They release a new version for each major feature roadmap they hit
So was the "major feature" they just hit two finger swiping, or support for font-stretch?
The reason we have versioning numbers is to give people an idea of what's been changed. 3.0.9, for example (assuming previous version was 3.0.8) would let people know that it's a minor bug fix. 3.1.8 would let people know it's a major bug fix. 4.0.0 would let people know they've done a large overhaul on the program. By making almost every minor change to Firefox lead to a new version, they've basically thrown out that system.