Firebase Authentication by default stores the user's credentials in local storage of the browser, and restores it from there when the page reloads. This requires a call to the server though, which means that if you access Firebase.auth().currentUser immediately as the page loads, it might not be set yet.

To prevent having this problem, use an auth state listener as shown in the first snippet of the documentation on getting the current user.

For v8 and earlier of the SDK that'd be:

firebase.auth().onAuthStateChanged((user) => {
  if (user) {
    // User is signed in, see docs for a list of available properties
    // https://firebase.google.com/docs/reference/js/firebase.User
    var uid = user.uid;
    // ...
  } else {
    // User is signed out
    // ...
  }
});

For v9:

import { getAuth, onAuthStateChanged } from "firebase/auth";

const auth = getAuth();
onAuthStateChanged(auth, (user) => {
  if (user) {
    // User is signed in, see docs for a list of available properties
    // https://firebase.google.com/docs/reference/js/firebase.User
    const uid = user.uid;
    // ...
  } else {
    // User is signed out
    // ...
  }
});