The menus don’t show up in Thunderbird any more … which renders it a bit difficult to use.
Is this because the upstream dropped the Unity menu patches from it? It still works for me on 20.04.
Ah, I found the menu : it’s been converted to the silly GNOME burger menu. So you can at least use it …
(and also enable it permanently in the tab bar, which is what I may have done elsewhere…)