I installed Thunderbird for the first time in a couple of decades recently. My impression was that it's very feature rich but also quite ugly and not friendly to new users. It comes with a lot of assumptions about what the user wants to do and how, and I found myself having to use cheats and workarounds from the outset. I wanted to import a batch of disparate .eml files that had been seperately exported, and after 15 minutes I was starting to think it might have been easier to just do it in Python.
I also didn't care for the tabbed panels, which make it feel as if the entire thing was just ported from a browser. It really needs some fresh design and user interface work.
Most of the smashing up and disfiguration of a perfectly good interface, uglification, waste and breakage of both data and compatibility were introduced in a rewrite 2023 ostensibly aiming for "fresh" design and user interface work.
I'd heed the call for donation simply for returning to pre-2023 design with up-to-date security patches. As it is, maybe it's merciful if development just comes to a standstill. Almost every visible step lately seems to move in the wrong direction.