How you address this sort of depends on what you take from the experience. Crucially do you think you would do the same again?
If you would do things differently
You can explain what happened, that you wouldn't do the same in the future and why. Nobody but a fool expects a candidate to never have made a mistake in the past - and the best candidates are the ones who can explain how they have used those experiences to improve.
If you wouldn't do things differently
You can explain what happened, that you stand by what you did and why.
From what you have posted I know what I would do and, (Spoiler alert!) it's not the second one. While your previous employer perhaps overreacted by going straight for the nuclear option of firing you (you kind of have to expect interns to do dumb things - the whole point of an internship is to give them the opportunity to learn how not to do dumb things when they enter the world of work proper) I have to say that what you did was pretty spectacularly bad.
Trying to extort your employer with the threat of refusing to do your assigned duties (when they are reasonably in scope of your role) isn't going to get you very far in most cases. Sure you might be able to pull it off when you hold the upper hand (i.e. they need you more than you need them) but when it's the other way around it's just a fancy way of asking to be fired.
In this scenario you were a kid with an empty water pistol telling an armored battalion to surrender or else.