I created an add-on to improve the thread view of mails in Betterbird/Thunderbird.
It shows all messages of a thread as cards in a tab of their own, newest on top, similar to the conversation view in Gmail.
Available in English and German (follows the Betterbird UI language, the German button is called “Mailverlauf”).
You can download the add-on from here:
https://gitlab.com/m0urs/thread-view
Thread View for Betterbird