Line 2 large baking dishes (I used 9 X 13 pans) with foil and coat with cooking spray. Fill each with 7 1/2 cups popcorn.
Combine butter, brown sugar, molasses, corn syrup, ginger, cinnamon, and salt in a large, heavy bottom saucepan. Heat over medium heat until mixture boils. Boil for 5 minutes; stirring frequently. Remove from heat and add baking soda. Pour over popcorn; toss to coat.
Bake at 250 for one hour, carefully stirring the warm mixture about every 15 minutes. Cool completely.