It has nothing to do with Chrome, and everything to do with 6-weekly updates.
Think of it: You have no way to know, going in, whether the changes in any given update will be big enough to "warrant" a major version change. Worse, what if those features aren't stable enough to release and are turned off? It's very difficult to go from version N to version (N-1).
For these reasons (and others), it's a lot easier for us to unconditionally bump the major version every 6 weeks.
Source: I am a Firefox developer and a Mozilla employee.
That's his problem: The numbers in versions have traditionally (and more usefully) been used to denote levels of feature change and bugfix. A time-based arbitrary version number is pointless; it'd make more sense to use a timestamp for a version number if they're going to do it that way.
