Polyurethane is oblivious to just about every chemical solvent after it cures. That is what makes it an awesome adhesive. Acetone will only work BEFORE it sets. Nitromethane is a good solvent and it will slowly etch into poly, but probably not enough to do any good and you'd probably need 98% pure nitro. UV will attack some polys, but there again, the process is too slow to do any good. The best solution is to mechanically remove the glue. In other words, you are going to have to cut out the firewall and grind away the poly.