It should also be added that the whole backend is proprietary (and was for GCM as well) and is being maintained and paid for by Google.
Having said that, making firebase-messaging GPL-compatible library seems to be something that should make a lot of sense for Google to allow GPL apps to be released on the platform.
Having said that, making firebase-messaging GPL-compatible library seems to be something that should make a lot of sense for Google to allow GPL apps to be released on the platform.