First, try updating to Firefox 12. If that fixes your issue great! If it doesn't, Try to start Firefox in Safe mode. This mode disables all extensions, user customizations, hardware acceleration, etc. temporarily for diagnostic and troubleshooting. If Firefox works fine in Safe Mode, then this is likely an add-on issue. Restart Firefox in normal mode, then type "About:addons" (without the quotes) in the address bar. Disable your extensions one by one, restarting Firefox between each one, until you find the trouble maker. Once you do, you can disable that add-on and report the issue to that add-on's developer.

I have just updated to Firefox 12 (it's strange it didn't automatically update, I have it set for 'automatic update'). So far, everything seems fine. Hope this continues. If not, I'll give the safe mode suggestion a try. Thanks so much.

The problem is back. I've checked the add-ons. They aren't the problem.

So if you start in Safe Mode, the issue still exists? Try to create a fresh testing profile (Profile Manager - Create, remove or switch Firefox profiles shows you how :)