This ext shows the number of unread Facebook notifications on the toolbar icon. You can also choose and enable SOUND notifications.
Image from store
(2020-12-26) Felina Faerlaingal: Can't use it, it says
"Application not configured: This application is currently under development, and you cannot access it. Use an authorized test user account or speak to an application administrator to gain access."
and won't log in
(2015-11-25, v:1.0) Александр Шишов: Icon The current icon is not so god. Please make something in classic FB style.