I have Thunderbird 45.3.0 on a MacOS 10.8.5 system. Under Preferences->Display->Advanced, I have "Automatically mark messages as read" and "Immediately on display" checked off and the radio button clicked, respectively (see attached screenshot). Yet, this only works for *SOME* messages in the same thread (i.e., the received messages are all located in the same IMAP account). Why do only some messages get automatically marked as read but not others even if all are displayed one-by-one (i.e., I click on each one to display it in the preview pane and even when I double-click it to open it up in its own tab or window)? This non-marking as read occurs no matter how long I leave them displayed (so it's not a timer issue). Even if I select the "After displaying for __ seconds" and make it 1 or 2 or 5 or 10 or 20 or 30 seconds, Thunderbird *STILL* won't mark some displayed messages as read even after the requisite time has elapsed. As a result, I have to manually mark them as read. I'm perplexed. Can anyone explain why this is happening and what I can do to fix it? This inconsistency when all the messages are in the same IMAP account doesn't make sense to me and is quite infuriating.