Stuck on Target Disk Mode due to Firmware Password
You'll need your firmware password to boot into anything other than macOS (which is why it's called a firmware password). If you can't remember your firmware password your only real option is to take your computer to the Apple Store. I believe I read online somewhere they'll have a special key or something like that so you can go in and reset your firmware password or remove it, then you'll be able to boot into target disk mode.