Instead of 18 weeks in total, the Firefox 5 development process will only take 13, with a final release on June 21.
Firefox 6 development starts in 5 days on April 12 when the Firefox 5 build is moved from mozilla-central to mozilla-aurora.
Mozilla expects to release Firefox 6 on August, 16th. There you have it, two new Firefox releases in the next four months, who would have thought that a year ago?
There may be changes to that schedule, for instance if development is delayed at some point. The general release schedule stands on the other hand. Probably the most interesting new aspects are the new aurora channel which is releases as nightlies as well, but supposed to be more stable than the mozilla-central nightlies, and that beta releases will be more frequent due to a weekly release schedule.
It is likely that part of the Firefox user base will move to the beta and aurora channels to stay on top of the browser development without having to face the stability issues that the initial nightly builds might be causing.