1999 Pontiac Firebird misfire

If the misfire occurs when engine is under a load. I would begin with checking the spark plugs and wires. Just because they were done last year doesnt mean they didnt arc out somewhere. A good way to check is to power brake the car while sparying water on the spark plug wires. If you an see them arcing or the miss gets worse, you have isolated the problem. Is the check engine light on?

Hi firebird19_99, Welcome to 2carpros and TY for the donation

Engine misfiring can be caused by the following, worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

You hold the brake pedal tightly, put the vehicle in drive and acceleate slightly to put a stationary load on the engine. You only want to do this for 20 seconds at a time to prevent transmission damage.

You check the primary and secondary winding resistances of the coil and resistances of the injector solenoids.

Yes you can check it by doing that, but the car will run worse when a good cylinders is disconnected. It will run the same when you find the dead cylinder. Once you find which cylinder it is. You can then swap parts from one cylinder to another to see if the mis travels