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 Windows 8.
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.