I created an app called "Better Crumbs" to demo firebase auth and firebase firestore. This app is a vanilla JS app which uses parcel as a bundler. This ensures that it works on old browsers and the code is optimized.
The app is open source under the MIT license. So you can check it out and learn about firebase auth and firestore. Click here to check out the repo on GitHub.
I also hosted it on Netlify. Check out the demo here.
I am open to constructive criticism on how I can improve the code quality and architecture.