Accidentally activated firmware lockscreen, how do I go back?
As you have already attempted the known ways of changing startup disk, it seems that the firmware password is necessary in order to proceed. However, there's no publically known way of bypassing the firmware password.
You can however take the device to an Apple Store along with proof of ownership, and they will unlock the firmware password for you. If you do not live near an Apple Store, call Apple and they can probably arrange mail-in repair or similar.
If it is a company device and you haven't got proof of ownership, you'll need to go through your employer. Most probably they will know the firmware password, and can proceed without having to contact Apple.
In some cases, managed devices (i.e. company issued laptops) can be set to require special access in order to do things that could circumvent managed device restrictions - such as for example trying to boot from a different partition. This could quite possibly be what you have run into. In this case you'll need to contact your employer to get it fixed.