EOL'ing Firefox 4 after just three months does seem pretty abrupt. Many commercial products will support security fixes for the current and previous versions. Of course, Firefox 5 will presumably EOL'd in three months when Firefox 6 is released...
If there had been something subtantial in 5, it might have worked. But it seemed more like an update for the sake of versioning.
They could probably have gotten away with this later, but first turn of the crank just opens them up for complaints.