If you're willing to be a guinea pig and you want a 64-bit version of Firefox to match your 64-bit version of Windows, Mozilla has made their Firefox 64 pre-release available for download.
Keep in mind, aside from questionable stability and a lack of automated installer (you'll need to grab Microsoft Visual C++ 2010 Redistributable Package to complete the install, and if that sort of thing freaks you out, just stop now), Flash has yet to be made available for this build, meaning FF 64 basically turn your computer browsing experience into a big iPad. And Mac users are out of luck at the moment as well as this version is Windows exclusive.
Having said that, please download Firefox 64 asap. I need you suckers to debug the product so it's rock-solid when the final release hits and I finally download it. [Armen Zambrano's battlefield via webmonkey]