If you were to do an inventory on all the secret (and not-so-secret) origins of heroes with fire-based powers, you'd see a common denominator. Most of them were the result of an accident, allowing the hero (or heroine) to not only project intense heat and gained immunity from their own flames.
Fireball was among the first of such characters, proceeded a couple years early by Timely Comic's Human Torch. While Torch was an android, allowing the reader to more easily accept his ability to cope with such temperatures, Fireball was fire fighter Ted Tyler bathed in an experimental chemical that mutated him into a heat-emitting hero. Decades later, the two-in-one hero Firestorm was composed of teen Ronnie Raymond and scientist Martin Stein, merged together by a nuclear plant accident. While his fire was more radioactive than other flame-based characters, Firestorm visually and conceptually is somewhat similar to Fireball. In a fair fight the younger hero easily out-classes his elder in both power and knowledge.