I'm using Firefox 1.5.0.1 and have no problems logging in to any forum I've tried so far. Maybe try firefox secrity settings regarding the cookie issues because I've had problems like that before with FireFox but on IE as well.
I'd suggest you kill your cookies and cache first in case there are residual settings that persist. First clear all cookies, cache, exit firefox, restart, check settings and try again.
I realise that by clearing the cookies that you will have to re-establish logins to all forums that you go to but it's a good way to start.
You might even backup your old cookies and re-introduce them one by one when the issue is fixed.
Also, ignore the smart-ass forum troll who keeps replying with nothing useful to say. I ignore him all the time
