From: Gary Britt 

I always had OE set to mark all folders and newsgroups read upon exiting
them.  That's the way I liked it, maybe not for everyone.  Anyway since I
moved away from Microsoft to Thunderbird, there was no setting for this in
thunderbird and you had to manually mark each group/folder read each time
manually.  A bit of a pain.

Anyway it turns out that Thunderbird can mark each folder read upon exiting.
  Its just that there is no GUI setting and no built in config editor
setting that allows this.  You have to manually enter these settings
(registry keys in windows speak sort of anyway) and then set them.  Maybe
everybody else here using TB already knows this but here they are.

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(s) to enter are:

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.

Gary

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