This might be a problem with firefox, but it might be a problem with your profile, especially if you have installed addons. Try running firefox in Safe Mode, and see if that solves the problem.

Thanks! That worked! Now I've got to find out what addon is causing the problem. Any advice?

I've got 15 Plugins and about the same number of Extensions.

Here's a list of problematic extensions, but if you want to list them here, I can help you find out which on is causing the problem. http://kb.mozillazine.org/Problematic_extensions

Thanks for that. It's been behaving itself since I did the 'safe mode' thing. I suspect it may have been Free Download Manager which is now disabled. I'll see how it goes for the time being. Things are looking good. Thanks for all your help, much appreciated! :-)