FirebaseToken mock implementation

What is the easiest way to mock a FirebaseToken ,usually returned by the FirebaseAuth.getInstance().verifyIdTokenAsync(idToken) method. My authentication function calls this method and returns the FirebaseToken. I would like to test my authenticated route, but unfortunately FirebaseToken isn't an interface. I wonder why this design decision was taken.

The constructor of FirebaseToken takes a FirebaseTokenImpl which in turn takes Header header, Payload payload, byte[] signatureBytes, byte[] signedContentBytes. Payload is a static class with private fields.

I think the easiest option would be to map the FirebaseToken onto my own token representation class in the real authentication function, so that I can mock it easily in the overridden fake authenticator.

The second option, without any useless overhead, would be to find a way to instantiate a FirebaseToken. Unfortunately I need help with that. How do I create an instance of FirebaseToken with my chosen uid, mail etc?