Bugzilla

(In reply to comment #2)
> However, now, when I have read all the visible messages in a group, I am left
> with with a message count showing some unread messages in the group. I'm
> guessing this is because the threads I've marked to be ignored are just being
> removed from my view, rather than actually having all the messages posted to
> the thread, previously *AND* into the future, being marked as read as well as
> being invisible to me.

~~Bug 448776~~ Bug 448876 is most likely what you want. That's an entirely different issue altogether.

On another note:
(In reply to comment #1)
> confirming, but likely a core:? issue

This would be a MailNews Core: Networking: News bug.