In order to free up hardware resources and reduce build wait times, Release Engineering are moving a chunk of the Firefox builds onto Amazon EC2 instances (see Bug 772446). As you may know, B2G and Fennec builds are already on EC2 instances.
The actual builds will run under 'mock', allowing us to create custom build environments as well as build either 32 or 64 bit builds on the same build machine. Under CentOS 6, libstdc++, glibc and gtk2 will get a version rev, too. See Bug 772563 for details.
Note that this applies only to the linux32 and linux64 platforms of mozilla-central, try, and all of the 'project' branches (such as mozilla-inbound).
It does not yet apply to aurora, beta, release, or ESR builds (these will ride the trains), or to Thunderbird builds (though Thunderbird comm-central and try-comm-central will be moved in the near future).
Among other things, this will unblock the reimaging of linux32 and linux64 physical build machines as win64 builders (see Bug 780022), thus reducing wait times on our Windows platform.
The move is planned for next week after the migrations have completed and will not require any tree closure.