We've recently had a discussion over here https://social.trom.tf/display/dbc8dc44-1364-c506-bca9-8ba414333506 about how Thunderbirdßs update is in fact not that "cool", but less functional, and how the theming and window managing is starting to become more broken on Linux because many developers seem to not think of their apps in terms of usability and respecting the user's choices and their system.