If you want to remove it for sound purposes, it would be much easier to remove the rear dash panel and cut holes in it to allow more bass through, and then just cover the hole with a speaker grille. The firewall is there mostly to protect you from flying battery acid in the event of a collision. I don't think it has much to do with structural integrity becuase some e36s have fold down seats. Also, while it can be removed, I think it serves as a mounting point for the rear seats if you dont have fold downs. Ergo, if you remove it then you wouldn't have a rear seat back, only a cushion.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>