They release a new version for each major feature roadmap they hit, I don't see what's so difficult to understand about it. It's a number, why does it bother you so much? Chrome has been doing it for years already. Previously Mozilla struggled to hit development targets and were falling far behind on features. This is exactly what they needed and it's let them hit feature parity with other browsers while improving on oft complained about things.
Before - Many features developed for significant revision, milestones constantly missed and dev targets late while features slip
After - One major feature for each major iteration instead of throwing the kitchen sink into each release, they are actually hitting release targets and bringing the browser back up to parity with competition
But blargghh that number bugs me rarrr! Pretend it's Firefox 3.0.9 or something, sheesh. The number doesn't matter in the slightest to the end user other than giving them something to identify, the update content does.