Allright, what year of Firebird was this again?
The raised area in the hatch is raised for a reason- to allow clearance for the Rear end and in some cases the independant rear suspension. Your likely area to install the subs is in the storage compartment right at the rear. It should be about 2 ft deep and about 18" wide and approximately 58-60" wide. Those are just rough measurements, however that area should provide enough space for a decent multi sub set up capable of impressive output. In the Firebird, due to the limited hatch space; Aiming the subs up or forward is likely the better option. Typically in hatchbacks aiming the subs towards the rear does give a better resonse, however, the firebird has a few too many constraints- especially if you value your rear vision. As always, just make sure that your Subs are powered clean and crossed over at the appropriate level.
Take it easy,
-zane