index.js

...
import { auth, providerGoogle } from '../lib/firebase';

function Index() {
  ...
  const handleAuth = () => {
    auth.signInWithPopup(providerGoogle).then((result) => {
      console.log(result);
    });
  };
  
 useEffect(() => {
  auth.onAuthStateChanged((user) => {
    if (user) {
      auth.currentUser.getIdToken(true).then((token) => {
        console.log(token);
      });
    }
  });
  }, []);
  
  return (
    <div className="container mt-5">
      <button type="button" className="mt-5 mb-5 btn btn-primary" onClick={handleAuth}>
        Google
      </button>
      ...
    </div>
  );
}

export default withRedux(Index);