Fiberglass doors actually have an aluminum frame with fiberglass sections. Like aluminum, fiberglass is very lightweight.

Other advantages of fiberglass are resistance to salt-air corrosion and translucency—fiberglass doors are sometimes chosen for locations where light transmission is important or in corrosive ocean climates.

For more standard uses, fiberglass isn't a very popular material because it is a poor insulator, it yellows with age, and it breaks relatively easily, especially when it's cold.