He'll still be paying it even if fired. He would need to seek out a job that is at a similar level and would get the alimony adjusted if it pays a little more. But you can't suddenly decide to quit a field you are qualified for, have plenty of opportunities to work in etc, to chase a fast food career. Even if you get fired and do that, the judge will see it as malicious - you wouldn't do that if you had a family to support.

Also, if you prove you cannot find a job and are looking, they'll adjust it too, and will let you find a shittier job in the meantime. It's just if you do it all maliciously where they will say "nope"