FacebookNotifier adds a toolbar item with a badge showing the current unread notifications you have on facebook.
Note: This extension was inspired by another extension, called FB Checker by UntosCZ. However, his extension stopped working, and the url from which I got it is no longer on the web, so I decided to rewrite it. Enjoy.
For more information visit http://cornellcampbell.com/extensions/posts/7/facebooknotifier/.