var ios = Components.classes['@mozilla.org/network/io-service;1'].getService(Components.interfaces.nsIIOService);
var fileHandler = ios.getProtocolHandler("file").QueryInterface(Components.interfaces.nsIFileProtocolHandler);
var chromePath = fileHandler.getURLSpecFromFile(Components.classes['@mozilla.org/file/directory_service;1'].getService(Components.interfaces.nsIProperties).get("UChrm", Components.interfaces.nsIFile));