The captcha actually looks fine on my machine - I am running Firefox 3.5+ on Mac Snow Leopard and there is no issue with that.
I just tried your form and this doesn't appear to have anything to do with Universal Email as it appears that the form (based on what it did when I tried it) is not validating and even sending the post data.
To troubleshoot, here's what I suggest (as this has helped find similar problems)... go in to the form and add error messages for server validation... It really looks like a server validation issue from what I can see. I would do something simple to get answers quickly like just add the word "error" beside all the fields and then highlight each one and add the server behavior show if server validation fails (of course selecting the correct field for each show if statement). Then run the form again and submit it to see if any server validations are restricting the continuation of sending the form.