Firefox 17 is a Beta version, and AFAIK this new feature was just added today (10/22) - with the latest update from Mozilla. Are you running a version of Firefox 17 beta that you installed from a Mozilla server, or a version obtained from the Ubuntu repository?
Ubuntu may not have yet released the latest updates yet.
"To get started just upgrade to the latest Firefox Beta and then visit Facebook like you normally would. When Facebook offers you Firefox integration, it’s a single click to install and give it a try. It’s just as easy to remove, and obviously if you decline the initial offer nothing will ever be installed. The Social API doesn’t change what social providers can see about your online behavior unless you explicitly decide to share it, but it does make it much easier to stay connected."