And we still don't know why McFadden was fired... :-)
Gates McFadden was fired because of disagreements between her and several of the show's writers [mainly Maurice Hurley] over the direction of her character and elements of the show she felt were sexist...here's what McFadden said on the blurays:
"I felt that some of the scripts were sexist and I guess I was too vocal about it because it ultimately cost me my job at the end of the first season. I was very naive about how studios work. I was used to working in theater ensembles where everyone was equal and you were allowed to say I hate this. It didn't mean it was going to change but you were allowed to say it. I guess I voiced my frustrations in a way that really pissed people off. A good lesson to learn."
She also said that she was surprised by her firing because a few weeks before Gene Roddenberry had told her that focus groups had showed she was the third or fourth most popular character on the show...she wasn't sure that was true. Apparently they had a first season wrap party on a Friday night and she got a call from her agent on Monday morning saying she wouldn't be asked back.