More bloat, unfortunately
I originally moved from Mozilla Suite to FF because it was 'smaller and quicker'. It's gradually got more and more flashy and bloated. Yes, it's still a good browser, but really, do we NEED curvy buttons, an 'awesome bar' (although the functionality of it searching your bookmarks admittedly is useful) and so forth. Also, I regularly have to kill it using task manager, it can sit there for a good 10 minutes after being closed, actually still running. Much of the slowness is due to the web2.0 badgers' paws etc.
The security updates, not an issue for me, I have it set to notify me on all updates and ask, I briefly glance to see what it's doing and ok it. As mentioned by Andy Moreton and John Ridley, it's a positive thing, and anyway, you can tell it to update silently if it annoys you that much.