How to query firebase from a node backend with the credentials of the client

Here is a starter example

https://github.com/zeit/next.js/tree/master/examples/with-firebase

Unfortunately the firebase-admin tool and credentials is like accessing firebase with a root user. I originally thought some magic would happen with the req.session.decodedToken, but it appears that this is solely for passing the user object as props.

The objective here is to have a server rendered page as it would be in the front end.

The firebase npm package states that it can be used in the backend:

https://www.npmjs.com/package/firebase

In the client the firebase package seems to be using data from local storage. After successfully authenticating I can see a entry in local storage firebase:authUser:TOKEN. I assume all firebase queries use the data in that key.

But how do you do this in the backend? Since this seems to happen automatically, I'm not sure how to tell firebase to use data from session for authentication.