From Baris Derin:
Twitter App brings Twitter right into your Firefox Sidebar. Get notifications, status and photos from the left sidebar of Firefox instantly. You can open Twitter sidebar by clicking View > Twitter App menu option from Firefox menu. Please login Twitterâ?¢ to start using Twitter App. The sidebar will remain open and you will be able to check your Twitter Favorites, Lists, Apps, Friends and get recent posts and notifications from Twitter without leaving your web experience.