Not a Megyn Kelly fan, but this sounds like a false equivalency. There's an obvious distinction between saying someone does or does not deserve to be fired based on their actions, and saying that employers should or should not have the ability to terminate employees for practically any reason.
So for an analogy, there's no contradiction in saying that alcohol should be legal and then criticising a family member for heavy drinking. Likewise, there's no contradiction for supporting at-will employment and then criticising a business that fires an employee for, what you perceive as, a poor reason. It only becomes a contradiction when you graduate to "therefore they shouldn't be allowed to fire this person."
My comment was intended to be snark.
However, a few things. First, Megyn Kelly isn't an actual employee of NBC. She's on contract, and NBC will end up paying her a hefty sum to break it.
Second, high-profile public figures are paid to make their "employers" (actually clients) look good. That's literally their entire job function. Presenting the "news" is purely a secondary function. They're actors, not reporters. So failing to look good, or doing the opposite and making your employer look bad in public, is not doing your job, which is always good cause.
And finally, this whole thing was almost certainly a preplanned media event. Kelly and NBC get a whole a newscycle in the limelight, she gets a fat severance and a book deal, and right wingers get another opportunity to complain about leftists and political correctness. Everybody wins except for the people who want real news about real issues and not tabloid crap.