Thunderbird 6.0, etc.
In a process driven by the Firefox developers, Mozilla has substantially redesigned their release process [1]. Thus, there will no longer be parallel branches after Firefox 3.6 and Thunderbird 3.1 have reached their end of support. Instead, each release will have a full version bump even if there are no or marginal feature changes (i.e., Firefox 5.0 will be the stability update for 4.0.1, then 6.0 will be the update for 5.0, etc.) [2]. Only in quick "firedrill" follow-up releases which become necessary shortly after a release to fix a missed issue, a minor "dot" release will be issued.
Thunderbird decided to follow that pattern, and in the process to re-align its numbers with Firefox again [3]. Consequently, the "Miramar" branch has been bumped from 3.3 to 5.0, and the next release after 5.0 will be Thunderbird 6.0 (formerly 3.4) around the same time when Firefox 6.0 is released. It yet has to be seen how this process turns out in practical terms, especially with respect to add-on compatibility as a stable API is no longer given as it was previously assured during the lifetime of a branch.
The user-facing changes between Thunderbird 5.0 and Thunderbird 6.0 should be close to none, given that there were only two weeks (May 10 vs. May 24, 2011) between the string and feature freezes of these two versions, even if the time between those two releases is longer.
