-Sigh- That's because orcs (which are the race of pretty much all barbarians and the clan, including goblins) were changed to be IMMUNE to fire for some silly reason I don't understand. That's not a problem with Tower Mages so much as the target being immune to fire; same problem arises from a normal mage or wizard casting fireball as well.
Only idea I know for dealing with that as things stands is the tower of elements, to make elementals have the strong promotion. Not certain, but I THINK that applies to fireballs, and that +1 strength is not typed to an element and thus they're not immune to it.