Up until now, Firefox lovers using a 64 bit operating system were restricted to using only the builds provided by their operating systems or, the ones they could get off the Mozilla-x86-64 project at http://wiki.mozilla-x86-64.com/X64_Build. This is going to change soon.
Mozilla has started rolling out nightly builds for Firefox on Mac and Linux and will soon start doing the same for Firefox on Windows as well. The 64 bit version is already available for Mac and Linux. The Windows version will arrive soon and the final version of the next release, Firefox 4 will see releases for both x86 and x64.
This can significantly improve the global browser-share of Firefox as people on x64 still keep struggling to get a feature rich and pluggable browser. Though, just a 64-bit build of Firefox would not do the trick as most plugins like the ones for Java and flash are available only for the 32 bit system.
The build is an experimental one and has many unresolved bugs. It can cause loss of data and of you still want to try it out, download it from the Mozilla Public FTP server.