Crie um arquivo '.env.local' na raiz da sua aplicação, com as seguintes variavies:
REACT_APP_API_KEY= REACT_APP_AUTH_DOMAIN= REACT_APP_DATABASEURL= REACT_APP_PROJECT_ID= REACT_APP_STORAGE_BUCKET= REACT_APP_MESSAGINGSENDERID= REACT_APP_APP_ID= REACT_APP_MEASUREMENT_ID=