That's funny. I installed AVG for two people who use firefox. They still use firefox. Hell it even gives firefox a toolbar.Must be your computer.
When Firefox installation finishes downloading, it says there's some Trojan.Banload thing.
Something on your computer might be trying to inject something into firefox.Probably why Chrome won't go anywhere either.
Safari/IE. Or Opera, but I never liked it, too much.
Would letting Spybot scan overnight work?
Try it and see.
I use Safari 4, it's the worlds fastest web browser, and it's very nice.