There is also one ugly but working way. Decompile FirebaseMessagingService.class and modify it's behavior. Then just put the class to the right package in yout app and dex use it instead of the class in the messaging lib itself. It is quite easy and working.
There is method:
private void zzo(Intent intent) {
Bundle bundle = intent.getExtras();
bundle.remove("android.support.content.wakelockid");
if (zza.zzac(bundle)) { // true if msg is notification sent from FirebaseConsole
if (!zza.zzdc((Context)this)) { // true if app is on foreground
zza.zzer((Context)this).zzas(bundle); // create notification
return;
}
// parse notification data to allow use it in onMessageReceived whe app is on foreground
if (FirebaseMessagingService.zzav(bundle)) {
zzb.zzo((Context)this, intent);
}
}
this.onMessageReceived(new RemoteMessage(bundle));
}
This code is from version 9.4.0, method will have different names in different version because of obfuscation.