
Google's mobile application development platform that helps build, improve, and grow app.
Google Firebase API is a free Development API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Developer documentation for Firebase
Details from firebase.google.com.

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The Google Firebase API is provided by Firebase (firebase.google.com). Developer documentation for Firebase
Yes. The Google Firebase API is listed in the Development category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Google Firebase API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Google Firebase API supports CORS, so front-end JavaScript can call it directly.
Yes. The Google Firebase API is served over HTTPS, so it's safe to use in production.