Well I got my new Mac mini just over a week ago.
I set it up and then used Carbon Copy Cloner to back it up to a 2TB SSD (A vast improvement over using a standard HDD). Having done the initial backup, I wanted to encrypt the disk. You have to do this by booting from the cloned disk and then turn on FileVault. Once the encryption starts you can reboot the Mac and the encryption will carry on once you reconnect the external disk.
All sounds good, but being a new Mac mini you have to enable booting from an External drive having started in recovery mode. I thought this looked potentially dangerous and so decided to set a firmware password. As I didn't want to risk forgetting it, I set it to my Mac sign-in password. All good so far.
I then rebooted in recovery mode to start from the external disk and the Mac asked for my firmware password, which I duly entered and was rejected!
After numerous attempts (still rejected), I realised that the input field wasn't accepting the \ [backslash] key which is part of my password.
After googling firmware passwords (where the general advice was don't set firmware passwords ) I discovered that when the Mac initially boots and asks for the firmware level, it assumes that you have a US keyboard attached. There is a subtle difference between the two that involves the \ [backslash] and Return/Enter keys: