Configure your firebase credentials on the official firebase console. And add these details into your react app enviroment (.env)
REACT_APP_API_KEY
= 'api-key-here'
REACT_APP_AUTH_DOMAIN
= 'app-auth-domain-here'
REACT_APP_PROJECT_ID
= 'project-id-here'
REACT_APP_STORAGE_BUCKET
= 'storage-bucket-here'
REACT_APP_MESSAGING_SENDER_ID
= 'messaging-sender-id-here'
REACT_APP_APP_ID
= 'app-id-here'