I use a few email accounts with one dedicated to junk and spam. Thunderbird is auto tagging them as spam/junk, but still displays them as unread mail. It's not wrong, but it makes the pop up and unread counter kind of useless as I don't know how much new mail I actually have!
I want Thunderbird to not count junk mail as unread messages and not to notify me of them.
Ubuntu 12.10, Thunderbird 17.0.
Edit->Preferences->Security Tab->
There is a checkbox:
"Mark messages determined to be Junk as read"