Firebase is meant to replace a whole backend server by itself. Sometimes though, you need to code some server logic yourself: that's where firebase functions come in.
Firebase let you code some server-side logic that triggers on certain events. To see the whole documentation go on https://firebase.google.com/docs/functions/callable.
One of those events are "https callables". You can trigger those via a GDevelop action. Use the trigger function action with the https callable name. You can pass arguments and let the https callable return an object.