Help > About Mozilla Firefox.
I'm on 3.0.6 and it's fine for me. Have you just upgraded it, or do you have any add-ons.
Best thing to do is to start Firefox in 'Safe mode'. If you find Firefox in your start menu, there will also be a link to Mozilla Firefox (Safe Mode). This starts it without any extensions, and it might be one of them causing the problem. Have a go at that and report back as to how you get on.
Working fine now but thanks anyway