After updating to Lion, can't re-install Snow Leopard or Lion
Firmware shouldn't have anything to do with it. I was able to install SL while I had Lion on. Maybe there is some problem with your disk drive on Mac? 

I haven't tried it but you might be able to use the disc from a network device but you will have to find more info on it. Also you might be able to revert the system sometime back using Time Machine. Have you looked into that?