Using Thunderbird 68.11.0 on Windows 10. To reproduce, set: Options / Display / Advanced / Reading: Check "Automatically mark messages as read" - After displaying for 6 seconds View / Layout: * Classic pane * folder pane * message pane # Click on an unread message. # As soon as it's loaded in the message pane, switch to another message. # 6 seconds later, the message you viewed for only a second will be marked as read. '''Here's the crazy part: it is marked as read exactly X seconds after loading, where X is what you have set in "After displaying for X seconds" ''' I change that to 4 seconds, the message is marked read 4 seconds after loading. 10 seconds, it waits that long. Note: if I uncheck "Automatically mark messages as read", that is honored. Also, the feature works fine in Safe Mode. However, I have very few add-ons and they are all very popular: * Cardbook * Lightning * Provider for Google Calendar Can't imagine any of those interfering. Thanks for any ideas.