Correct Firebase Development environment variable names and update README

Hi @ctrixcode ,

I just wanted to point out that in the Firebase configuration inside firebase.js object, the environment variable names are incorrect. In the README file, only the production environment variables are documented correctly not for the Development environment.

Also, AUTH_DOMAIN has been written as AUTHDOMAIN, which might cause issues for new contributors when setting up the project. So can I update the README to include the correct variable names for both development and production environments to avoid confusion.