Install the Thunderbird add-ons [TBsync](https://addons.thunderbird.net/en-US/thunderbird/addon/tbsync/) and [Provider for CalDAV & CardDAV](https://addons.thunderbird.net/en-US/thunderbird/addon/dav-4-tbsync/) and follow [this guide](https://docs.nextcloud.com/server/latest/user_manual/de/pim/sync_thunderbird.html?highlight=caldav#recommended-method). As Server URL enter https;//faircloud.eu/nextcloud