In a posting on Google Groups, Mozilla engineering manager Benjamin Smedberg announced that there's been a change of plan regarding the foundation's move to kill of the 64-bit version of Firefox. Due to negative feedback on the Internet, Mozilla decided to continue building 64-bit nightly builds, but these builds will not be officially supported. All users of the 64-bit nightlies will be migrated to 32-bit in the near future, and will need to download the 64-bit version if they want to keep using it after the migration point.
Here's his full post:
After I announced my decision to disable 64-bit Windows nightlies, there
was significant negative feedback. After reviewing that feedback, and
consulting with Release Engineering, I believe that we can keep a set of
users happy by making a modification to the original plan.
Most importantly, it seems that there are users who regularly run into
the 4GB memory limits of 32-bit builds. These users often have hundreds
or even thousands of tabs. These users are using the 64-bit nightlies
not primarily to be part of our testing community, but because those
builds are the best product available.
At this point, the Mozilla project does not have the resources to
actively support this use case. Making these builds, however, is not a
significant burden on our Release Engineering group. Therefore I have
modified my original plan in the following way:
* Migrate all existing users of win64 nightly channel builds to the
win32 nightly channel builds via automatic update.
* Continue to build win64 Nightly builds and updates on the nightly
channel. Users who need the 64-bit builds will have to download it after
the migration point (date TBD).
** Change the default first-run and update page for win64 builds to
explain to users that they are not supported.
** Disable the crash reporter for win64 builds
** Enable click-to-play plugins by default in the win64 builds.
* Discontinue the win64 tests and on-checkin builds to reduce release
engineering load. By default, do not generate win64 builds on try.
* win64 builds will be considered a "tier 3" build configuration. 
We will continue to test the win32 builds and make sure that they work
well on both 32-bit and 64-bit versions of Windows. Specifically, all of
our testing on Windows 8 is planned to be done on the 64-bit version of
I do hope that the projects and developers who are interested in win64
will work together to maintain this build configuration. I am interested
in hearing from volunteers who want to become the 64-bit build
maintainer. I will also set up a discussion list specifically for win64
issues, if that would be valuable.