I think you're referring to the new Unified Toolbar. You're right; at least for now, it is not optional. This is a blank canvas to place a variety of links. At first, I felt it useless, but now I wouldn't want to be without it. This link may give more info: https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova#w_2-unified-toolbar

If the menu bar was on top of that unified toolbar, it would still be acceptable. But toolbar on top of menu confuses me. I know, most of people usually hide menu, only a fraction use it novadays. But i'm disapointed if thunderbird developers don't care about users who have established habits using their software.

If the menu bar was on top of that unified toolbar, it would still be acceptable.

Not to hijack this, but let's make it clear you're speaking for yourself there.

Monitors are nearly twice as wide as they are tall, and a growing number are even wider than that. Vertical space is at a premium, and that one icon is wasting a huge amount of it - in return for adding literally no value at all, as the icon is already duplicated elsewhere onscreen .