If you want a 64-bit build of Firefox right now, you might want to check out Waterfox, which I've been happily using for a while on Windows 7:

https://www.waterfoxproject.org

I moved from Firefox to Waterfox during a period where Firefox was frequently crashing on me. Waterfox seems to be more stable if you have lots and lots of tabs open.

good to know. i've also seen people mention pale moon.