Firebase .set() promise stuck on pending

I'm trying to do a simple write to the fb database like so:

return app.register( email, password ).then( f => {
  return app.login( email, password )
} ).then( f => {
  return app.currentUser()
} ).then( user => {
  return app.firebase.db().ref( `users/${user.uid}` ).set( {color: 'red'} )
} ).then( f => {
  console.log( "This never triggers" )
} )

Where app.register is:

( firebase, email, password ) => firebase.auth().createUserWithEmailAndPassword( email, password )

Where app.login is:

( firebase, email, password ) => firebase.auth().signInWithEmailAndPassword( email, password )

And app.currentUser is:

firebase => {
    return new Promise( ( resolve, reject ) => {
        firebase.auth( ).onAuthStateChanged( user => {
            user ? resolve( user ) : reject( user )
        } )
    } )
}

App.firebase is an initialised firebase reference.

I am getting no errors, but the .set() never resolves.

I'm very much stuck, any help appreciated.

via Mentor