By default, a server set up will defer authentication to the Mozilla-hosted accounts server at [https://accounts.firefox.com](https://accounts.firefox.com). So you will still have to authenticate at Mozilla, but _the storage of your information will be done on your host_.
Once installed, reaching `http://domain.tld/path` should show a page explaining how to configure it. Otherwise please refer to the [Yunohost page](https://yunohost.org/#/app_ffsync).