how do i find out what month my 91 firebird was produced?

It should say inside the drivers door jamb on the VIN tag, it will give the month and year.

1991 was an unusual production year. Most 91's were built in 1990 due to production problems on the 1990's. Pontiac decided to go forward with the 1991's instead of building more 90's late in the model year. That said, try asking the question on the thirdgen.org site. Many experts there focussed just on 1982-1992 Firefirds and Camaros. The date does not appear in the door jam in 1991 like earlier models but if you provide your VIN someone on that site can give you a pretty close estimate. The last 6 digits of the VIN (sequence number) will be important to them.