What is the rest of the home made of? Fiberglass isn't going to melt at a considerably lower temperature than the steel unit will fail. If the house has a metal roof and some sort of cementitious cladding, perhaps metal is more indicated (and with a fire rating) in that case. If the construction is otherwise based on norms like asphalt roofs and non flame resistant cladding, I am not sure it makes a difference. Generally speaking, I prefer Fiberglass over the steel option.

house is stucco with a fire-rated roof. It has aluminum windows.

Real stucco or EIFS? If real stucco, then the fiberglass door would likely be the weak link along with the windows.

real stucco. Keep in mind, this house was literally destroyed by a wildfire, this is the rebuild. This is an area of southern California at risk for wildfire. The need to protect against fire is serious here. Windows are double-paned aluminum clad. They would have a fire rating of 20 minutes A steel door would have 90 minutes and the fiberglass, no rating. The front door, however, is not at the riskiest spot in front of the canyon. It faces the street.