I also have this problem with FOP2 2.26, and solved it by disabling websockets in firefox, but I am not able to fix by modifying presence.js.
I have inserted this line into /var/www/html/fop2/js/presence.js:
var disableWebSockets = true;
But this will not resolve the issue. I also tried adding the same line to /usr/local/fop2/fop2.cfg, and again did not solve the issue. Is there a syntax error? Do I need to reboot the server? I have also tried clearing cache in firefox, and 'force updating' presence.js by navigating directly to it in the browser. More info?
John