[joedrew]

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.