Q: how to open firm ware lock on mac pro

You enter the firmware password.

If you forgot the PW then you need to

If you can't remember the firmware password for your Mac, schedule a service appointment with an Apple Retail Store or Apple Authorized Service Provider. Bring your proof of purchase (original receipt or invoice) with you. If you plan to visit an Apple Retail Store, make a reservation (available only in some countries).

If you lost or forgot your firmware password - Apple Support