weird! MacBook Air got a firmware password with keyboard shortcut

Solution 1:

What you are describing is not possible unless at some time in the past you, or someone else deliberately booted into Recovery mode, ran the Firmware Password Utility from the Utilities menu and then set a Firmware password. No key combination creates and sets the Firmware password automatically.

Have you powered down the Mac by holding the power button until it shuts off?

Do you now get the Firmware password on boot up?

Is it possible you might have a friend or colleague with enough (evil) knowledge to boot your machine into Recovery mode and set the Firmware password as a prank or as deliberate sabotage? If this is genuinely your computer, (and I have no evidence that it is or isn't your computer from my side of the LCD screen,) this is another possible reason your Mac suddenly has a Firmware password set.

Note: If you go to a repair center to have them remove the Firmware password, you may have to prove ownership of the MBA for them to unlock it.