Edit 11-25-2011: still no official x64 Firefox; nightly is still updated, and Flash supports x64 now.
I tried x64 Firefox a while back: buggy as Hell. June 2011 Nightly: working so far! There is an x64 Flash plugin in the wings, and there's an addon to help enable addons that might still work (StumbleUpon does).
* Firefox nightly x64 (look for x86-64 EXE)
* Add-on Compatibility Reporter
* Flash plugin non-IE x64
* x64 Java
If you're using an "IETab" plugin, those are still 32-bit only and won't work; use IETester for those sites.