Firminy is a commune in the Loire department in central France. It lies on the river Ondaine, 13 km southwest of Saint-Étienne by rail. The anci…Firminy is a commune in the Loire department in central France. It lies on the river Ondaine, 13 km southwest of Saint-Étienne by rail. The ancient name of the town was Firminiaco or Firminiacus. The name was first recorded in a 971 charter by the King of Burgundy.