracker.init();
-
-  for (let line of readSync(data.resourceURI.resolve("chrome.manifest")).split("\n")) {
-    let [directive, ...args] = line.trim().split(/\s+/);
-    if (directive === "overlay") {
-      let [url, overlay] = args;
-      windowTracker.overlays.get(url).add(overlay);
-    }
+  // Only browser chrome tests need help starting.
+  let testRoot = Services.prefs.getStringPref("mochitest.testRoot", "");
+  if (testRoot.endsWith("/chrome")) {
+    windowTracker.init();
   }
 }
 
 /////// Desktop ///////
 
 var WindowListener = {
   // browser-test.js is only loaded into the first window. Setup that
   // needs to happen in all navigator:browser windows should go here.
@@ -111,18 +86,17 @@ function loadMochitest(e) {
   win.loadURI(url);
   if (flavor == "mochitest") {
     return;
   }
 
   WindowListener.setupWindow(win);
   Services.wm.addListener(WindowListener);
 
-  let overlay = "chrome://mochikit/content/browser-test-overlay.xul";
-  win.document.loadOverlay(overlay, null);
+  loadChromeScripts(win);
 }
 
 function startup(data, reason) {
   if (AppConstants.platform == "android") {
     androidStartup(data, reason);
   } else {
     let win = Services.wm.getMostRecentWindow("navigator:browser");
     // wait for event fired from start_desktop.js containing the

deleted file mode 100644
--- a/testing/mochitest/browser-test-overlay.xul
+++ /dev/null
@@ -1,12 +0,0 @@
-<?xml version="1.0"?>
-<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
-<!-- 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/. -->
-
-<overlay id="browserTestOverlay"
-         xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
-  <script type="application/javascript" src="chrome://mochikit/content/chrome-harness.js"/>
-  <script type="application/javascript" src="chrome://mochikit/content/mochitest-e10s-utils.js"/>
-  <script type="application/javascript" src="chrome://mochikit/content/browser-test.js"/>
-</overlay>

--- a/testing/mochitest/jar.mn
+++ b/testing/mochitest/jar.mn
@@ -1,13 +1,12 @@
 mochikit.jar:
 % content mochikit %content/
   content/browser-harness.xul (browser-harness.xul)
   content/browser-test.js (browser-test.js)
-  content/browser-test-overlay.xul (browser-test-overlay.xul)
   content/chrome-harness.js (chrome-harness.js)
   content/mochitest-e10s-utils.js (mochitest-e10s-utils.js)
   content/shutdown-leaks-collector.js (shutdown-leaks-collector.js)
   content/ShutdownLeaksCollector.jsm (ShutdownLeaksCollector.jsm)
   content/harness.xul (harness.xul)
   content/redirect.html (redirect.html)
   content/server.js (server.js)
   content/chunkifyTests.js (chunkifyTests.js)

--- a/testing/mochitest/moz.build
+++ b/testing/mochitest/moz.build
@@ -37,17 +37,16 @@ TEST_HARNESS_FILES.testing.mochitest += 
     '/build/sanitizers/lsan_suppressions.txt',
     '/build/sanitizers/ubsan_suppressions.txt',
     '/build/valgrind/cross-architecture.sup',
     '/build/valgrind/i386-pc-linux-gnu.sup',
     '/build/valgrind/x86_64-pc-linux-gnu.sup',
     '/netwerk/test/httpserver/httpd.js',
     'bisection.py',
     'browser-harness.xul',
-    'browser-test-overlay.xul',
     'browser-test.js',
     'chrome-harness.js',
     'chunkifyTests.js',
     'gen_template.pl',
     'harness.xul',
     'leaks.py',
     'mach_test_package_commands.py',
     'manifest.webapp',

--- a/testing/mochitest/runtestsremote.py
+++ b/testing/mochitest/runtestsremote.py
@@ -193,30 +193,16 @@ class MochiRemote(MochitestDesktop):
         if savedTestingModulesDir:
             options.testingModulesDir = savedTestingModulesDir
         self.localProfile = options.profilePath
 
         restoreRemotePaths()
         options.profilePath = self.remoteProfile
         return manifest
 
-    def addChromeToProfile(self, options):
-        manifest = MochitestDesktop.addChromeToProfile(self, options)
-
-        # Support Firefox (browser), SeaMonkey (navigator), and Webapp Runtime (webapp).
-        if options.flavor == 'chrome':
-            # append overlay to chrome.manifest
-            chrome = ("overlay chrome://browser/content/browser.xul "
-                      "chrome://mochikit/content/browser-test-overlay.xul")
-            path = os.path.join(options.profilePath, 'extensions', 'staged',
-                                'mochikit@mozilla.org', 'chrome.manifest')
-            with open(path, "a") as f:
-                f.write(chrome)
-        return manifest
-
     def buildURLOptions(self, options, env):
         self.localLog = options.logFile
         options.logFile = self.remoteLog
         options.fileLevel = 'INFO'
         options.profilePath = self.localProfile
         env["MOZ_HIDE_RESULTS_TABLE"] = "1"
         retVal = MochitestDesktop.buildURLOptions(self, options, env)