Place popcorn and pretzels in a large bowl; set aside. In a large heavy saucepan, combine the brown sugar, butter, corn syrup and salt; cook over medium heat, stirring occasionally, until mixture comes to a rolling boil. Cook and stir until candy thermometer reads 238° (soft-ball stage). Remove from the heat; stir in baking soda. Quickly pour over popcorn and mix thoroughly; stir in peanuts.
Turn into two greased 13-in. x 9-in. baking pans. Bake at 200° for 20 minutes; stir. Bake 25 minutes more. Remove from the oven; add candy and mix well. Remove from pans and place on waxed paper to cool. Break into clusters. Store in airtight containers or plastic bags.