If they can still be fired, then what does it even mean to say that they can refuse to follow an unconstitutional order? Refusal to follow any order is not illegal. If the consequences for refusing to follow an illegal order are the same as the consequences for refusing to follow a legal order, then there is no sense in saying civil servants can refuse illegal orders.

The consequences for following an illegal order include being sued, being held in contempt of court, or being criminally prosecuted by a subsequent administration. They don't have the same immunity that presidents do because they don't have a direct vesting of authority under Article II.