Firmware Password suddenly required for startup commands - Sometimes without setting a Firmware password - Cannot access Recovery Mode or Boot Disk Selection
I have encountered the situation in subject, when due to a crashed OS update, I found myself locked out of all options, only being able to get to the gray lock logo, asking for the firmware password, which I don't recall ever setting, hence, there was no chance for me to know it.
I read about it quite a lot, with no success of actually resolving it, and was just about to give up and accept I now have a nicely designed paperweight. The only visible option was to take it apart and go through a quite extreme hardware work, or take it to Apple, in hope they will even try to fix it, without me holding any proof of purchase, and even if they do, I understood this is quite costly.
Eventually, I ran into some forum where it was assumed that when setting up the OS for the first time, the "Find My" service, or whatever other names this service previously had, might set the firmware password, as part of the security measures. So, I simply tried to use my iPhone unlock code, and, amazingly, it was indeed the password. Now all is back to normal.
Assuming anyone who encountered this problem may also have an iPhone, or another Apple device, I thought you might find this post useful. Simply try your unlock code as the firmware password, and you have a good chance to recover.
Hope this will help someone :-)