diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/browser_upgrade_backup.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/browser_upgrade_backup.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,49 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+
+function test() {
+  waitForExplicitFinish();
+
+  Task.spawn(function task() {
+    try {
+      // Wait until initialization is complete
+      yield SessionStore.promiseInitialized;
+
+      const PREF_UPGRADE = "browser.sessionstore.upgradeBackup.latestBuildID";
+      let buildID = Services.appinfo.platformBuildID;
+
+      // Write state once before starting the test to
+      // ensure sessionstore.js writes won't happen in between.
+      yield forceSaveState();
+
+      // Force backup to take place with a file decided by us
+      Services.prefs.setCharPref(PREF_UPGRADE, "");
+      let contents = "browser_upgrade_backup.js";
+      let pathStore = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js");
+      yield OS.File.writeAtomic(pathStore, contents, { tmpPath: pathStore + ".tmp" });
+      yield SessionStore._internal._performUpgradeBackup();
+      is(Services.prefs.getCharPref(PREF_UPGRADE), buildID, "upgrade backup should be set (again)");
+
+      let pathBackup = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.bak-" + Services.appinfo.platformBuildID);
+      is((yield OS.File.exists(pathBackup)), true, "upgrade backup file has been created");
+
+      let data = yield OS.File.read(pathBackup);
+      is(new TextDecoder().decode(data), contents, "upgrade backup contains the expected contents");
+
+      // Ensure that we don't re-backup by accident
+      yield OS.File.writeAtomic(pathStore, "something else entirely", { tmpPath: pathStore + ".tmp" });
+      yield SessionStore._internal._performUpgradeBackup();
+      data = yield OS.File.read(pathBackup);
+      is(new TextDecoder().decode(data), contents, "upgrade backup hasn't changed");
+
+    } catch (ex) {
+      ok(false, "Uncaught error: " + ex + " at " + ex.stack);
+    } finally {
+      finish();
+    }
+  });
+}