Believe it or not, Firefox on Windows has been a 32-bit affair until now -- while Google and Microsoft made the leap to 64-bit web browsing a while back, Mozilla hasn't been quite so quick off the mark. All's well after today, though, as Mozilla has released a 64-bit version of Firefox for Windows. So long as you're running at least Windows 7, the browser can take better advantage of processors from recent years. The software won't necessarily run faster, but you should get improved security, greater app compatibility (some intensive web apps won't run otherwise) and other upgrades that come with the added headroom.
Source: Mozilla (1), (2)