It is possible to set up billing alerts to prevent this from happening, but it's done in the Google Cloud console, not the Firebase console. To set up alerts, go to the
Cloud console, choose your project, and then follow the instructions under "create a budget"
here.
Also, support should be able to help out with the big bill from this month. Just
send us a description of your bug and what you've done to fix it, as well as the amount you'd like to be credited/refunded :)