I wish Thunderbird fix their plain text editor (it is at level of old Notepad, and chrome for it looks ugly, and line wrapping is a mess, especially with in-line quotation), add ability to store Folder properties (including Identity used for this folder, retention period and such) as IMAP properties and not locally to have same settings on different devices.
And, yes, proper support for Sieve, including per-folder Sieve. Sieve is a pain after they changed something and 3rd party Sieve plugin died (become Electorn Application).
Now Thunderbird has so many rough edges (I named only my top-3, but I'm sure anybody can add others!), but still one and only usable cross-platform e-mail client.
Oh, yes, development pace is unbearable slow: after killing "Manually sort folders" plugin it takes more than year (!) to add this as "core" feature with huge help from aforementioned plugun's author. Very slow process of review, integrating, releasing which takes MONTHS to integrate ready feature. It should be very discouraging for contributors.
Thunderbird now provide like 10% of features of old and almost forgotten (but still alive) windows-only client "The Bat!" from end of 1990s, beginning of 2000s and was written by team of like 5 people.
But still, I've donated!
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.