A firefighter in south suburban Matteson died while performing CPR on a shopper at Wal-Mart. The firefighter, identified as 49-year-old Mark Zielinski, was called to the store at 21410 S. Cicero Avenue in Matteson Friday afternoon for a medical emergency. He had been with the Matteson Fire Department for 13 years, after spending many years working for the fire department in Harvey. His cause of death has not been determined. The condition of the person he was helping is not known.