As the message says on the bottom, 'change your password.'

Thanks Warboy; for the context of other readers, we're following up on the potential unauthorized access in a private bug. If anyone else has received a similarly unexpected "New sign-in to Firefox" email, I invite you to file a bug at [1], let us know the email address associated with the account, and mark it as "security sensitive" for privacy. [1] https://bugzilla.mozilla.org/enter_bug.cgi?product=Cloud%20Services&component=Server:%20Firefox%20Accounts