They changed their development cycle a while back to facilitate much faster (but "smaller") releases. At the same time, they changed the criteria for what calls for a major version bump in order to get version numbers that look similar to those of IE and Chrome. The thought within Mozilla was that Joe Blow would look at Chrome and see that it's version 14 and then at Firefox, which was at version 3.5 at the time, and think "14 > 3.5, thus Chrome > Firefox."
will it prevent babby?
This still hasn't made it into the trunk. =\