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.

Source: LWN.net – The future of Thunderbird