First, it's not entirely clear who it was who made these comments to me. Can you please edit down your post quite a bit to just explain who said what to whom, and in what context? You have a lot of "this person" and "they" and I can't tell who you're referencing in each instance.
We only need to know:
The person who made inappropriate comments. Who are they to you/company. What they said, and in what context.
Have they made anti-semetic comments to you or in your presence in the past? Do you normally work with them?
In short, you cannot be fired for reporting this, and this is your remedy in the immediate. Go to your direct supervisor (or HR if this person is the supervisor you normally report to,) explain only the specific facts - i.e. "Person A said these specific things to Person B in my presence." Let them know you found it upsetting. That is all you need to do. It cannot hurt to do this through email, but in either case, just take note of the date you spoke to them, and with whom. Then wait to see what happens, if anything.
If the company fails to protect you from further inappropriate behavior from this person, or you believe they retaliated again you such as demoting you, then you would file a complaint with the EEOC in the state where you work.
As an aside, and I am not entirely sure how to phrase this delicately. My own backstory is such that I assure you that I'm not being dismissive of how inappropriate the comments were, but you should not use hyperbole like "nuclear level" in describing the antisemitic comments, nor go into a rant like you have here in listing everything inappropriate or concerning that you know of this person. Stick to just telling them the facts and only things you were present for. Do not give them your observations/thoughts of their mental health or decline; do not repeat rumors or things that you've been told. It is an uncomfortable situation; I know, and it's not easy to approach. If this is a large company, I can assure you that they have resources as well as legal counsel to help advise them on how to deal with it. It's extraordinarily unlikely your job is at risk, even if this person "knows" that you spoke to your supervisor. It serves you well to be as professional as possible about, so understand that your only duty here is to inform your own supervisor that it happened, and that you were bothered by it. Their job is to deal with it, and ensure that you're not retaliated against or further exposed to it. You're not tasked with figuring out how they can resolve it, and can and should continue concentrating on your job rather than that issue. It seems you rarely encounter this person, so it shouldn't be an ongoing concern for you at this time.