For anyone having this problem I was able to solve it by Restoring Toolbar Defaults (right click on the toolbar, select customize, select Restore Toolbar Defaults).

In fact, restoring Toolbar Defaults does NOT permanently fix this problem. It has come back. I am unhappy with the recent updates of Thunderbird. The changes I've noticed are not improvements, they're just sort of an annoyance.

Restoring toolbar defaults that way only restores the toolbar buttons. It might be worth trying another reset by launching in safe mode (hold Shift when launching TB), checking 'Reset toolbars & controls', 'Make changes & restart'.

Thanks, sfhowes. I followed your suggestion but nothing changed. When I open Thunderbird and there are new messages they remain hidden and I have to manually scroll to see them. Very frustrating.

I have no active addons.

This issue seems to have a long history:

https://bugzilla.mozilla.org/show_bug.cgi?id=218935

Try this add-on: Auto Select Latest Message.

The auto select add-on seems to work! sfhowes, I can't thank you enough for your persistence in helping me find a solution. Very much appreciated.

After a recent update of the auto select add-on the add-on stopped working. I have raised the issue with the author but have not received a response. So, back to square one, no auto scrolling to the newest messages. Very frustrating!

It seems to work here in 78.6.1 after I just installed it, so maybe remove/install will restore the operation.

Tried uninstalling/reinstalling several times, still doesn't work. A real mystery!

Interesting development. When the message window is sorted latest at the top (message dates ascending) the add on seems to work. When sorted with the latest message at the bottom (message dates descending) it doesn't.

It's hard to believe that the author intended that the add on work that way.

When I first installed the add on it did work when the latest message was at the bottom. It seemed to stop working with that configuration after an update.