I'm a bit of a newcomer to this stuff, but I had a problem recently that took ages to get sorted out (with the help of this site - Imar in particular), and the problem turned out to be corrupted HTML code in my page. I cleaned up all of the junk (which had accumulated after trying different things) and hey presto - it suddenly all worked.
So, from my limited exprience, I would check the HTML code looks Ok (no dulpications etc) and try again. If that doesn't work, check via Google for any info that is along the lines of your problem. When I did that, I couldn't find anything that really matched my problem. That's significant, because unless you are the first person to find a new bug (unlikely), then chances are you've got something fundamentally wrong! If you still can't suss it out, start again from scratch (new site if possible) as that way you are not being 'blinded' by an error that you can't see.
P.S. I've had firewall problems in the past too. My solution was to totally disable any firewalls (in my case Norton and the Windows XP firewall - probably best to disconnect from the Internet first if you can) and then try to access the site. When I did that it worked, proving the the site was OK and that the firewall(s) were definately the problem.