If you've been fired, be forthright about it:

So many people have been laid off through no fault of their own in the past ten years that it's no longer a stigma to have been fired--unless it was for justifiable cause (e.g.,- you socked your boss). Answer directly, but without a "charge" in your voice. Expressing your bitterness over being let go tells the interviewer (rightly or wrongly) that you can't accept the realities of modern free enterprise -- that downsizing is acceptable and often necessary.

From: http://topten.org/public/AB/AB8.html