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The following documentation
here, mentions that the service account you would like to create is normally created by default when you enable the Compute Engine API for your project. You may check the documentation provided above for further details on what the responsibilities of this service account have.
As for your inquiry regarding Firebase and Google App Engine: standard, would you be able to clarify your exact use case is? Are you calling an API created on Firebase from GAE? This information will help me accurately assist you with your issue.
I look forward to your response.