They do. There are 4 branches: nightly, aurora, beta, "final". Every one of them follows a 6 six week cycle. I'm currently running firefox 9 nightly, for example, and current versions 8 and 7 are very stable already.
You have to take into consideration that, in browsers, implementing or fixing a particular css/html feature is prone to break existing apps that rely on the wrong behavior. IE6 is still used in many environments for this reason exactly, but in the case of Firefox changes are very small between releases.