How to read and write prefs.js (Preference) Firefox

Have to call the below JavaScript from the jsp page onload

function load() {

      try

      {

          netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");

          var prefManager = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefService).getBranch("extensions.ietab2.");

          if (prefManager.prefHasUserValue("filterlist")) {

              prefString = prefManager.getCharPref("filterlist");

              if (prefString.indexOf("http://127.0.0.1:7001/*") == -1){

              prefManager.setCharPref("filterlist",prefManager.getCharPref("filterlist")+" http://127.0.0.1:7001/*");

              }

          }

          else {

              prefString = "0";

          }

          window.open("http://127.0.0.1:7101/Application/login.jsp",'_SELF');

      }

          catch(err)

          {       

          }

      }