The last two times I logged onto MEF I have had my read messages to me shown to me again.  The first time, I saw I had unread messages to me after Id finished reading new messages and when I looked at them, I'd seen them already.  This last time, it showed them to me as new messages, but it wasn't showing unread messages to me.  This only happened on MEF, not on FF.

ETA: oh, also happened just now in Support--repeated seen messages but wasn't showing unread to me, only as unread messages.