There is likely a team working on this. The job posting said for someone to come on board.
And yes, hiring a single developer to work all alone by himself would be akin to killing the project itself as just understanding the codebase, design, technology and architecture would be too much for one person to handle.
We’re looking for an amazing developer to come on board to help make Thunderbird the best Email client on the planet!
Thunderbird was moved to "community development" with Mozilla providing only stability and security updates. https://web.archive.org/web/20161003075603/https://blog.mozi... In the interview it says that employees might only work on the project part-time.
Quite a bit of that code was shared between Thunderbird, Firefox and Seamonkey. But now Firefox don't want to play along any more, and dragging the other two along for the ride...
