Firefox 16.0.2 (the latest version) was working ok when I installed it, but today the whole system slowed down and only worked intermittently and screens kept losing focus before I could type in passwords, whenever Firefox was running.
I uninstalled Firefox and all plugins & settings and just ran IE for a while and everything runs smoothly. So I downloaded Waterfox (a 64 bit build from the Firefox code) and installed that. It runs perfectly now too.
I don't know what was wrong with Firefox, but I didn't have as many addons in it as I had before in Windows 7. Windows 8 was a clean (reformatted HD) installation, so there's no rubbish from the old system there.