Why a firmware password?. Well, to protect your machine in case of being stolen/lost. The password will stop the boot process if the boot disk is not the one that was present when the password was set. Also stop booting to recovery/USB/Super Drive.
This firmware plus FileVault (disk encryption) will prevent getting your data or use your mac without permission.
There is also a second firmware password set when you use “Find my Mac” which locks your computer and asks for a 4 digit password at boot as soon as you report your mac as lost/stolen in iCloud.
If you loose your firmware password then you’ll have to go to an Apple authorized specialist with proof of ownership, then they can remove the password, but in my case that means 6 hours driving to the nearest apple support specialist ?.
I used all the passwords that I remember, none seemed to work… until I realized that the password was entered with a Spanish keyboard layout and the password entry have no choice over the layout, English only. So the special (nonascii) characters didn’t match.
I didn’t removed the password, but now I know why it wasn’t working.
The layout thing.. I hope it helps you if you are in the same situation as I was.