Depending what engine will determine if the same mount is used for left or right or a different mount on each side. Checking online parts listings for NPD, AMES, Classic Industries or Rock Auto will show the same part number for left or right side engine mounts on 1975 -1981 Firebirds, with a Pontiac 301, 350, 400, 455, or Oldsmobile 403. My 1980 TA with 301 had the same mount on both sides. When I replaced that engine with a 400 block I installed new mounts from Rock Auto, they sent me two of the same part number mounts. The fit both sides without issue.
1970-1974 Firebirds had different mounts on each side. Later 2nd generation Firebirds with certain engine options used different mounts on each side. The metal bracket that connects the engine block to the engine mounts on 1975-1981 Firebirds with Pontiac V8's had a different metal bracket on left and right side.