Firefox has always done a good job of handling updates. It is not in the background, but it is painless and automatic once you agree to upgrade. It will never be anything like IE6 - how many people do you see using Firefox 2? or even 3.0?

The upgrade to 5 isn't much different than going from 3.8 to 3.16. Mainly, they've changed their numbering scheme, so there's no reason to make a big deal about it.