This is an example of using Nuxt.js in SSR mode with Firebase Authentication
/plugins/auth-cookie.js/plugins/firebase-client-init.js/services/firebase-admin-init.js/middleware/auth-check.js/store/index.js
For the Sign-In to work, you will need to add your production site url to the Firebase's Authorized Domains in the Authorization settings of the Firebase Console.
I got a lot of help understanding what needed to happen by looking at this so special thanks to the contributers of that project! Firebase Function - Handlebars Example