Helpful Information

• You can test Firebase Cloud Messaging using this test application*. Push notifications do not require account registration.
• You can add an account through the system settings. Some applications might ask you to do so, if you don't.
• Apps that use Firebase Cloud Messaging must be installed after GmsCore, or else they will not work. Some applications that can run with microG GmsCore is installed in the correct order: TextSecure/Signal, Play Music, YouTube
• If you are using AdAway, make sure to put mtalk.google.com on your whitelist, or else problems are likely to occur when using Firebase Cloud Messaging. Thanks @benstyle1 on XDA for the hint.
• If your device is having trouble registering with Firebase Cloud Messaging, you may need to open the system phone app and dial *#*#2432546#*#* (or *#*#CHECKIN#*#*) to manually register the device as described here. if typing via the keypad does not work, check this out: (execute as root) adb shell am broadcast -a android.provider.Telephony.SECRET_CODE -d android_secret_code://2432546 --receiver-include-background

*possibly inaccurate if not up-to-date; if you're having issues considering testing with a more up-to-date application