Firmware issue

If you don't recall the firmware password, then you need to take the laptop to an Apple Store or an Apple Authorized Service Provider along with valid proof of purchase and valid ID to prove ownership of the laptop. Contact the Apple Store or AASP to find out exactly what items you need to produce. If you purchased the laptop directly from Apple, perhaps they still have this information on file (or perhaps the vendor or store you purchased the laptop will still have this information and provide you a copy of the receipt).

If you cannot do any of this, then you will never be able to boot from any external media again on this laptop which includes USB and Recovery/Internet Recovery Modes as well as the Apple Diagnostics. You can continue booting to the internal drive for as long as it works. Some people will remove the internal drive and reinstall macOS using another compatible Mac, then re-installing the drive in the locked laptop.