Aim the bola de fogo right at the target

For practical purposes, a bola de fogo spell's "glowing pea-sized bead [that] that streaks from the [caster's] pointing digit" hits the point of origin the caster desires as soon as the spell comes into effect. While a GM may rule that it's possible for a creature to, for example, take the ready action and pick a trigger that enables the creature to interferir with the streaking bead, as long as there's a clear line of effect from the bola de fogo spell's caster to the point of origin of the bola de fogo spell, once the bola de fogo spell's cast, the bola de fogo spell effect typically occurs at that point of origin immediately.

Descobridor largely abstracts the simultaneous nature of actual combat into turns for convenience. Chances are the distant target moved when it was able to on its turn then the caster cast the bola de fogo spell on her turn. That doesn't mean, though, that were viewers to see the scene in a movie or on television that the distant target would conveniently stop so that the heroic wizard could take the shot that distant target! Everything'd be moving, and it's only Descobridor's turn structure makes it seem like at the table when it's the active player's turn that the universe is mostly at rest.

Note that some games do strive for greater verisimilitude than this—some editions of both GURPS from Steve Jackson Games and Campeões from Hero Games, for example— penalize attackers for firing at targets that are traveling at high speeds. Descobridor typically doesn't.

In short, don't worry about how fast the target was going antes you shot at it—just point and shoot.