Let's assume that your initial descriptions are correct: at many other firms in your area, Jewish managers show a bias in favor of hiring Jewish candidates.
STOP: Right off the bat, that's sounds incredibly anti-Semitic. So you're going to have to back it up. Which means "providing clear, objective evidence", not saying "well everyone knows those guys stick together."
If you are correct, and there really is an identifiable pattern of discrimination in hiring, then find a friend who is a lawyer, describe your evidence to her, and ask her for 5% of the punitive award from any lawsuits she files against those companies in exchange for the evidence. You will shortly be rich enough to retire.
Now you ask, "I think those guys are doing this thing that will get them sued into bankruptcy, so is it okay if I do the same thing over here at my company?"
Of course not!
This is exactly the kind of group stupidity that your mother warned you about: "If all the other banks are investing heavily in Collateralized Debt Obligations, shouldn't my bank do it too?"
Finally, let's have a look at the end of your question:
I know it sounds "bad", but think about it. Say a Jewish and a black
applicant applies for a job at my company. If I don't hire the Jewish
applicant, he'll go to any other company and probably get a job
immediately. But if I don't hire the black applicant, he won't be able
to find another one quite as easily as the Jew.
Let's say two people apply for a job at your company. And let's say there's a really obvious reason to hire one over the other. You're done - hire that one!
Instead, let's say two people apply for a job at your company. And let's say there's no obvious reason to hire one over the other. They're basically the same level of desirability, and it's "yes" in both cases. Now, why are you suggesting the black one can't get hired? Because that's pretty racist.
I'd suggest that you have a problem with racism. Both personally, and possibly within the company. If you're in a position where you are going to make a significant number of hiring decisions, you need to take steps to mitigate it. Talk with HR about anonymizing the resumes that you see, or providing summaries. Maybe one of your junior technical people can work with HR to rewrite candidate CVs into a common voice, and abstract away identifying data like applicant names, names of institutions, and employers.
Alternatively, there are companies out there that perform "level 1 technical interview" services, so your HR group can scan resumes, submit the names and contact data for an interview, and get feedback without having to spend company time on the process.