Why do people use Firefox? Most users claim extensions. What breaks extensions? Frequent updates. Effectively annulling the most compelling reason to use Firefox.
This is certainly my experience. Pentadactyl, the most compelling reason for me personally to keep using Firefox is more broken than not. Every single update in the last year has broken it and sometimes in non-trivial ways, and stretching my patience to the limit. If I have to abandon Pentadactyl, I really don't have a reason to use Firefox anymore.
UI changes proposed in Australis are not something to look forward to either esp. if you like hiding Firefox UI elements and basically just keeping undecorated minimal window with Pentadactyl.