Preheat oven to 350 degrees F (175 degrees C). Stir the hash browns, sour cream, Cheddar cheese, celery soup, onion flakes, 1/2 cup melted butter, and salt together in a 9x12-inch baking dish. Stir the bread crumbs and 1/2 cup melted butter in a small bowl; top the hash brown mixture with the bread crumb mixture. Do not mix topping until right before you put the dish in the oven, otherwise it will become soggy. Bake in preheated oven until the topping is golden brown, about 45 minutes.