It's pretty rare that someone, at least a lead, is fired after the opening. Usually firings come during rehearsals or previews, or out of town back in the day.
A firing during the Broadway run, but in a supporting role, was Doretta Morrow in The King and I. She was taking a six-week leave of absence (after having already taken a 10-week leave of absence for film work) because of a throat ailment, but she appeared on television during this time. She was fined by Equity and presumably was fired from the show as her understudy then took over.
Len Cariou was fired post-opening from Ziegfeld in London, although producer Harold Fielding said that Cariou was not fired, it was merely suggested to him that he might want to negotiate a withdrawal. Well, can you blame him for then telling the cast that he'd been fired? This was crazy as they had no one lined up to replace him, and the understudy played most of the run. Eventually, Topol took over (by which time the show had been significantly revised but it quickly closed after Topol took over anyway), getting perhaps just slightly better reviews than Cariou had gotten, but the problem was the show and the role, not the actor(s).