Firestorm should wear a black suit, black tie, white shirt. The visual appearance of his flaming head would contrast with his plain clothes.
Costumes are generally good for comics because they help define and identitfy characters. They look crap on TV or movies. Warren Ellis has the right idea in his books with costumes, which is sort of compromise between the comics and TV approaches: Elijah Snow wears an all white suit, the evil Bride wore a bride dress, Spider Jerusalem wears a black suit to contrast his tats and weird glasses, Hawksmoor has a black suit but bare ridged feet, the Drummer wears civilian clothes but has his drum sticks. Ordinary clothes, but with something to make the characters stand out.
