The Tor Browser / file revision

 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);