Tony Vargas
Legend
Not really disagreeing, but...Quite the contrary, you'd be hard pressed to find a wizard in fantasy that casts fireballs prior to D&D inventing the trope. Gandalf is far and away one of the flashiest wizards in fantasy prior to D&D and he's the original inspiration for the fireball and in part the original inspiration for the non-occult wizard, but even he doesn't actually cast a fireball at any point in the story.
In Harryhausen's 1973 Golden Voyage of Sinbad, the evil Prince Koura - a wizard, who, among other things, makes a bat-winged 'homunculus' (nothing like the classical fetus-like thing of Paracelsus), but very much like the illo of the '77 AD&D MM and sees through its eyes, animates statues, and suffers aging side-effects for casting spells - also, off screen, sends a 'great ball of fire' to incinerate a room of the sultan's palace, burning off a character's face.