Rob Sayre, Mozilla's Engineering Director has posted an upcoming release schedule of Firefox -- Now, 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.
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. Likely, we could also see Firefox 6 by October, and Firefox 7 by the end of the year.
More Info: Mozilla Firefox: Development Process