Okay, that's scary. It's like a one half of a conjoined twin trying to remove the other half from their lives. Mozilla started transitioning Thunderbird out of their umbrella while TB is still highly dependent on Firefox code. Yeah, scary.
Maybe we need a successor to Thunderbird? Something that's not tied to Firefox so much? Because it seems like we can't trust Mozilla and however many unpaid/underpaid volunteers to maintain it well enough.
If you are referring to Mozilla Messaging and the period to 2012, no the only transitioning was in terms of product functionality. The community organized in 2014, unaffiliated with Mozilla, and it's not until about 2018 when staff were hired that community leadership were able to seriously reduce technical debt. But seriously reducing dependency on Firefox code is some distance away.
> Maybe we need a successor to Thunderbird? Something that's not tied to Firefox so much?
You are looking a millions of dollars paid developer time.
> Because it seems like we can't trust Mozilla and however many unpaid/underpaid volunteers to maintain it well enough.
Mozilla are not running the show here, the community is.
Perhaps it's difficult to keep up with the history, but https://blog.thunderbird.net/ has past relevant postings, plus developer and other mailing lists. To cite a couple, big changes in encryption https://blog.thunderbird.net/2019/10/thunderbird-enigmail-an... https://mail.mozilla.org/pipermail/tb-planning/2019-December... and development priorities http://lists.thunderbird.net/pipermail/maildev_lists.thunder... which includes a new address book.
There is a significant professional development staff which is nicely paid, and the number of developers will soon double. https://www.thunderbird.net/en-US/careers/
Unlike a web browser, email has not changed much in the last 15 years, so I don't see why development needs to advance at a high speed.
I doubt that any UX fashion statements are going to be enough to make up for the huge loss of functionality.