I have Firefox running on Vista, XP, 2000, 2003, Mac OS X, OpenSUSE, Mandriva, Ubuntu, and others. Firefox versions 2 and 3.
My experience is that the Auto Update mechanism in Firefox is flawed. A number of these PC's never trigger to be updated even if they are months behind. One of my Windows 2000 servers often takes about a week before it's auto updated.
Experience shows that it doesn't check for an update at every launch. And that sometimes it gets stuck, something gets corrupt, and not until you ask it to check will it check again.
Granted, this is much better than most software. However the update mechanism needs work.
Microsoft signs/encrypts and then checks the IE package signature. As much as a dog Microsoft, their update mechanism is one of the best.