Seen Blocker for Facebook™ by Morni

Seen Blocker for Facebook™ allows you read your Facebook messages without letting Facebook from reporting your seen status to the sender by blocking the request. As a result your friends will not see the 'Seen' label when you read their messages. The extension can simply be activated from the toolbar button and it will preserve its last activation status across sessions.

FAQs:
How can I activate the extension?
Once this extension is installed, there's nothing really much you need to do. Just click on the toolbar button. When the the color turned to green, it will block specific web requests to disable read receipts from Facebook.