In my experience with tshirt design, one can not be at fault for copy write infringement for a parody. The fact that Firestone is in the "clothing business" (which to be honest, I don't know if that can prove that having fan gear is the same is being in the business. You don't see firestone clothing stores around) makes it a little more difficult, but I don't believe Firestone will win this one. As PhanArt said they have to prove that this hat was detrimental to their business, which I think they will have a hard time corroborating.