It seems all Firefox did is replace the incremental-type releases (3.5, 3.6) with a full version number release rather than staying at incremental. Not sure why they felt the need to expedite full-version releases when the incremental system was working fine.
I'm REALLY shocked to see M$ with a 10 preview already, 9 has only been out a month, maybe 2? Talk about accelerated development cycles. Yes, i realize it's not a release in any way, but Win 7 has been out for 18 months and there's still only speculation on the next version.