Thunderbird 115 has so many faults. One of these is the Toolbar customization of an email window.
For example there is no "Delete" button as an option to be placed beside the "Tag", "Mark", "Junk", "Address Book" etc.
So it is not the small, narrow button row on the right!
It is the BIG buttons on the top left!
Thunderbird 102 has 37 buttons, Thunderbird 115 has 24.