Sorry, just discovered a bit more info. This appears to be related to unified folders (unified inbox). If you de-select unified folders, Thunderbird behaves as expected, opening with the newest message selected. Unified inbox is however crucial for me. I monitor over a dozen accounts.

I appreciate any help.

Try resetting Unified view: https://support.mozilla.org/en-US/questions/1426387

I'm not certain that add-on works in this version. After enabling, I couldn't find any menu next to Unified Folders heading anyway.

I attempted the manual instructions, and this didn't help. I reset the startup cache while I was at it and saw no change from that either.

The add-on works here in 115. Select View/Folders and enable Unified Folders and All Folders, then click the 3-dot icon in the header (see picture).

Ok. Learned that I had to have "All Folders" selected in addition to Unified Folders in order to see that heading. Got add-on installed, and chose to reset Unified Folders, but the behavior is the same. If I exit TB, upon restarting, the messages pain goes to the top rather than to the newest message.

This issue is still present. I have tried using "Troubleshoot Mode" then just enabling Unified folders. Should make it pretty stock I would think, but the issue is there.

Sfhowes, I appreciate your efforts at helping! Can anyone using this version and unified folders confirm that they are NOT having the issue? Maybe that would lead to an answer?

I see the same behaviour with Unified view, and it has been reported here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1845934

sfhowes, thanks again. I'll keep eyes open for the next update.