Frerotte finished with 311 yards passing, connecting on 23 of 35 attempts.
St. Louis (3-9) has won three of its last four games.
Chris Redman replaced Joey Harrington at quarterback in the fourth quarter for the Falcons (3-9). Redman led a comeback effort with two touchdown passes, but he was intercepted near the end of the game.
Roddy White caught 10 passes for 146 yards and a touchdown for Atlanta.
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