I'm using the firebase function signInWithPopUp() to handle user authentication in my web app. When a user initially signs in I can get an access token from the response but after that I can't figure out how to get it. If the user comes back to the site a day later they will still be signed in but they won't have an access token anymore. I found this and a few other answers but they make it seem like you have to change your entire authentication flow (i.e. I can't use signInWithPopup()). Is this true?

I thought I could use firebase.auth().onAuthStateChanged((user) => {}) to check for an access token and then retrieve one if necessary, but I can't figure out which endpoint to hit for it.