Users of 64-bit operating systems who wanted to run a native 64-bit version of Firefox were restricted to those provided by their Linux distribution, or, the Mozilla x86-64 web site, for Windows.
However, it seems Mozilla is getting ready to finally provide 64-bit versions as part of its core set of Firefox releases. Mozilla’s John O’Duinn posted that current Firefox 4 nightlies, are now being built for Windows 64-bits, in addition to Linux and Mac OS X 64-bits versions introduced a few weeks ago.
Keep in mind that very few plugins are available in 64-bit editions (Java, and Flash in non-final status), so you will be running mostly 32-bit plugins.
I haven’t had the chance to try the builds yet, but a known bug so far is a missing Visual C++ redistributable library on Windows, which you can get from Microsoft.
And this is just one of many bugs you can expect at this stage. “This is still just a very very very early experimental build”, warns John. If you understand that things may not work as expected, and you may even lose some or all of your data, check Mozilla public FTP server to get Firefox 4 64-bit nightlies (along with regular 32bit editions)