Growl is a notification system which allows applications to send notifications about events. Firestorm can use this to alert you of received IMs, when you are working in other applications. This is set in Preferences → Notifications -> World -> Enable Growl Notifications.
NOTE: Do NOT enable Growl in the viewer this unless you actually have Growl installed and active. Enabling this option without having growl installed will result in small freezes when someone messages you or goes on/offline.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.