On my 64-bit Win7 laptop, I've had problems with Firefox for months, to the point where the browser is paralyzed in an endless loop whenever I try to access any site. It might have something to do with my overabundance of anti-everything utilities (AVG, iObit, Malwarebytes, etc.), but I still can't trace the specific root of the problem. I'm now using Google Chrome, which feels light and nimble but is also still feature-deficient. I'm dumping my history and download manager by hand every other hour!
But on my XP workstation, Firefox works just as fine as it always has.