Most people develop acne when they become socially aware. Firefox just gets an add-on. Mozilla Contacts is an experimental new add-on for the browser that provides one-click access to your address books, provided that you've synchronized them.
Right now, the add-on has fairly limited features and functionality. It can sync contacts from Gmail, Twitter, LinkedIn, and Plaxo. Mac users can also sync with with local address books, and there are plans to incorporate Thunderbird, locally stored contacts in Windows and other social-networking systems.
Once installed, Contacts can be accessed only from your Tools menu. It opens up a new tab that's limited to entering in user accounts, although helpfully it already has a button for wiping that data. Your contacts list can be viewed in list or icon mode and will default to Twitter avatars as it merges contact data.
Currently, you have to give a site permission to use the data from Contacts, but some popular sites are supported. These include Flickr, Amazon, Yelp, and Gravatar. The add-on will automatically discover Webfinger and HCard data, and if you're looking at a contact in list view there's a search button that should search the Web for information about that person. This feature wasn't working for me, but it's a good idea.
Mozilla Contacts is fun to play with, but unless you're an early adopter there's not a lot here just yet to excite the average user. It is good to see that Mozilla is addressing a potentially killer feature in Google, given the easy ties between Chrome and Gmail.