The breaking changes, broken extensions and other bugs stopped me from using it altogether... I still have it on my phone (that version is based on K9 though), but long ago stopped using the shared dropbox profile. The profile rolled forward a version and I could no longer revert to an older version because of bugs in the app itself.

I used to love Thunderbird... I also used it a lot with BBS centric NNTP hosts... at some point those features largely broke as well, and extensions to correct the behavior fell farther and farther behind as well.

The lack of a good calendar/contacts server solution is also a massive pain point imo.