No place sees more wear and tear in any car than the seats and the carpet. Whether they are cloth, vinyl or leather, the seats will see a lot of action and over time simply wear out.
Leather cracks and splits, cloth ravels and frays, vinyl is probably the most resilient with the minimum amount of maintenance, but even vinyl dies. Replacing the seat covers is really the only fix, as repairing seat covers just doesn’t work. The problem is that fourth-generation Firebirds don’t get much love in the upholstery market.