Firefox 3 has just barely gotten out the door and Mozilla is already readying the alpha release of the next version, 3.1. Of course, as a point-release, it will be a much less ambitious project than the leap from 2 -> 3.
In Firefox 3.1 status meeting notes
from 6/24, the ship date is now estimated to be Q4 of this year or Q1 09. First alpha is in July, while first beta is in August. The release date is close to what had previously been stated by Mozilla. They had earlier said, however, that it would be date-driven, and it looks like a little flexibility has been added to that.
Earlier Mozilla had also said the features in FF 3.1 would consist of those that had missed FF 3 but were nearly complete. Additionally the notes indicate more changes to better meet the Acid 3 test (currently FF 3 scores 71 / 100) and changes to the Location Bar (which, depending on who you ask, is either the best or worst feature of FF3). According to Mozilla's count, there have been 25 million downloads of FF3 at the time of this writing.