One possible way around the 'fire-mages are abominable' vs. 'fire mages are Really Useful' dilemma is to have an underclass of people who are used for their useful talent while at the same time despised for it. An analogous situation is the way that morticians and handlers-of-the-dead can be considered both vitally important to a culture's way of dealing with death while at the same time being considered unclean or untouchable, or the way that medieval Christian Europe (in which lending money was forbidden for good Christians as usury) were willing to bank with Jewish people while at the same time despising them for it and concocting bigoted stereotypes about them. Especially if fire mages occur naturally -- ie, they will pop up in this society even if the society stigmatizes them as barbaric -- fire mages might be raised to be useful but also raised under abuse and control that keeps them as second-class citizens.
Not something you need to do, of course, but a possibility.