Selecting a startup disk when you put a firmware password on a Windows single-boot Mac

Usually, when you put a firmware password on a Mac, you can double-check in System Preferences > Startup Disk to see if the proper startup disk (or any startup disk) is selected.

Since the default behavior of the firmware password is you needing to enter the password in order to boot from anything other than the startup disk, you probably want to have a startup disk selected (otherwise, you'll get the folder with a question mark inside of it when you boot up).

In most cases, this isn't that difficult, but if you set up a single-boot Windows installation, you may not always see it available for selection if you boot to recovery mode or boot to macOS from an external drive.

Instead, you may want to just boot into Windows (hold down the option key and enter the firmware password if you need to this one time), and then launch up the Bootcamp Control Panel, and you can set the startup disk there.

Terminal command to see the Startup Disk in macOS

If you want to see what the current Startup Disk is on your macOS installation, you can certainly go to System Preferences > Startup Disk.

But if you want to use the terminal instead of the GUI, this command will return the current Startup Disk:

bless --getBoot
If a Startup Disk is set, you'll see something like this:
/dev/disk0s2
If no Startup Disk is set, you'll see this error message instead:
Can't access "efi-boot-device" NVRAM variable
or this one:
Could not interpret boot device as either network or disk
Can't interpet EFI boot device
And, yes, there's a typo in the error message (as of macOS 10.12.6, anyway). That should say Can't interpret instead of Can't interpet.