So use the one that works !
Firefox works fine for me too.
I don't use Firefox, but I do use the "full-fat" version of Mozilla (1.2.1 on this machine, and I think it's 1.4 on another). No troubles at all for me.
I would suggest firewall issues, but if you can post with Firefox on other internet sites then it's probably not that. Stumped thereafter...!
A