Firefly is not trained on historical, religous, or public figures. You can describe the person you are trying to depict but you can't ask it to render someone specifically by name.
Just to add what others have said. Give detail to the way you want your character to appear (colour, hair - length/colour, eyes, etc). Generally, if you ask for a man or woman, it will create that. Firefly does inject diversity so if you simply state ask for a group of people, you're likley to get a mix of genders and cultural diversity.