If you're a person like me (who constantly changes browsers and does not use any chat client) then (oh, and that uses Gmail) during the time interval where Firefox is the default browser and you occasionally chat with a buddy using the built-in chat in Gmail, it's a serious drag when you switch windows, continue your everyday activity and don't receive any alert that your buddy wrote something in the chat window.
Unfortunately, this feature is only Chrome available. But, by installing this Firefox extension, you can receive chat notifications even during the Firefox time interval. (the notifications UI is not as good as the one in Chrome, but it sure beats no notifications)
Thank you and Enjoy!