https://blog.thunderbird.net/files/2020/07/compose-compariso...
Old: Clear that you can enter multiple "To" addresses. Obvious that the formatting buttons are buttons.
New: Single-line "to" implies you can only enter one address. "Flat" buttons don't look like buttons.
https://blog.thunderbird.net/files/2020/07/account-setup.png
Extra white space makes the window nearly twice the size for no apparent reason. I wouldn't say the new one is any clearer. Maybe even less clear, since the grey textual tips next to the fields have gone.