The Mozilla accounts server provides a centralized database of all user accounts for accessing Mozilla-hosted services.
By default, Firefox will use Mozilla’s hosted accounts server at https://accounts.firefox.com. This configuration will work well for most use-cases, including for those who want to self-host a storage server. User who want to minimize their dependency on Mozilla-hosted services may also self-host an accounts server, but this setup is incompatible with other Mozilla-hosted services.
Protocol documentation: https://github.com/mozilla/fxa/blob/main/packages/fxa-auth-server/docs/api.md
API server code: https://github.com/mozilla/fxa/blob/main/packages/fxa-auth-server/
Web interface code: https://github.com/mozilla/fxa/blob/main/packages/fxa-content-server/