Since 2010, Mozilla has at been developing a 64-bit versions of Firefox for Windows, Linux and OS X. For the latter two operating systems, this is an official release, however, the 64-bit version of Firefox for Windows has not been released yet.
Now it is clear that there will be no official 64-bit version for Windows in the foreseeable future. Following a discussion initiated by Benjamin Smedberg, Engineering Manager at Mozilla, he announced that further development of the 64-bit version of Firefox for Windows has been shut down.
The main reason for the shut down seems to be that this version of Firefox has been a low priority and as a result it still has major deficiencies. There is also a continuing problem with plugins, which are seldom compatible to the 64-bit version.
There are few or no problems running 32-bit browsers with 32-bit plugins in a 64-bit operating systems. This is what most people use today, even though both Internet Explorer and Opera are available in 64-bit versions for Windows.
Mozilla has recently launched several new projects, such as a Modern UI version of Firefox, and a smartphone operating system called Firefox OS. It has led priorities and resources away from other projects, including the popular e-mail client Thunderbird.