My guess: They're fast-tracking the version numbers of Firefox to "catch up" with IE and Chrome. So I don't think this really has anything to do with code or features.. But rather, with trying to compete in the minds of users who actually think they can compare two totally different software applications by comparing their version numbers. Sadly, I have encountered many such people. They've also been allowed on the Internet.
Edit: Here's some actual meat I found on the subject:
The rapid-release cycle, in which
Firefox issues four new versions a
year, is intended to bring new
features to people sooner. That could
be better performance, new Web
programming technologies, or user
With the older style, a version number
change was a rare event that signified
major change. As a result, releases
often were pushed back by months as
programmers worked to include and
debug their new features. With the
rapid-release approach, new versions
of Firefox ship quarterly with
whatever new features are done. The
consequences to missing the release
train are lower, since another train
will come around again soon.
"By releasing small, focused updates
more often, we are able to deliver
improved security and stability even
as we introduce new features, which is
better for our users, and for the
Web," Needham said.
The idea is based on how Google
develops Chrome, a browser that in
less than three years has won over one
out of every eight people on the Web.
Last year, Chrome switched from
quarterly releases to an even faster
While that doesn't blatantly support what I said, I still think that's really going on is what I've said. But I think you'll have a hard time finding a public statement from Mozilla that admits my interpretation. I still think it's all about getting more users and moving version numbers in line with the competition, annoyingly, won't hurt.