This has happened to me more than once, over several builds. Most recently today. After substantial mail activity over several days (don't know if this is actually a factor), go to a local folder. Dialog box offers to compact mail folders. Say yes. When complete (did I wait long enough? - not sure), go to INBOX. Immediately the unread messages, one after another, lose bolding and go to the read state. Very bad!
mail issues belong over in the mailnews product, so punting. not sure what the proper component would, so move as needed.
This is a dupe of bug 100781 "Unread messages changed to read when compacting folder", and was just (04/15/2002, 15:41) fixed on the trunk by bienvenu. *** This bug has been marked as a duplicate of 100781 ***