Not that it answers your question, but the move happened in 2020 to "hire more easily, act more swiftly, and pursue ideas that were previously not possible".

https://blog.thunderbird.net/2020/01/thunderbirds-new-home/

So here more than 6 years later, did they act more swiftly or pursue new ideas? The development pace seems unbearably slow.

Judging from the side it seems like there aren't a lot of developers and the current few have their favorite subsystems, those get almost all of the attention. The rest is kept as-is and does not progress. I also don't know if there's a trivial way to find how many external contributions they get from BugZilla, if they even get any?