In 4-quart saucepan, heat butter, corn syrup, brown sugar, granulated sugar and salt to boiling over medium-high heat, stirring constantly. Cook 6 to 8 minutes, without stirring, to 234°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a soft ball that flattens when removed from water. Remove from heat. Pour over popcorn mixture; stir with wooden spoon until well coated. Cool slightly.