All changes to Firefox source code are initially integrated in the mozilla-central Mercurial repository. At scheduled intervals (typically 6 weeks), the changes are imported from mozilla-central to one of three other channels (larger features and projects are usually initially developed in other repositories which track mozilla-central). In addition to mozilla-central (currently referred to as nightly), there will also be firefox-experimental, firefox-beta, and Firefox (release), each backed by its own Mercurial repository. These names are currently placeholders and Mozilla may still change them.
The firefox-experimental channel will get new features at regular intervals, but some of them might be disabled if they look like they need more work. The beta channel receives only new features that are slated for the next Firefox release. New features are never directly added to the firefox-experimental or firefox-beta channels. In general, each stage of the process (and activity pertaining to a particular version) will last for 6 weeks, but because of the development overlap, we can expect a new version every 6 to 12 weeks.
Firefox 5 (pictured above) will be slightly different from future releases due to the lack of a development overlap with Firefox 4. Mozilla announced last month that Firefox 4, Firefox 5, Firefox 6, and Firefox 7 would all ship in 2011. That looked nearly impossible after the delays of Firefox 4, but with this new schedule, the company may manage after all.