Practically it adds some handy features like autofill, page rank and gmail.
You can decide if you gonna use them often.
Personally, I don't like it, because it takes some space on the screen and I already have installed some plug-ins and I don't feel like having another one in front of my eyes all the time.
But hey, have a look here:
http://www.google.com/tools/firefox/.../en/index.html