How do I detect if a user is already logged in Firebase?

https://firebase.google.com/docs/auth/web/manage-users

You have to add an auth state change observer.

firebase.auth().onAuthStateChanged(function(user) {
  if (user) {
    // User is signed in.
  } else {
    // No user is signed in.
  }
});

You can also check if there is a currentUser

var user = firebase.auth().currentUser;

if (user) {
  // User is signed in.
} else {
  // No user is signed in.
}