I purchased a MacBook Air from someone 4 months ago and everything was working perfectly. There is no user name or password needed to log in and I've never had problems using it.
Now I would like to reinstall OS X but I can't do it because it's asking for a firmware password when booting with Option ⌥ held down. It's 4 months now and I've lost the seller's phone number to ask for password.
I've tried to reset NVRM but those keys are disabled. I can't boot from external HD when I turn on the system. What is the reset procedure?