The only thing you need to do is block the URL: Chrome Extension. facebook unseen app You can install this extension in your Google chrome like you install other extensions.
Open the page, then click on the FREE button. After the application is saved and installed,you will notice a new icon. It is in the installed apps part at the top-right area. User may also click on the icon to toggle Facebook Unseen On and off. This will disable seen feature in facebook messages