Users of (reversed) threading have always requested to show the most recent message in the thread as the collapsed root, see this Bugzilla and this GitHub report. This has now been implemented.
Note that the most recent message in a thread isn't necessarily the last displayed message in this thread:
To enable the new functionality, set preference mailnews.collapse_thread_to_most_recent. This works with and without reversed threading. Note that when this preference is set, mailnews.sort_threads_by_root has no effect, since the root and the most recent message are the same.
As a reminder, the preference to switch on reversed threading is mailnews.reversed_threading.
A preview release (lastes build 7) is available for Windows and Linux. This will ship in the next release.
This is now shipping in Betterbird 140.11.0-bb23.
Please let us know your thoughts.
Update: And of course there was an issue with cross-folder views which we fixed in 140.11.0-bb23 "latest build".