Combine sugar, corn syrup, butter, and molasses in a medium saucepan; bring to a boil over medium heat. Cook 5 minutes, stirring once. Remove from heat; stir in vanilla, baking soda, and salt. Place popcorn in a large bowl; pour sugar mixture over popcorn in a steady stream, stirring to coat.
Spread popcorn mixture into prepared pan. Bake at 250° for 1 hour, stirring every 15 minutes.
Remove from oven; stir to break up any large clumps. Cool 15 minutes. Serve warm or at room temperature.
Note: Store in an airtight container for up to 1 week.