It was several years ago since I used Thunderbird, but I just tried Solarflare on the Linux laptop I recently got and enabled the new "cards view" and one huge flaw stood out immediately:
* When I went to reply to an e-mail which was a threaded view, I was very surprised to not see a reply button!
It appears that when Thunderbird collapses messages to a thread, and you click on that thread in the list, the right part of the screen just shows... I don't know how to describe it... an old, 1990s style static webpage? It does not look modern at all!
Worse, this 1990s text based threaded summary does not respect the ascending/descending sorting option you've chosen for your list of e-mails!
Continuing on this bad behavior, I have to choose a mail from this horribly ugly page and then click a second time to get the reply button to show up. Why the latest mail isn't chosen automatically and why the reply buttons aren't always visible are strange UX design decisions I just can't get over.