http://getsatisfaction.com/mozilla/topics/how_to_display_favicons_in_ubiquity

test_favicon.ub.js

CmdUtils.CreateCommand({
  name: 'test favicon',
  preview: function(pb){
    const FS = (Cc['@mozilla.org/browser/favicon-service;1']
                .getService(Ci.nsIFaviconService));
    var uri = Utils.uri(CmdUtils.document.URL);
    try { var faviconUri = FS.getFaviconForPage(uri) } catch([]) {}
    var dataUrl = faviconUri && FS.getFaviconDataAsDataURL(faviconUri);
    pb.innerHTML = <><img src={dataUrl}/><br/>{dataUrl}</>;
  },
});