hypnosec writes "Plans for 64-bit Firefox for Windows have been put on hold by Mozilla in a bid to concentrate more on the 32-bit version. Eliminating the 64-bit nightly builds was proposed by Benjamin Smedberg, a Firefox developer, last week. Some of the reasons Smedberg cited include missing plugins for 64-bit version; lack of windowproc hooking which facilitates smooth functioning of whatever plugins are available; and the inability to work on the crash reports submitted for the 64-bit versions because they were not on high priority. The proposal, it seems, has been accepted as is evident from this bug report."
The bug tracking system seems unable to differentiate between 64-bit and 32-bit builds, causing a few issues since Windows 64-bit builds are much buggier. They also intend to reintroduce 64-bit Windows nightlies some time next year.