To troubleshoot:
1. Check the headlight switch: This is the most common culprit. If it's faulty, it won't send power to the motors even if the fuses are good.
2. Check the relays: There are likely headlight relays in the underhood fuse box. These are small square or rectangular components that click when activated. A bad relay can prevent the motors from functioning. You'll need your owner's manual or a wiring diagram to identify the correct relay(s).
3. Inspect the wiring: Look for any broken, frayed, or corroded wires running to the headlight motors. Pay close attention to the area where the wiring harness enters the headlight assembly.
4. Check the underhood fuse box: While there isn't a dedicated "headlight motor" fuse, there might be a fuse related to the headlight circuit that could be causing the problem. Use your owner's manual to find the fuse box diagram and check any fuses related to headlights, parking lights, or the headlight switch.
Without a wiring diagram specific to your 1991 Firebird's model and trim level, pinpointing the exact fuse (if one is directly involved) is impossible. A Haynes or Chilton repair manual for your vehicle will be invaluable in this process. They contain wiring diagrams and fuse box layouts specific to your car's year and model.