Hi Felipe,
thanks for the contribution. Fastlane plugins have built-in system for passing arguments from ENV or command line. Advantage is that users can list these arguments and see what they can pass to every action. If you want to add the password to the plugin you can add it as:

FastlaneCore::ConfigItem.new(key: :password,
                                     env_name: "FIREBASE_PASSWORD",
                                     sensitive: true,
                                     description: "Password to your firebase account",
                                     optional: true),

in here

In every action you will need to get it from params and pass it to the login function as an optional parameter. Also, please do not re-indent the files, we might need to do some proper indentation in future but I would rather do it all at once.