diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/offlineByDefault.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/offlineByDefault.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,17 @@
+var offlineByDefault = {
+  defaultValue: false,
+  prefBranch: SpecialPowers.Cc["@mozilla.org/preferences-service;1"].getService(SpecialPowers.Ci.nsIPrefBranch),
+  set: function(allow) {
+    try {
+      this.defaultValue = this.prefBranch.getBoolPref("offline-apps.allow_by_default");
+    } catch (e) {
+      this.defaultValue = false
+    }
+    this.prefBranch.setBoolPref("offline-apps.allow_by_default", allow);
+  },
+  reset: function() {
+    this.prefBranch.setBoolPref("offline-apps.allow_by_default", this.defaultValue);
+  }
+}
+
+offlineByDefault.set(false);