From the "we know what's best for you" files:

Firefox 3 has been out since June 17th, but to date existing Firefox 2 users have not received an upgrade notice for the new version. Instead users have had to go out and download Firefox 3 on their own (which over 30 million people have already done).

The vast majority of Firefox users however (somewhere in the 120 million user range) still use Firefox 2 though. That's where the Major Update notification comes in which will pop up on Firefox 2 users screens next week.

Considering that Firefox 2 may well be at risk from a Jinx attack (check out my article from Black Hat on that), the need to move to Firefox 3 for security reasons alone is very obvious to me.

Firefox 2 is still set to be supported by Mozilla till the end of the year when Firefox 3.5 is expected to be released.