Sounds like something's up with one of your addons that you've installed.
Install it again, and in the menu it creates in Start menu -> Programs, there should be a "firefox (safe mode)" item.
If so, try running that. If it isn't there, try this:
http://kb.mozillazine.org/Safe_mode#Windows
Basically, safe mode disables all add-ons, so that just firefox starts. The issue is unlikely to be firefox itself, and more likely either an extension not working with version 3 correctly, or the mover program from version 2 to 3.