If a message is cross-posted to several newsgroups, mozilla won't mark all the occurrences as read if you read one occurence of it. I believe such cross-post detection is common in news-readers, at least i think it's a nice feature to have. I'm not sure which component this is, but other cross-post related bugs were filed under Mail Back End, so i picked that one.
Known limitation for 6.0 release. Duplicate of 432778 *** This bug has been marked as a duplicate of 43278 ***
Marking verified as a duplicate.