It's possible to use a Mac invited account with firmware password enabled?
Firmware password prevents the startup of any disk other than the designated startup disk, so it should not prevent startup of guest or other accounts.As long as the account is using your startup disk, it should not ask for the firmware password.

However, if you have file vault enabled, guest accounts can only access Safari, and the network connection. File vault encrypts the hard drive (SSD), preventing the guest from accessing it. If file vault is off, guest accounts function like a normal account with limited permissions.