How about the browser's back button?

If someone sends you an email via that form, the email contains a link which takes you directly to http://musicbrainz.org/user/username/contact

Ah. If the use case is to go back to the user page, perhaps a direct link to the profile page would make more sense in that context than a cancel button?

Why not just keep the profile navigation bar ("Profile", "Subscriptions", ...) on the contact page?

Fixed, contact form now uses the main users profile layout