Running Windows 7 32 bit and when I launch Firefox 13.0.1, for a split second, it has the appearance of 'aero peek' in that you can see through it before it turns to a 'normal' display. I have tried turning off 'aero peek' but it still does it. Once properly launched it works perfectly.
My lap top running the same version of Firefox behaves normally.
I have tried all sorts of things, including new graphics drivers but nothing seemed to work - until I moved the tabs to 'Tabs On Top'. It worked!! I've no idea why it should make a difference but it's ok now.