Playing with mozIAsyncFavicons https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/mozIAsyncFavicons#getFaviconURLForPage%28%29

main.js

const { Cc, Ci, Cu } = require('chrome');
const FaviconService = Cc["@mozilla.org/browser/favicon-service;1"].
                       getService(Ci.nsIFaviconService);
const IO = Cc["@mozilla.org/network/io-service;1"].
           getService(Ci.nsIIOService);
const asyncFavicons = FaviconService.QueryInterface(Ci.mozIAsyncFavicons);
let url = IO.newURI('http://www.mozilla.org', null, null);

// PRINT:    URI: null len: 0 data:  type: 
asyncFavicons.getFaviconDataForPage(url, function (aURI, aDataLen, aData, aMimeType) {
  console.log(
    'URI: ' + aURI,
    'len: ' + aDataLen,
    'data: ' + aData,
    'type: ' + aMimeType);
});

// PRINT:    URI: null len: 0 data:  type: 
asyncFavicons.getFaviconURLForPage(url, function (aURI, aDataLen, aData, aMimeType) {
  console.log(
    'URI: ' + aURI,
    'len: ' + aDataLen,
    'data: ' + aData,
    'type: ' + aMimeType);
});