Code to access Firefox prefs:

var prefs = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch);//for access to firefox preferences system

To set a string pref:

prefs.setCharPref("EXT.KEY.NAME","VALUE");

To get a string pref:

VARIABLE = prefs.getCharPref("EXT.KEY.NAME");

To check if a string pref exists:

prefs.getPrefType("yubnub.url") == prefs.PREF_STRING