Thunderbird Notifications
Apparently this is a known issue.
I stated "Thunderbird … (updated to latest)" - BUT I was mistaken! The Update only updated to a version of 60. There seems to be no Update option.
There is apparently a version of 68 which does NOT have this problem BUT disables most of my Add-ons.
It seems I have a choice of displaying unread email OR working Add-ons (which is the whole point of Thunderbird).
Maybe I should try another email client (I find the side by side display of Mail unusable)