Update, nope, it does tell you there's an upgrade, though (at least on OS X). I think the next step for the FF team is getting those cool assembly-text-diff things like Chrome has working. Until then, there's just fragmentation.
https://wiki.mozilla.org/Software_Update:MAR
It downloads the update automatically, but it will prompt you to restart a while after the update is downloaded (if you haven't restarted on your own). We are actively working on making the whole process less intrusive and more seamless.
http://dev.chromium.org/developers/design-documents/software... has some of the details. Note the ~90% reduction in size going from bsdiff to this.
I want to use this for my own stuff, because that's cool as hell, but the Courgette source has pretty major dependencies on the rest of Chromium.
Even when update servers aren't dying under heavy load, my internet connection isn't very fast. There are many people who don't have fast connections, both in the US and elsewhere. Programs downloading things in the background render web browsing noticeably slower and make voip unusable. Update size matters.
1. (pdf) http://broadband.cti.gr/el/download/World%20Broadband%20Stat...
2. http://en.wikipedia.org/wiki/List_of_countries_by_number_of_...
Ran Firefox, went to the about window and it was already downloading the update. It also does autoinstall, but you will get notified if you still have the default Vista/7 notification settings (i.e. 'Blah wants to run this. Yes or no').