This seems a bit excessive and weird. Why does Mozilla assume that the About Firefox window is where users go to see if an update is available? Even if that's true, why hide the version number? Chrome doesn't, and it's hardly confused people there.
One of the primary reasons I declared Chrome my browser of choice (this was among other reasons, mind you) was that just around Firefox 3 there came a time when I had to update it seemed almost daily. To keep my foot out of my mouth, I checked to see if maybe there were settings on my system that were coming undone upon shutdown and boot, negative. I grew quite annoyed that every time I launched I had to go through an update process again.
This is a change I personally am in favor of, and can only assume is the case for other non-power users of the browser. Thankfully though they're making that information available still through the about:support page for people who find that information relevant.
Those numbers aren't really version numbers. It's closer to build/realse numbers now. They used to mean major releases with major changes, now they don't. I can see why they are getting rid of them. You still can find the release number if you want (as you can with chrome).
