I added the "Mark" button years ago and I liked the way it worked. Select one or more eMail's and press the button. It would then simply toggle the read/unread status. Clean and simple. With the new release, it brings up the total menu. Why would I want a menu? If I wanted a menu, I'd use the menu bar.
All previous versions had a border around the main window. If I center click on the mouse, the main window would go to the back and I could see other windows. Now, the center click does nothing (maybe because there is no border?).
This is not unique to the upgrade (?) but the spell checker is a problem. When I forward or reply or reply all and then do a spell check, it checks the forwarded (or replied) text (including the separator line). Why oh why would I ever want to spell check someone else's eMail? Thunderbird knows where the forwarded eMail text starts because it put it there. So why not stop the spell checker before it gets to the forwarded text?