Interchangeability, is that even a word???
I need to know if the rear impact bar is the same for camaros and
firebirds. The part under the rear bumper cover. Mine is rusted out and there is a rust free one for a camaro at the junkyard.

I would think so. However Firebirds have thier license plates in a molded section of the rear bumper. Camaros have a solid bumper as they mount the license plate in the tail light assembly. So maybe not.

Firebirds and Camaros shared the same rear impact bar up to '90 ('birds). The camaro used the same impact bar 82-92. The license plate 'notch' on the 'birds is in the plastic absorber.