Heat oven to 200º. Place popcorn and almonds in very large roasting pan or divide popcorn mixture between 2 ungreased rectangular pans, 13x9x2 inches.
Heat brown sugar, butter, corn syrup and salt in 2-quart saucepan over medium heat, stirring occasionally, until bubbly around edges. Continue cooking 5 minutes without stirring; remove from heat. Stir in baking soda until foamy.
Pour sugar mixture over popcorn; toss until evenly coated. Bake 1 hour, stirring every 15 minutes. Spread on aluminum foil or cooking parchment paper. Cool completely, about 30 minutes. Store tightly covered.