How to restore a Mac if you forgot recovery password
Apple documents firmware passwords and how you want to schedule an in person appointment if you lose the password:


https://support.apple.com/en-us/HT204455



  If you forgot your firmware password
  
  If you can't remember your firmware password or passcode, schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase.