Recently there was a requirement in a project I worked on that required Firebase as our backend for saving progress and login for a simple RPG game. I had never worked with Firebase before but I presumed that it wouldn't be that hard. To my surprise, there was no plugin ready to use for Firebase in WebGL.
Creating the plugin
To make testing easier, since Firebase only works in web and not in the editor, I used a UNITY_EDITOR directive to define some dummy implementation for the login and progression save. This actually made our development much quicker, since other parts of the game didn't require the backend.
I created a WebGL template which implemented the necessary imports and configurations for Firebase, this also checks the login status so when the Unity app launches I can check that and automatically move forward in the ui. https://firebase.google.com/docs/web/setup?hl=es
Demo github repo
Here is a demo with the scripts I used. This project has some syntax errors as of now, mostly because there are some plugins from the asset store which I used but are not included in the repo (Doozy UI is a great plugin for managing your UI!). Hopefully this will suffice.