The Wikipedia page on Sexual Harassment lists a few early cases.
Williams v. Saxbe is the first one chronologically listed. That seems to be about a male supervisor firing a female underling for refusing his sexual advances, so pretty textbook sexual harassment employment discrimination.
Looks like Williams (the woman) won damages, but there’s no indication in the court decision that the harasser faced termination for his behavior.
I expect that some people were fired for actions like this prior to modern law terming it “sexual harassment”, but the firing probably went along traditional power lines.
Like, probably no executive was fired for grabbing the secretaries, but I bet some junior somebody-or-other got fired for treating the boss’s daughter in an unladylike fashion.