Well yes this notifies you of new gmail message with a pop up like most do. But it also allows you to interact with the new mail without having to visit your inbox! That one little thing is so convenient its great! Look at the screen shot below for explanation on how it works:

As you can see in the above image, it shows the Subject of the email, and also a quick snippet of text from the email. Then there are a bunch of buttons in red between the subject and the snippet of text. The read; Open, Mark as read, archive, Report spam, and Delete. These are great because you can delete archive etc. all from here and not even having to open up a Firefox window and go into gmail. yes its a small feature but its worth its weight in gold for me!