I recently upgraded from Thunderbird 38 portable to Thunderbird 45.1.1 portable.
Now it is no longer automatically marking the message as read after they are open.
The settings at OPTIONS -> DISPLAY -> ADVANCED ->
"Automatically mark messages as read" is properly set.
I tried different settings, like "Immediately on display" and "After displaying for X seconds".
At "About:config" I found some keys, that are also properly set:
I still can mark as read/unread by right-clicking the message and selecting "Mark -> As Read" or "Mark -> As Unread".
For me this seems a bug.
Any idea on how to fix it and having messages automatically marked as read again?