Mozilla introduces awesome new contacts integration -- get the add-on now!
What are the advantages? Well, you'll be able to let sites grab your contacts via API, which makes setting up new social accounts a lot easier. Your Mozilla Contacts database will also stay in sync, which theoretically means you could add a friend as a contact on several new sites at once. Another perk is that email autocomplete will suggest recipients from your contacts on any web form.
It sounds like a pretty good idea and it's built on open standards -- so other browsers can easily implement Contacts, too.