I did something stupid... is it possible to be fired over this comment on Facebook?

Well DO NOT ask the friend to take it down, because that would make him feel like he has some sort of leverage over you. I'd sy just ignor it and him until things cool down. If your extremely worried and want to go the extreme you can delete your account and that comment of yours will go away (I believe. if not it at least makes it so you cant see who wrote it).