When a recipe calls for “firmly packed” brown sugar, it should look compacted in the measuring cup. Push down with your fingers or a tablespoon firmly on the brown sugar in the measuring cup.
Brown sugar is a soft textured sugar made with a combination of white sugar and molasses. The two most common types of brown sugar are light and dark. The lighter the brown sugar is, the more delicate the flavor. Dark brown sugar has a deeper color and stronger molasses flavor than light brown sugar. Brown sugar tends to clump because it contains more moisture than white sugar.