Retrieving the favicon url of the websites from firefox extension
var faviconService = Components.classes["@mozilla.org/browser/favicon-service;1"]
                               .getService(Components.interfaces.nsIFaviconService);
var favicon = faviconService.getFaviconImageForPage(gBrowser.currentURI);
alert(favicon.spec);

nsIServiceManager serviceManager =
Mozilla.getInstance().getServiceManager();
nsIFaviconService service =
(nsIFaviconService)serviceManager.getServiceByContractID("@mozilla.org/brows
er/favicon-service;1", nsIFaviconService.NS_IFAVICONSERVICE_IID);