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.