I'm not aware of any calendar app that syncs with S Planner. What I would recommend instead in using [https://calendar.google.com/calendar/r Google Calendar] because it can be synced across devices. You will need to install the [https://addons.mozilla.org/en-US/thunderbird/addon/lightning/ Lightning addon] and the [https://addons.mozilla.org/en-US/thunderbird/addon/provider-for-google-calendar/ Provider for Google Calendar addon] on Thunderbird. You will also need to install the [https://play.google.com/store/apps/details?id=com.google.android.calendar&hl=en Google Calendar app] on your mobile device. There is no builtin calendar feature for Firefox at the moment, but you can easily use the web-based version of Google Calendar and pin the tab if you want to always have it open. Hope this helps.