Wonderful article. I agree that is a very tough question to answer.
I will share as an HR Manager - one of the worst things to do is to get defensive about the separation. A lot of people use the opportunity to talk bad about the former employer, tht's not the way to go.
The best thing that you could do is acknowledge your part in the reason for being fired. Keep it short and sweet, then move on to another question or skill that you're great at.
Also keep in mind that some companies will let you resign in lieu of being terminated...see if that option is available to you if you find yourself in the position of "about to be" terminated.