The first update for Firefox 4.0 is scheduled for release later this month. Firefox 4.0.1 code named Macaw (yes, Mozilla is now giving the updates code names as well) is scheduled for April 26th. Further updates for Firefox 3.5 and 3.6 will also be released on this date. These updates had been scheduled prior for April 19th. Source: Mozilla Links

Mozilla is planning a major release day on April 26th. Besides updates for Firefox 3.5, 3.6 and 4.0 there will also be an updated for Thunderbird 3.1 as well. The release of Thunderbird 3.1.10 is directly connected to the releases of Firefox, so should these become delayed the Thunderbird update will be delayed as well. Currently there is no information available regarding the next release of Miramar (Thunderbird 3.3). Alpha 3 was released about a month ago on March 15th. Not sure if the next release is going to be Alpha 4 or Beta 1.