The future of Thunderbird

The Thunderbird email client blog has a
plan for where the project is going.

Throughout the next 3 years, the Thunderbird project is aiming at
these primary objectives:

Make the code base leaner and more reliable, rewrite ancient code,
remove technical debt.
Rebuild the interface from scratch to create a consistent design
system, as well as developing and maintaining an adaptable and extremely
customizable user interface.
Switch to a monthly release schedule.