We're not done with Thunderbird Supernova!
As promised, we will continue to improve the UI and functionality of message threads and Cards view.
We outline the upcoming changes in our Thunderbird UX mailing list here: https://thunderbird.topicbox.com/groups/ux/Tdb49a4666565e68e-M4aba7d876e0ced9c8d66ca58
A mockup with developer comments is also available from the link.
💡 If you have valuable feedback to offer about this, please reply to the mailing list so our developers & community see it!