Mozilla details new Chrome-like release schedule and channels, Firefox 5 soon
Instead of major releases every 12 to 18 months, Firefox will shift to a four-channel system, just like Google's Chrome browser. The nightly channel (mozilla-central) will remain in place, but the beta audience will be split into Experimental and Beta channels. The final channel will be a stable build (Firefox 4, Firefox 5, etc.)
Every six weeks, code will be shifted from one channel to the next. At week 6, code will move from mozilla-central to firefox-experimental; firefox-experimental will move to firefox-beta; and firefox-beta will move to Firefox, the stable channel. Around week 12 and 18 the same process will occur, and so on, until the end of time. This means that we could see a major Firefox release every six weeks, once the machine starts turning.
Rather than waiting 16 to 18 weeks for Firefox 5 to emerge, however, we might see a rushed release in just 12 or 13 weeks, sometime around the middle of June. We could then see Firefox 6 by October, and maybe Firefox 7 by the end of the year (but it's unlikely).
It should be noted that the details of this new development process are subject to change -- Sayre's document is clearly labeled 'draft' -- but at this stage, we're almost certain that Mozilla will be moving to schedule-driven releases, and that Firefox 5 will be released in just a few months.
For more details about QA, localization, security patches and support for extension developers, be sure to read the full document -- there's a lot of important information in there.