I have an 86 gt it looks like the firebird seat is a little wider than the fiero seat, I think they only had the speakers in the seats for 84 and 85, maybe just 84, b/c my 86 has the speakers in the sail panel.
We had an 85 GT, speakers in the headrest...back in the day the sound system work well, but the cockpit was small. Not sure how they would sound in a bigger car or a convertible.