User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:41.0) Gecko/20100101 Firefox/41.0 Build ID: 20150917150946 Steps to reproduce: I'm using macbook 10.10.5 yosemite, TB 38.2.0, thunderbird conversations 2.10. In options, I select automatically mark messages as read and immediately on display. In view dropdown menu, select conversation view. Select an unread message. Actual results: For some messages, the message is marked as read immediately. For some other messages, the message is never marked as read. There does not seem to be any specific pattern as to which messages are marked as read and which not. Whether the message is part of a message thread or not does not play into this. Whether the message is marked as important or not does not play into this. The current size of inbox or the compaction does not seem to matter either. Right click on the message and selecting mark as read always works. If conversation view is not selected, the messages are always marked as read when selected/displayed. Expected results: Whenever a message is selected and displayed, it should be marked as read in conjunction with the preference (mark messages as read / immediately on display).