Is push support in Service Workers dependant on Firebase (and Apple's equivalent)? All the tutorials I find have a step where you register a Firebase account, but for our webapp this is a no-go as it will be used at premises without internet access. 

I would have assumed that it was possible to register a URL that conformed to some protocol that the OS would register with, but I cannot find any information of the sort.

If it is the case that one needs internet access for service worker push support I assume the only option for a web app to receive background notifications is to wrap it in a native web view and use that to call out to on-premise services.