The firefighters’ union is seeking an injunction in state Supreme Court to block the city from a new policy that prohibits firefighters from being called into work when their colleagues call in sick. The City Council last week decided in executive session to establish a new policy that will not allow firefighters to be brought into work when others call in sick, a move that City Manager Sharon A. Addison admitted violates the union contract. The union promptly filed a grievance. The city and the union are in the midst of a bitter three-year contract dispute. The new practice of “not backfilling” sick firefighters began Sunday. But attorneys for the Watertown Professional Fire Fighters Association Local 191 filed court papers seeking the injunction on Monday afternoon.