Yes. This is definitely citizenship discrimination.  You cannot fire a protected individual under the INA because s/he has a temporary work permit as opposed to legal permanent residency.  A protected individual is a U.S. citizen, national, permanent resident, temporary resident, refugee, or an asylee.  In any event, your firing decision cannot be based on this factor.  Otherwise, your actions will be considered discrimination by the Office of Special Counsel for Immigration Related Unfair Employment Practices.