1992 Pontiac Firebird my Firebird will not start

Welcome, I don't know what testing you have done? Are you saying it cranks good but won't start? Did you visually check for spark at the plugs, also, use a gage and check fuel pressure? Are there any trouble codes? You may have already checked all that, I have no way of knowing, unless it is in your post.

Is there a load on the starter circuit, with key in the crank position? Watch the lites, headlite-- dash lite or something. If there is a load on the starter circuit, the lites should react. If no reaction and they stay bright, you have an open circuit to the starter.

The purple wire at the starter solenoid, should be hot with the key in the crank position.

Because you said everything has been tested and is ok. The only other thing I can think of, is to pull the plugs and access belts, see if you can turn the crank by hand. Goodluck.