Install it from the Chrome Web Store
Sick of the new "seen" feature of facebook? Block the read receipts with this extensions and your friends will no longer be able to see if you read their messages. You will still be able to see if they read your messages. Additionally you get a button to mark messages explicit as read.
Attention: If you are loading the "full conversation" page (Messages » Name of your friend), it will get marked as "read" and your friend will see it!
One drawback of this extension is that all your messages will not be marked as read for yourself, too, and the "unread messages" notification stays visible as long as you do not reply, open the full conversation, or mark it explicity as read.