64-bit Firefox for Windows lands in Mozilla's nightly trunk builds
You'll need the Visual Studio 2010 redistributable for msvcr100.dll, but other than that, just grab the ZIP from Mozilla's latest nightly trunk and off you go! (Both of those are direct download links, by the way.)
Regarding the SunSpider test, the 64-bit version clocked in at 418ms, while 32-bit came in significantly faster at 373ms. Of course, the SunSpider test might not be designed for 64-bit browsers... and this is still early days! There isn't even a 64-bit Flash plug-in for Windows...
Still, if you're an enthusiast, grab the 64-bit version of Firefox and give it a go!