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 (mixture will foam). Quickly pour over popcorn and mix well; stir in nuts.
Transfer to two greased 13x9-inch baking pans. Bake at 200* for 45 minutes, stirring once. Remove from pans and place on waxed paper to cool. Break into clusters. Store in airtight containers.