rem: var prefBranch = prefService.getBranch(null).QueryInterface(Components.interfaces.nsIPrefBranchInternal); 
add: var prefBranch = prefService.getBranch(null).QueryInterface(Components.interfaces.nsIPrefBranch2); 
context: function RemovePrefObservers(){  var prefService = Components.classes["@mozilla.org/preferences-service;1"]                              .getService(Components.interfaces.nsIPrefService);  var prefBranch = prefService.getBranch(null).QueryInterface(Components.interfaces.nsIPrefBranchInternal);  prefBranch.removeObserver(kPrefMailAddrBookLastNameFirst, gMailAddrBookLastNameFirstObserver);}
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/10234/361daac9367ad464688c4da3c3206d18cb2ae94e/addressbook.js/buggy/mailnews/addrbook/resources/content/addressbook.js