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.

Mozilla

This just makes sense you have 64 bit PC you should be running 64-bit apps.