64-bit Firefox is the New Default on 64-bit Windows (mozilla.org)
An anonymous reader shares a blog post: Users on 64-bit Windows who download Firefox will now get our 64-bit version by default. That means they'll install a more secure version of Firefox, one that also crashes a whole lot less. How much less? In our tests so far, 64-bit Firefox reduced crashes by 39% on machines with 4GB of RAM or more.
Also when people had 4GB RAM max on their computers there was no advantage of a 64bit OS. The 64-bit applications are larger and they are not faster. If the 32-bit version of Firefox crashes more then it's because they aren't spending as much time maintaining it.
64-bit applications can be faster if the code benefits from the twice available registers.