Seems that this PHP form is only not posting data successfully in Firefox Mac on the Thank You page. Any idea why? Thanks in advance.
There are a couple questions that you should answer to help you and us better asses the problem. The first question is what do you mean by not successfully? Is it not submitting anything? The next question, supposing that it is submitting something, is what is it submitting and how does that differ from what you expected? Also, you may want to tell us what other user agents it does work with. I'm assuming that you really haven't tried all others (Lynx, elinks, Mosaic, etc.).
Your forms should be very robust and expect all sorts of incomplete or badly formed data, so this could be a good thing in that it reveals a flaw in your form processing script. Keep in mind that a user can send in a form without any user agent and could just send the request by hand, thus sending in potentially arbitrary or harmful code.
Without having more information, I can tell you that some browsers handle submit buttons differently. This is particularly true of image submit buttons such as the one that you use in this form.
Finally, to be a little picky, I did notice that you didn't properly close your <input /> tags. I don't know if that has any effect, but you are using an xhtml doctype, so it wouldn't hurt to do it correctly. Also, for accessibility, try replacing the form's <p> tags with <label> tags.