diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/src/TabStateCache.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/src/TabStateCache.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,82 @@
+/* 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 = ["TabStateCache"];
+
+/**
+ * A cache for tabs data.
+ *
+ * This cache implements a weak map from tabs (as XUL elements)
+ * to tab data (as objects).
+ *
+ * Note that we should never cache private data, as:
+ * - that data is used very seldom by SessionStore;
+ * - caching private data in addition to public data is memory consuming.
+ */
+this.TabStateCache = Object.freeze({
+  /**
+   * Retrieves cached data for a given |browser|.
+   *
+   * @param browser (xul:browser)
+   *        The browser to retrieve cached data for.
+   * @return (object)
+   *         The cached data stored for the given |browser|.
+   */
+  get: function (browser) {
+    return TabStateCacheInternal.get(browser);
+  },
+
+  /**
+   * Updates cached data for a given |browser|.
+   *
+   * @param browser (xul:browser)
+   *        The browser belonging to the given tab data.
+   * @param newData (object)
+   *        The new data to be stored for the given |browser|.
+   */
+  update: function (browser, newData) {
+    TabStateCacheInternal.update(browser, newData);
+  }
+});
+
+let TabStateCacheInternal = {
+  _data: new WeakMap(),
+
+  /**
+   * Retrieves cached data for a given |browser|.
+   *
+   * @param browser (xul:browser)
+   *        The browser to retrieve cached data for.
+   * @return (object)
+   *         The cached data stored for the given |browser|.
+   */
+  get: function (browser) {
+    return this._data.get(browser.permanentKey);
+  },
+
+  /**
+   * Updates cached data for a given |browser|.
+   *
+   * @param browser (xul:browser)
+   *        The browser belonging to the given tab data.
+   * @param newData (object)
+   *        The new data to be stored for the given |browser|.
+   */
+  update: function (browser, newData) {
+    let data = this._data.get(browser.permanentKey) || {};
+
+    for (let key of Object.keys(newData)) {
+      let value = newData[key];
+      if (value === null) {
+        delete data[key];
+      } else {
+        data[key] = value;
+      }
+    }
+
+    this._data.set(browser.permanentKey, data);
+  }
+};