Mozilla is indeed working on a 64-bit version of Firefox for Windows users, but some third-party developers out there moved a little bit faster and already brought out 64-bit browsers based on Mozilla's very own engine.
This is the case of Waterfox, whose developers claim is the fastest 64-bit browser out there. The application today reached version 30 after a long time of inactivity, which made many believe that the project could be abandoned.
Well, it wasn’t, and Waterfox 30 is here for all Windows users who'd like to install a 64-bit browser and thus benefit from enhanced stability and performance, taking advantage of the newest technologies and OS features.
Release notes are yet to be disclosed, but we've already installed the app and everything seems to work amazingly well, so you should really give it a shot if you're looking for a full-featured 64-bit browser.