We had a pair of Firestone tyres do exactly the same as what the OP has described, it was the cords across the tread failed. The first one went flat, pulling the tyre off revealed steels cords projecting into the inner tube and the odd shape, the second was when the old man got home with the truck one day and we noticed that a second had gone miss-shapen, in the case of that one wires were projecting out of the inner sidewall. From that point we wouldn't touch firestone tyres, are they even still made?