The difference is one is a Chevy & the other is a Pontiac.
Firebirds weighed more than Camaros.
Pontiac had there own rear axles for most of them.
Different interior. Dash boards are different. 69 Firebird dash looks closer to a 67-68 Camaro dash. Angle of the sheet metal.
Most parts will interchange between the two.
MBHD