Thunderbird 17.0.2 contains 20 fixes for known security vulnerabilities and various other bug fixes, but no new features.
The reason is that Mozilla has changed the release scheme for Thunderbird (i.e., Firefox and SeaMonkey will stick with the rapid-release 6-week interval scheme) and returned to a quasi-branch scheme again which was in place for 3.x and earlier. Thus, 17.0.x releases will come for a period of about 42 weeks until 24.0 is reached which will include new features accumulated until that cycle. Then, a new 24.0.x branch is opened for another 42 weeks, etc. Beta releases will continue to be current with feature developments and numbered by the respective cycle (19.0 beta, 20.0 beta, etc.).As a consequence, new features will hopefully be more mature before they hit the releases, thus related bugs and regressions sufficiently fixed. The stable interfaces should also avoid issues with extension updates and custom-style modifications. There may be an intermediate 17.1 release to catch new features considered important, but that's hypothetical at this time and not planned.
This switch was also introduced due to an organizational change within Mozilla, which reduced the commitment of paid developers to Thunderbird and Mail/News Core development substantially, expecting the community to take over much of the feature development. Security and stability fixes will still be provided by Mozilla itself. That's in essence the status where SeaMonkey has been for years now (and rather successfully), thus no reason to panic.
See viewtopic.php?f=29&t=2033027 for more details on the rapid-release model and the extended-support releases.
