Firefox has adopted a faster release cycle. This basically means instead of us seeing new versions every, say, six months, we’ll be seeing them on a far shorter period of time (probably 16 weeks intervals.)
More versions in less time means less (more frequent) changes. These changes may not be very big or major, and may very well be in the back-end (performance enhancements and so forth.) So you might be told Firefox has bumped to version 5.0 overnight and you see zero changes at first glance. I personally don’t mind that.
What I do mind however, is my addons compatibility.