diff -r 000000000000 -r 6474c204b198 browser/modules/CustomizationTabPreloader.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/modules/CustomizationTabPreloader.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,217 @@
+/* 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 = ["CustomizationTabPreloader"];
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+
+const HTML_NS = "http://www.w3.org/1999/xhtml";
+const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+const XUL_PAGE = "data:application/vnd.mozilla.xul+xml;charset=utf-8,<window%20id='win'/>";
+const CUSTOMIZATION_URL = "about:customizing";
+
+// The interval between swapping in a preload docShell and kicking off the
+// next preload in the background.
+const PRELOADER_INTERVAL_MS = 600;
+
+function createTimer(obj, delay) {
+  let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  timer.init(obj, delay, Ci.nsITimer.TYPE_ONE_SHOT);
+  return timer;
+}
+
+function clearTimer(timer) {
+  if (timer) {
+    timer.cancel();
+  }
+  return null;
+}
+
+this.CustomizationTabPreloader = {
+  uninit: function () {
+    CustomizationTabPreloaderInternal.uninit();
+  },
+
+  newTab: function (aTab) {
+    return CustomizationTabPreloaderInternal.newTab(aTab);
+  },
+
+  /**
+   * ensurePreloading starts the preloading of the about:customizing
+   * content page. This function is idempotent (until a call to uninit),
+   * so multiple calls to it are fine.
+   */
+  ensurePreloading: function() {
+    CustomizationTabPreloaderInternal.ensurePreloading();
+  },
+};
+
+Object.freeze(CustomizationTabPreloader);
+
+this.CustomizationTabPreloaderInternal = {
+  _browser: null,
+
+  uninit: function () {
+    HostFrame.destroy();
+
+    if (this._browser) {
+      this._browser.destroy();
+      this._browser = null;
+    }
+  },
+
+  newTab: function (aTab) {
+    let win = aTab.ownerDocument.defaultView;
+    if (win.gBrowser && this._browser) {
+      return this._browser.swapWithNewTab(aTab);
+    }
+
+    return false;
+  },
+
+  ensurePreloading: function () {
+    if (!this._browser) {
+      this._browser = new HiddenBrowser();
+    }
+  }
+};
+
+function HiddenBrowser() {
+  this._createBrowser();
+}
+
+HiddenBrowser.prototype = {
+  _timer: null,
+
+  get isPreloaded() {
+    return this._browser &&
+           this._browser.contentDocument &&
+           this._browser.contentDocument.readyState === "complete" &&
+           this._browser.currentURI.spec === CUSTOMIZATION_URL;
+  },
+
+  swapWithNewTab: function (aTab) {
+    if (!this.isPreloaded || this._timer) {
+      return false;
+    }
+
+    let win = aTab.ownerDocument.defaultView;
+    let tabbrowser = win.gBrowser;
+
+    if (!tabbrowser) {
+      return false;
+    }
+
+    // Swap docShells.
+    tabbrowser.swapNewTabWithBrowser(aTab, this._browser);
+
+    // Load all default frame scripts attached to the target window.
+    let mm = aTab.linkedBrowser.messageManager;
+    let scripts = win.messageManager.getDelayedFrameScripts();
+    Array.forEach(scripts, ([script, runGlobal]) => mm.loadFrameScript(script, true, runGlobal));
+
+    // Remove the browser, it will be recreated by a timer.
+    this._removeBrowser();
+
+    // Start a timer that will kick off preloading the next page.
+    this._timer = createTimer(this, PRELOADER_INTERVAL_MS);
+
+    // Signal that we swapped docShells.
+    return true;
+  },
+
+  observe: function () {
+    this._timer = null;
+
+    // Start pre-loading the customization page.
+    this._createBrowser();
+  },
+
+  destroy: function () {
+    this._removeBrowser();
+    this._timer = clearTimer(this._timer);
+  },
+
+  _createBrowser: function () {
+    HostFrame.get().then(aFrame => {
+      let doc = aFrame.document;
+      this._browser = doc.createElementNS(XUL_NS, "browser");
+      this._browser.setAttribute("type", "content");
+      this._browser.setAttribute("src", CUSTOMIZATION_URL);
+      this._browser.style.width = "400px";
+      this._browser.style.height = "400px";
+      doc.getElementById("win").appendChild(this._browser);
+    });
+  },
+
+  _removeBrowser: function () {
+    if (this._browser) {
+      this._browser.remove();
+      this._browser = null;
+    }
+  }
+};
+
+let HostFrame = {
+  _frame: null,
+  _deferred: null,
+
+  get hiddenDOMDocument() {
+    return Services.appShell.hiddenDOMWindow.document;
+  },
+
+  get isReady() {
+    return this.hiddenDOMDocument.readyState === "complete";
+  },
+
+  get: function () {
+    if (!this._deferred) {
+      this._deferred = Promise.defer();
+      this._create();
+    }
+
+    return this._deferred.promise;
+  },
+
+  destroy: function () {
+    if (this._frame) {
+      if (!Cu.isDeadWrapper(this._frame)) {
+        this._frame.removeEventListener("load", this, true);
+        this._frame.remove();
+      }
+
+      this._frame = null;
+      this._deferred = null;
+    }
+  },
+
+  handleEvent: function () {
+    let contentWindow = this._frame.contentWindow;
+    if (contentWindow.location.href === XUL_PAGE) {
+      this._frame.removeEventListener("load", this, true);
+      this._deferred.resolve(contentWindow);
+    } else {
+      contentWindow.location = XUL_PAGE;
+    }
+  },
+
+  _create: function () {
+    if (this.isReady) {
+      let doc = this.hiddenDOMDocument;
+      this._frame = doc.createElementNS(HTML_NS, "iframe");
+      this._frame.addEventListener("load", this, true);
+      doc.documentElement.appendChild(this._frame);
+    } else {
+      let flags = Ci.nsIThread.DISPATCH_NORMAL;
+      Services.tm.currentThread.dispatch(() => this._create(), flags);
+    }
+  }
+};