A 9V battery is just six quad-A (AAAA) 1.5V batteries pressed together in a case. As others have noted, they wouldn't burn on their own (unlike lithium-ion rechargeables); but if shorted out by aluminum foil, I suppose they could be enough current to create a brief burst of heat and ignite tinder. The conditions for causing a fire would have to be unbelievably perfect -- I would rate it as a freak occurrence. There are things in the average house and attached garage that are more dangerous by orders of magnitude.