The last few months Mozilla firefox has been on a rally to catch up with its biggest rival Google Chrome. From version 3.6.xx it has jumped to 11.0. On the other hand users demand an 64bit version and that is why projects such as waterfox and palemoon seems to be very popular. Mozilla team has been working since version 13.0alpha1 on a 64bit version and they are already on their second version 14.alpha1.
This release is a nighlty release which means it even newer than the Aurora ones but none the less it seems stable enough even for every day use. Moreover it is update automatically almost daily and it has reasonable memory footprint.
For more information check the following url
Over here the Windows 64bit version is not yet visible but none the less you can download it from here
the file for your concern right now 19/03/2012 is this one
With an official 64bit Mozilla Firefox seems to be back in the game.
*1 for an 64bit Flash, Java and Silverlight version please check here http://waterfoxproject.org/downloads/
*2 in case that some of your plugins are not working properly get this one which overrides any compatibility problems Nightly Tester Tools
*3 You can not run concurrent your old 32bit version with the 64bit version. Only one of them should be running but you can have both of the installed and choose accordingly