On some of the older macs, you can reset the firmware password simply by removing the RAM. If that doesn't work for your model, you'll need to take it into Apple to get the firmware password reset. A firmware password is esentially a password on the logic board, not something that can be bypassed.
If you need to take it in to Apple, proof of ownership is needed.
Hi, thanks for reply but I have really big problem now. I programmed the computer for next boot from /Volumes/mac the mac was my bootable USB stick (installed mountain lion). I restarted computer but didn't work. Now I can't change the boot settings because I can't start the computer. If I try open with startup manager (Command+R) there is firmware password. With out any button combination there is a folder symbol on the white screen with question mark. So the computer compeletely off-line now. Can't use it.