Most users don't even know what a "browser" is. That means it doesn't matter to them if they have a choice to upgrade. I say, force-feed them, and do it transparently so they don't have to restart anything if possible. If an update requires anything like a restart or other interruption, ask the user when to do that (but without popups or alert boxes!). Possibly, for more advanced users provide an opt-out of these automatic updates.

I think Google Chrome does a great job (even though it's mainly used by tech-savvy peeps, one of the reasons for the fast upgrades).