
Source How to tell if Firefox is 32-bit or 64-bit The installer will upgrade Firefox to the 64-bit version but install it to C:\Program Files (x86), breaking Windows conventions of installing 64-bit applications to C:\Program Files. This is a bug in the first release of the 64 bit version: If you need to run 32-bit Firefox or manually install 64-bit Firefox, you can simply download and re-run the Windows (32-bit or 64-bit) Firefox installer from the Firefox platforms and languages download page.I notice it still goes to folder Programs (x86) You can still manually install 64-bit Firefox, if you choose.

Here’s the key thing to know: 64-bit applications can access more memory and are less likely to crash than 32-bit applications.

What’s the difference between 32-bit and 64-bit?

How much less? In our tests so far, 64-bit Firefox reduced crashes by 39% on machines with 4GB of RAM or more. That means you get a more secure version of Firefox, one that also crashes a whole lot less. Users on 64-bit Windows who download Firefox can get our 64-bit version by default.
