At this time, Mixpanel’s Android push services leverage Google Cloud Messaging (GCM) instead of Firebase Cloud Messaging (FCM). Mixpanel does plan to transition push infrastructure to FCM in the next year as Google winds down GCM and moves toward FCM.
In the meantime, Mixpanel still supports FCM for new implementations. Pass in your FCM Sender/App ID into the initPushHandling function, and use your Firebase API Key for delivery.
Moving from GCM to FCM
If you are switching from GCM to FCM, your users’ existing push tokens will not work with your updated FCM API key. You can either:
Switch to FCM and regenerate updated push tokens (recommended to ensure functionality after April 2019).
Stick with your current implementation and use GCM API keys (keeping in mind GCM will be deprecated after April 2019).