Okay, so this is less of a "how-to" and more of a "buy-this-contraption-and-shoot-fireballs-from-your-wrists" kind of post. Behold PYRO, whose actual tagline is "Shoot Fireballs From Your Empty Hand"...
No longer simply the realm of Marvel characters, the PYRO was designed by Adam Wilbur of Ellusionist.com and it appears to deliver. It consists of a wristwatch-sized gauntlet of sorts with a separate trigger that can shoot up to four fireballs (made of flash cotton and paper) roughly ten feet in the air before you need to reload. And each pack contains enough ammo to use more than fifty times. The video below feels a little over the top to me, but I can sympathize with his excitement.
Kinda.