I do connect through a firewall, but I've used the same one for over 3 years, and never had this problem before. I tried upgrading, still no luck.
However.. one of my friends (I have lots of geeky friends! its awsome) set up a CGI:IRC client causes me to use his ISP and IP instead of my own, and it works fine..
It it would seem to me,
1. Its not the software, since i've updated MIRC, and tried several others.
2. Its not my ISP, because someone I know with the same ISP can connect fine
3. Its not my computer, since I can connect through an alternate program that routes me through someone elses IP, while still using my computer
The only other thing I can think of that would block me is IRC itself, blocking my individual IP out.