env.js

var ENVS = {
  prod: {firebase: {firebaseURL: 'https://my-app-prod.firebaseio.com'}},
  staging: {firebase: {firebaseURL: 'https://my-app-staging.firebaseio.com'}},
  dev: {firebase: {firebaseURL: 'https://my-app-dev.firebaseio.com'}},
}

window.__env = (function() {
  switch(window.location.hostname) {
    case 'localhost':
      return ENVS.dev;
    case 'my-app-staging.firebaseapp.com':
    case 'my-app-staging.com':
      return ENVS.staging;
    case 'my-app-prod.firebaseapp.com':
    case 'my-app.com':
      return ENVS.prod;
  }
}());