I too had this issue (but did not know it). I have used Thunderbird for past several years and have only ever seen the search box at the top of window, with the menu bar below that. Recently I was working with an AI to help figure out why I could not add a mailing list. This opened up a new world for me in that I could actually customize the toolbar which I had thought of a search bar only. This seems like a pretty significant omission. The default toolbar should at least have a visible 'customize' button. In my opinion by default some basic buttons should be visible also (Get messages, next, previous, next unread, move and delete come to mind). This would be a significant useability improvement.
