Spread popcorn evenly in a lightly greased 15-x 10-x 1-inch jeilyroll pan; set aside.
Combine molasses, corn syrup, and vinegar in a medium saucepan; cook over low heat, stirring occasionally, until mixture reaches soft crack stage (270).
Remove from heat; stir in butter.
Pour syrup over popcorn; stir until all popcorn is coated.
Let mixture cool slightly; shape into 2 1/2-inch balls.