We get a lot of requests on howto use the MatriX XMPP SDK with Google Firebase Cloud Messaging.

The following are the settings for your XmppClient object

• The XMPP domain is: gcm.googleapis.com
• The hostname is: fcm-xmpp.googleapis.com
• It looks like there are no SRV records in place, this is why you need to set the hostname manual and disable SRV record lookups
• Firebase does not support XMPP’s StartTls feature and requires to initiate a TLS connection. This is how we have done it in the old days when the protocol still was called Jabber. The OldStyleSsl property takes care of this in MatriX’s XmppClient.
• Firebase is using a non standard XMPP port, because no SRV records are in place we have to set port 5236 manual in the code
• the username is your Firebase Sender Id and the password is your server key. You find them in the Firebase portal under Settings => Cloud Messagging
• the contact list and presence feature is not require. So we can disable it by setting AutoRoster and AutoPresence to false

XmppClient xmppClient = new XmppClient
{
    XmppDomain          = "gcm.googleapis.com",
    Hostname            = "fcm-xmpp.googleapis.com",
    Port                = 5236,
    ResolveSrvRecords   = false,
    OldStyleSsl         = true,
    Username            = "YOUR_FIREBASE_SENDER_ID",
    Password            = "YOUR_FIREBASE_SERVER_KEY",
    AutoRoster          = false,
    AutoPresence        = false
};

xmppClient.Open();

You can find a basic example also here in our GIT repository:
https://gitlab.com/matrix-xmpp/samples/tree/master/csharp/FirebaseClient

See also:
https://firebase.google.com/docs/cloud-messaging/server#implementing-the-xmpp-connection-server-protocol