Solution 1:

Hope someone finds this useful, it helped in my case.

Currently sending push notifications from device to device isn't possible with solution Firebase offers. If you want that type of thing, you should try OneSignal which is free and very easy to implement.

Solution 2:

There are two features in the new Firebase to send notifications to users:

  • Firebase Cloud Messaging - the next generation of Google Cloud Messaging
  • Firebase Notifications - built on top of Cloud Messaging, which allow you to easily send notifications to users through the Firebase Console

To send messages with Cloud Messaging, you need to specify an Authorization Key in a request. Since knowing this key allows sending of messages on your behalf, you'll want to limit access to it to trusted processes. This indeed can be a server, but any trusted process is fine.