Mozilla has a detailed new development model for Firefox, with the current draft saying Firefox 5 will make its way to fans on June 21, 2011 and Firefox 6 not that long after on August 18, 2011. These dates may well change but right now this is what Mozilla is hoping for. All changes to the Firefox source code are initially integrated in the mozilla-central Mercurial repository.
At scheduled intervals (usually 6 weeks) the changes are then imported from mozilla-central to one of three other channels. In addition to mozilla-central there will also be firefox-experimental, firefox-beta and Firefox (release) with each one backed by its own Mercurial repository. The names below are placeholders and Mozilla may still change them:
The firefox-experimental channel will receive new features at regular intervals, but some of them have the option of being disabled if they look like they require more work. The beta channel will receive new features that are set to be included in the Firefox release. New features are never directly added to the firefox-experimental or firefox-beta channels. Each stage of the process lasts for 6 weeks but because of the development overlap, we should see new versions every 6 to 12 weeks.
Mozilla announced last month that Firefox 4, Firefox 5, Firefox 6 and Firefox 7 would all ship in 2011, it may sound insane but with this new schedule we might just see that happen.