The Firefox Version Naming System

I don't understand how Mozilla comes up with the version numbers for new releases. Would it be so difficult to use a Debian-style six month release cycle? Why not steadily increment the version number? Why are the nightlies of Firefox 4 called 3.7? Will there be a 3.8? A 3.9? How is Chrome wich was released several years after the first version of Firefox already on version 5? Safari, is on version 4, and it too is younger than Firefox.