@thunderbird I think the macOS version has always lacked in UI integration. In fact, I've been using Thunderbird on other OSes except on the Mac, until v102. This new version felt a huge leap forward, yet it still doesn't stand a chance to overtake the fact people are just used to the (terrible) OS-integrated Mail.app.

Months ago I read some sort of roadmap, and I remember the goal was to match Mail.app's GUI, more or less. Can't wait.