Firefighters scrambled to contain a small fire that was discovered on Saturday after it had apparently smoldered overnight amid construction debris and plywood on the 89th floor of 1 World Trade Center in Lower Manhattan, the authorities said. One firefighter suffered minor injuries in the fire, which was discovered shortly after 7 a.m., according to James Long, a Fire Department spokesman. The ”main body” of the fire was extinguished by 8:19 a.m., said Assistant Chief James E. Esposito, the department’s top commander in Manahttan.