Firefox Accounts is the account system for Firefox users to access hosted services provided by Mozilla.
A user can sign in with a Firefox Account to any of her "Foxes" - Firefox on Desktop, Firefox for Android, and Firefox OS - to access integrated services such as Firefox Sync and Firefox Marketplace. She can also sign in to services on the web using a standard OAuth flow.
For information on integrating a service with Firefox Accounts, visit the Firefox Accounts portal on MDN.
For information on contributing to Firefox Accounts development, visit the developer documentation.