No way to know exactly what was said behind closed doors, but it wouldn't have done Ross any good to humiliate Parcells with a pink slip. IMO, Parcells knew what the litmus test was for this season, and knew they weren't going to hit it, so he left.
That makes him a coward in my book. We didn't hire him to slink off into the night to avoid getting fired. The Tri-****ed-us has done exactly that.