We had a returning customer come into the shop a few days ago. Everything was working fine on his computer apart from Firefox.
His operating system was Windows 7 Home Premium 64bit.
When he loaded up Firefox, his homepage appeared straight away. But any time he would click on one of the menus, or any of his favourites on his favourites bar, Firefox would go Not Responding, the little Windows 7 ‘waiting’ circle would appear and spin round, and the Firefox window would change to that opaque white colour that tells you Windows 7 is struggling with something.
But if he tried to access the menu again, or click on a bookmark, same problem would happen again.
Internet Explorer was working absolutely fine on this computer, without any similar problems.
The problem turned out to be conflict between a couple of his real-time spyware/malware scanners.
When I disabled these scanners, the Firefox problem went away. So I removed one of the spyware scanners, problem solved.