How to restore motherboard firmware on an unresponsive MBP mid-2009?
A search for fixing broken firmware suggested that you can use a "firmware restoration CD". These are some instructions for creating and using one (although I don't see your machine listed). If you can't find the right CD for your machine, maybe you can take it in to an Apple store; they should have all the tools necessary to fix firmware if it exists. If they fix it, see if you can find out how.