Every Firebase project has an associated Google Cloud Platform project. If you’ve added billing to your project in order to enabled any paid features or APIs in the Cloud project, Firebase will auto upgrade to Blaze (pay as you go).
Under Blaze, you will still get all the free tier usage from the Firebase Spark plan, described in the Pricing Guide. You will only be charged if you exceed the free tier.
To switch back to Spark or Flame plans:
To setup budget alerts, follow the budget how-to guide.