in Preferences under General, there are tons of options for notifying you a message was received in the background.
You can have the Fire icon bounce, make it stop bouncing either when you bring Fire to the front, or when you bring the specific message window to the front, you can have the Fire icon flash in the dock, or the coolest one of all . . .
Flash chat windows - this causes the message window that is trying to get your attention to turn translucent and back, fading in and out like the sleep light on a PowerBook. Very unobtrusive and effective.
That being said, one very cool feature of the official AIM client is the way its individual message windows work in the dock.
I love Fire, great piece of software.
-- Jason