Firefox appears to be blocking Cross Origins Requests so cannot actually successfully sign-in: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://apresolve.spotify.com/?type=dealer&type=spclient . (Reason: CORS request did not succeed).
> Cannot register to Track Playback with non-premium user.
However, the "iPod" just keeps telling the user to sign in. It might be good to have some indicator that you need a paid account to use Spotify on this iPod.