I use Thunderbird every day and use and appreciate many of the features. However, I don't need any new features, I would just like to have some bug fixes and behavior changes.
Spam detection and separation in Thunderbird is really sub-par.
I use BlueMail https://bluemail.me/ on my phone and it works great to detect spam and move it into a different folder. It lets me tell it that a message is spam, optionally tell it to block the sender's address, or block entire domain in the future. Works great. However, in my experience, Thunderbird's spam detection never learns when I mark a message as spam or unmark it. New identical messages from same sender and with almost identical content end up back in the spam folder no matter how many times I have un-marked similar messages. Training simply does not work.
Thunderbird gets all confused when editing or replying to some messages that contain HTML:
It won't let some text styles be changed. Highlight some text and "Remove All Text Styles" will simply fail depending on some unknown factor.
Sometimes it is very difficult to delete a portion of a table.
Can't change the background color of some text as would be possible with a <span ...> some text </span>, background color changes always affect the entire container or simply does nothing at all.
Why not also give us an option to edit the HTML directly?
Thunderbird gets confused with numbered and bullet lists:
Sometimes won't let you end a list with two carriage returns, or at all.
Won't let you increase the margin between <li> lines
Can't specify default behavior so it creates list just like this one,
numbers then letters then bullets
This editor in your web page is better with lists than Thunderbird.