The new UI has a compact mode, I think you can get an even more compact UI than before. I've used thunderbird forever and I'm not lost at all. I think they did a really good job not breaking us.
I guess they can't please everyone but I don't know how they could have done better in this respect honestly.
There were several minor amateur moves however. The menu bar in the wrong place for example—not sure what they were thinking. Also hardcoded the highlight color to blue (on purpose), disrespecting the system UI theme. Same thing happened ~twenty years ago with firebug and I was happy to drop it for FF dev tools.
Every few years a new batch of "senior" engineers needs to earn xp points.
Personally, I find it a bit jarring, because contextually, that bar only contains stuff related to mail, but that bar and some stuff on it continues to appear even when I switch to Calendar. Thankfully, some CSS takes care of this.
Otherwise, it is a quite nice change. Dark mode now also applies to mail contents as well, which is a great change, though I would like a shortcut to swap to light mode.
1. the designers never considered keyboard navigation, which is now even more awkward than it was already
2. the interface is now very visibly slower - startup and general usage (on two of my systems)
This ignores the very long standing issues like:
1. searching is very confusing; there are several different types of searches
2. writing an email still requires opening another window. compose emails like it's the 90s!
Inbound email is 99.9% notifications. I read and delete most of them with K9 on my phone before downloading what's left with Thunderbird. BTW my mailbox is on a POP3 server. I clear it after downloading the messages.
I personally never like the old UI, and though it still has ways to go, it's miles ahead of the old UI.
To each their own, I guess.