I tend to run bleeding edge software everywhere and when I can, I run the most native applications I can. This is why I'm running Office 2010 x64 on my x64 laptop. Primary reason is that most of the annoying Office plugins I don't want do not have a x64 counterpart yet. This includes the annoying iPhone plugin that iTunes installs that can make Outlook stall/crash on exit. Bad plugins can cause bad behavior.
Of course, there isn't a lot of 'gee-whiz-wow' aspects of running Office 2010 x64 other than it being a native app and not going thru the WOW64 emulation. You also get some of the security protection features that are not available with x86 applications.
Anyway, if you are daring and want to feel like an Apple Mac users for a while (No working Flash support), grab one of the FireFox nightly builds here.
If the Tinderbox directory seems to be empty (sometimes the build process gets broken and stale files removed), you can always check the trunk builds for x64 here. The x64 installer can be found here at the trunk location and the zip can be found here at the trunk location. Many times, the .zip file is newer than the x64 installer version.
To be able to use the executables, you will need the Visual Studio Runtimes for x64 which you can download here.
What I did was take the zip file, unzip it into "%ProgramFiles%\Mozilla FireFox" (NOT "%ProgramFiles(x86)%\Mozilla FireFox", unblock the executable since it was downloaded from the internet (oh no!), and launched the application. It does seem to run relatively faster overall and use less memory but maybe that it due to the lack of Flash.
You can always check the jemalloc stats by pulling up "about:memory" and seeing just how bad something might be leaking.
With "about:config", add "gfx.font_rendering.directwrite.enabled" (boolean) and set this to 'True'. Add "mozilla.widget.render-mode" (integer) and set this to '6'.
A noteworthy Windows 7 hotfix related to Direct2d and DirectWrite can be downloaded from here. It seems to make behavior more consistent for applications outside of just Visual Studio 2010. I have personally been running this hotfix with Firefox with DirectWrite enabled for a long time now with no major issues.
After restarting Firefox, the fonts on the page and browser should be anti-aliased. Websites with large images should load faster, and any SVG examples should be much faster.
If you run into problems with Direct2d and DirectWrite, check out this thread here.
If you run into Add-On Manager problems, unrelated to 64-bit but with the new AOM framework, check out this thread here.