Every once in a while, someone asks if Mozilla Thunderbird can automatically mark a folder or newsgroup read when you exit that folder or newsgroup. (Why: I don’t know. Automatic marking of all messages as read on exit, is one of the reasons I dislike web-based forums.)
Thunderbird 1.5 can do this, but the setting is hidden. Go to Tools -> Options -> Advanced -> General, and click on “Config Editor.” In the resulting window, right-click on any of the listed preferences, and select New -> Boolean. The preference name to enter depends:
mailnews.mark_message_read.nntp for Newsgroups
mailnews.mark_message_read.pop3 for POP3 email folders
mailnews.mark_message_read.imap for IMAP email folders
mailnews.mark_message_read.rss for RSS folders
mailnews.mark_message_read.none for folders in “Local Folders”
Set the value to true. Close the Config Editor, and click on “OK” in the Options panel, and you’re done.