The rules carry from State to state in the US. Many states have "at will" employment, in which case there's no implied contract between the employee and the business, and either party can end the employment at any time for no reason at all.
In these states, even if a company wants to fire you for cause, they will not give you a reason. If they give a reason, it has to be a valid reason under labor laws, not discriminatory, etc. If they give you no reason, then they're in the clear unless you want to sue for discrimination if some sort.
If they fire you for wearing a diaper, and they tell you that you were fired for wearing a diaper, them you could have grounds to file a complaint under ADA alleging that they had to offer you reasonable accommodation. If they fire you and tell you it's because your services are no longer required, you would have a very difficult time filing a complaint because of the difficulty of proving that it was because of an ADA violation.
Here's the kicker with all this: looking to wear diapers is NOT a protected disability under ADA. If they find you're wearing diapers and your manager wants to fire you for that, he or she may do so and there's nothing you can do about it. If they don't tell you a reason, then you'll have a hard time proving that it was because of a protected disability (because, let's be honest, it wasn't). If they do give you a reason, you'll still have a hard time proving that it was because of a protected disability (again, because it wasn't). Either way, you're out of a job, and have no legal recourse.
If you're worried that you'll be fired for wearing diapers at work, or wetting them, or smelling like pee or other body waste, then you should not engage in those actions.
If you are incontinent, that may be a different situation. I do not know if that's protected under ADA, but I suspect that it would be. If they fire you for incontinence or related things (like too many bathroom breaks) and you can prove that you're incontinent, you may have grounds for an ADA complaint. You will still have to fight it out, though - it's not like the employment police are going to swoop in and force your manager to give you back your job and all your back pay.
If you don't need to wear diapers at work, you're worried about being fired for wearing them, and you want to keep your job, you'd be well advised not to wear them. If you want to wear them anyway, consider finding a job where you won't worry about being fired for wearing them.