Facebook Connect is a secure and private application that allows users to log in and access other websites and apps with their Facebook credentials. It offers a single sign-on service, which allows users to log in and access numerous websites and apps with their existing Facebook login credentials. This is a convenient and secure way for users to quickly log in and access their favorite websites and apps without having to create a new account. Facebook Connect also provides users with the ability to share their activities and data with their friends and family on the social network, while keeping their personal information safe and secure.
Notice discontinued on the official site: "persona is no longer actively developed by Mozilla. Mozilla is committed to operational and security support of persona.org services until November 30, 2016. On November 30, 2016, Mozilla will shut down the persona.org services. Persona.org and related domains will be taken offline." See https://developer.mozilla.org/en-US/Persona