I forgot my iMac firmware passcode.

Hi,

If you forgot your firmware password

If you can't remember your firmware password, 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.

kaz-k is spot on.

see > Set a firmware password on your Mac - Apple Support

So you forgot your firmware password and your local login password? Did you forget your Apple ID password as well? (Forgetting that could cause problems as far as resetting your local login password.)

Apple Support – If you forgot your Mac login password

But first you need to fix the firmware password issue. If Best Buy can't, or won't, reset the firmware password, contact Apple and ask them who (if anyone) can.

Official Apple Support