The heater fan in my firebird doesn't work on the high speed setting. The other settings seem to work just fine. I used to have a cutlass that had an inline fuse under the hood just for the high speed setting. I had to replace it several times. Does the firebird have one of these?

 

You're close...it has a fusible link to provide power through a switching relay for the high-speed setting.
I'm not sure where the relay is on an AC-equipped thirdgen - I'm thinking it's handy to the blower motor - but it has orange, black, red, and purple wires on the connector. Black is grounded; orange gets 12V when the switch is in HI; purple goes to the blower motor; and red is 12V through the fusible link.
First one to check would be for 12V in the red.

 

It is a relay that will not let it turn on high. If you look where the A/C dryer is, there will be two relays. It is the one that is closest to the fan.