Mozilla has revealed that they are pretty much halting development of their Thunderbird email client.
They would continue to provide security updates through their Extended Support Release process. But you are unlikely to see any new features in the future.
They are however asking the open source community to step in and take the development process forward.
Mozilla representative Mitchell Baker spoke about their decision:
Much of Mozilla’s leadership — including that of the Thunderbird team — has come to the conclusion that on-going stability is the most important thing, and that continued innovation in Thunderbird is not a priority for Mozilla’s product efforts. (For more information about the path to this conclusion, see the “Background Information” section below.) As a result, the Thunderbird team has developed a plan that provides both stability for Thunderbird’s current state and allows the Thunderbird community to innovate if it chooses.