The reason is the underlying Gecko engine being constantly yanked from under them — they rely on parts of Firefox's codebase and when that changes, they have to keep up, risk having security vulnerabilities or manually backport relevant security patches, in effect maintaining their own forked browser engine.
This, in addition to some of their other UX changes (like moving to SVG icons) - just makes it feel like after 10 years I am no longer their user base, and from what I've read the team is unwilling to be flexible with these changes. Thanks for the middle finger. They're breaking things and I'm supposed to just be happy about it.
Regretfully I've fallen out of love with thunderbird, and i am exploring other clients. My needs are not complex, outside of perhaps the google auth song & dance.
I want thunderbird to be the FF of email clients.... I guess in a way they are. Open source disappointment that pushes people to non-open solutions.
That isn't to say that you should give to thunderbird, since they are currently disappointing you, but it is a bit of a vicious cycle.
Well, not exactly, because while it was on low-effort maintenance mode, there was no such changes; it was just doing the same job in the same way since forever. Problems arose when a new team with a new mission, new funding, and worst of all, "new" ideas ("new" being the same fate other pieces of software underwent in the last decade: inserting more web tech, changing the look into something more modern, i.e. more web-like), became active.