[patio11]

What should someone who's been fired tell potential employers why they're no longer with a previous employer?

There is virtually no answer to that question that makes you sound like a better candidate than you were before it was asked. I'd be strongly inclined to say some variant of "It's a tough economy.", which is non-specific, virtually guaranteed to be a contributing factor, and avoids faulting either party.

The canonical answer is some variant of "We weren't a good fit for each other." Then, again, refocus the conversation on how you'll be a great fit with interviewer's company.

[tomjen3]

Why not just say 'they hired me for a different role that I was not suited for'?

[patio11]

Because a) that sounds like you have a problem and b) that sounds like you blame your problems on other people, including people that the decisionmaker empathizes with more than he empathizes with you. That sets him to thinking whether you're going to blame him for hiring you in 6 months after you're fired from his company.

There's just no percentage here for you. (An American idiom: basically, no outcome of this line of thinking is going to help you.) Just handwave towards the economy and start talking about happy subjects.

[Avenger42]

Keep in mind that they're going to go back to the company, and if your characterization of your parting is different than that of the company, it will reflect negatively on you. If (and only if), during your parting, your boss says something along those lines, then you're probably safe in repeating it.

[larrys]

"virtually no answer to that question that makes you sound like a better candidate "

My first reaction to this was to think that someone could somehow turn this into a positive if they could solve this dilemma as in "No! I was fired for (insert reason) but I overcame...and now I"m ..."

But then I realized that similar to breaking up in a relationship if you reveal the reason to your new partner and say that you learned from it, and aren't the same person, all that will do is alert the other party to be especially vigilant to the particular behavior, and in fact then you are automatically disadvantaged. Everything you do is magnified greatly. "Oh boy. Here we go again just like their old job"

About the only thing I can think of (and in fact recommend this when trying to get rid of a sales person) is to produce a reason that can't happen again. Such as "I was fired because I had to take care of my sick mother. But she died and I now live alone and don't have any other relatives". Etc.