There are a lot fewer factors in shipping a new firefox for OS X than there are for Linux. On OS X, for the most part, all of the important dynamic libraries are frozen at a single or at one of a small number of versions, everybody is either ia32 or amd64, there aren't distro patches applied to things, there isn't the wide array of Desktop Environment configurations that there is in other Unixes, etc, etc. This isn't to say that rolling releases aren't possible; it's just trickier for Linux distros.

Also, you have to update the glibc sometime.