Odd though I've never heard of it, only just had a thingy saying why don't I upgrade to a new 3 version.

Judging by previous FF versions it'll probably be a while until the automated update prompts start encouraging you to move to FF4, and they'll most probably do a couple more security updates to the 3.xx track before freezing that. So, at the moment, the upgrade to FF4 is a manual operation.

But that's all based on previous experience, as I jumped ship from FF to Chrome about six months ago and haven't seen anything in FF4 to make me switch back. It'll be interesting to see how much longer Google carry on bankrolling Mozilla now they've got Chrome established as a browser, and once the Google money dries up Mozilla will, sadly, disappear like a punctured balloon ....