Ajax Petition Confirmation Not Working in Firefox/Safari

My petition is live here: http://savethebonfirerings.com/

It works great in Chrome (mac), but when the form is submitted in Firefox or Safari, the ajax confirmation is not working. It just spins without any confirmation. On the back end, the user’s emails are coming in as expected, but on the front end the user is left without any confirmation.

It must be a script conflict, but I can’t figure out what it is. I’ve tried deactivating my plugins and nothing works.

Please help!

http://wordpress.org/extend/plugins/speakup-email-petitions/