3.6 doesn't force you to auto-upgrade. You're encouraged, but never forced.
(still on 3.6 at home and at work for reasons of inertia -- not broke, won't fix, yet)
I'm a great believer in that. However, bugs have been fixed, memory leaks plugged, rendering times improved. Personally the promise of more effcicient memory use has me chomping at the bit - FF runs like molasses on my 768MB Athlon 1.1GHz.
3.6 doesn't force you to auto-upgrade. You're encouraged, but never forced.