Much respect to Firefox for being the torch bearer in the darkness and all that, but their secret auto-updates in the background are why I don't use them.
I use a Mac not because I'm some techno-illiterate neo-lithic throwback, but because I'm a raging control freak. On the Mac I feel like I'm in control. Programs that go and do random weird stuff behind the scenes break that fragile illusion. :D
The worst was when Firefox's stealthy self-updating broke itself. I don't remember which version that was (sometime in 2008? 2009?) and it might have been on a work machine (Windows? Vista?) but there was basically no going back (couldn't use previous versions because of stealth updates...), and no going forward (wasn't gonna sit around waiting for them to patch it).