Please let whoever steers Thunderbird development and road map also steer Firefox. Thunderbird is at the moment the pinnacle of user-centered, focused and down-to-earth development of open-source software.
Apart from the UI which is crap since their last major update. There are menu options everywhere, two ribbons on the top, a hamburger menu on the right and another on the left. For a long time you opened Thunderbird and it didn't default on the last message that you received but somewhere in the middle of the heap.
The old UI was criticized by some for being outdated, a mix of old and new styles, didn't fit well with new OS/app styles, etc. It was crap. So they update the UI and it's still crap... for other users. Damned if you do and damned if you don't.