It defaults to the bizarre Gnome look (no native controls, toolbar buttons haphazardly strewn through the title bar), but you can change it in the preferences if you hate that as much as I do.
I have a relatively niche use case where thunderbird would no longer find my smime certificate on my smart card. But only in the compose/send UI, not in settings.
While I understand not many people are doing this, it also... Used to work, and I would have thought that detecting regressions in X.509 cert validation should be well covered by tests.
In comparison, evolution works.
My only note with that is the caldav and exchanges internal handling of event invites will fight with each other, so don't try to accept invites from within your imap mail client.