Whenever I get an overview of all active apps, I noticed that the calendar is always among these apps, even though I am sure I didn’t start it myself. It seems like it is selfstarting.
Well, it needs to be active so it can alert you of events.
Yes, this is by design.
There’s a few things going on. The first is https://bugzil.la/821604 causes the calendar app to periodically wake itself up using the mozAlarms API even if there are no remote servers to synchronize with. The second is that the calendar app does not window.close() itself when it’s done doing whatever it does. The email app, for example, closes itself if it started itself up and you didn’t switch to the app. There may or may not be a bug on enhancing the calendar app to behave more like the email app.