There are rivits around the upper and side of the firewall going into the body frame structure. There are a couple of difficult to reach ones that go on the front section of the A pillar facing the front bumper. You can usually chisel them off from the back side (in the A-pillar where you bolt/unbolt the hinges on).
There is at least one per side on the very top of the hinge pillar section, then about 20 or so of them across the top. There is also sealant that will need to be pried loose.
You seperate the floor out, then wiggle and jiggle until it comes off. I assume you have the front clip already removed?