Thunderbird 38 is now released (actual initial version is 38.0.1 to maintain compatibility with equivalent Firefox releases). This release has some significant new features, as well as many, many bug fixes. Some of the new features include:

• Calendaring is now shipped by default. This continues to be implemented as the Lightning extension, but that is now enabled and installed by default.
• Chat now supports Yahoo Messenger.
• Messages can be filtered when sent and when archived.
• You can now search multiple address books.
• Gmail users can now authenticate using Google’s preferred OAuth2 authentication (which means that new GMail users should work with Thunderbird without special configuration).

This is a significant milestone for the Thunderbird team, as it is the first release that has been fully managed by our volunteer team rather than by Mozilla staff.

Mozilla is still heavily involved with this release, as we still use Mozilla infrastructure for the build and release process. Thanks to the many Mozilla staff who helped out to fix issues!

Thanks to all of the volunteers who have contributed to make this release possible!

https://blog.mozilla.org/thunderbird/2015/06/thunderbird-38-released/