Contacts in the Browser is a Firefox add-on which provides a contacts database that can be used within the browser.
The program has built-in support for importing your GMail, Twitter or Mac OS address books, and then stay in sync with them automatically. And if you use another address book, don't worry, a generic import routine may be able to grab the details you need.
And with that done, you get an easy email autocompletion tool that should work on any website. The browser detects that you're entering a friend's email, maybe to invite them to join this great new site you've discovered, and automatically offers you the full address so you don't have to type (or remember) it all yourself.
Sounds interesting, but beware - this is pre-alpha code, and bugs are to be expected. Uninstall the add-on if you have any problems.