I solved this by using the photoURL that can be found within: user.providerData[0].photoURL
Where user is the user data returned on
firebase.auth().onAuthStateChanged( user => {})
And [0] is the index of the Authentication Provider of interest. In my case I only have Facebook authentication, thus the index is 0.
Seems that this URL is somehow being updated... (maybe on user sign in?)
Edit: It seems that the URL is indeed being updated on user sign in. So be aware that if the user is "remembered" by the browser, with this method the image will eventually expire, if the user isn't forced to sign in again before expiration.