This plugin allows the user to turn on the display of "(read/unread)" links to
the right of any folders in their folder list that contain at least one
message. Clicking on the link will then mark all messages in this folder as
read/unread. Note that you may turn the "read/unread" link on and off by going
to the Folders page. You may even turn only the "read" or the "unread" link on
or off. It also displays a "mark all read/unread" link below the folder list to
 mark all folders read/unread that are selected on the Folders page.

WWW: http://www.squirrelmail.org/plugin_view.php?id=218