I was pulled into a meeting with HR. They asked me a bunch of inappropriate questions.
Such as?
Prior to the interview, I contacted the managing partner at the previous firm and asked him what I should say. His advice was to deny anything about what happened at the previous firm
So...he advised you to lie? Interesting.
you can't be fired based on unsubstantiated rumors.
This is quite untrue. You
can be fired for any reason and can
legally be fired for anything that is not expressly illegal (which unsubstantiated rumors are not), unless you fall within one of the few exceptions (which you likely don't).
HR asked if it was true that my letter of reference did not disclose I was in a romantic relationship with the writer or that I was fired. They also asked if it was true that I assaulted a lawyer's wife previously. I said all of it was completely untrue and someone was out to sabotage me. (Yes I know I lied but I don't see any way that they can ever prove that I had affairs or not or what happened with the lawyer's wife when they all happened in private quarters, and I don't see how it's relevant to my current job anyway).
Well...for starters, a good investigator could probably find this thread, and I'd be willing to bet the lawyers you used to work with won't lie under oath if it comes to that. More importantly, they don't have to "prove" anything to anyone. Also, your opinion about relevance is...well...irrelevant.
my understanding from speaking to the previous managing partner (he does not do employment law so he was only able to give me the basics) is that they can fire me after my probationary period is over however they have to pay me in lieu.
Pay you in lieu of what? Since you're not a union member or a civil service employee, you can be legally fired for any reason that is not expressly illegal, and your employer will owe you nothing other than pay for time worked. The only remaining exception would be if you have an employment contract that limits your employer's ability to fire you or requires certain post-termination compensation.
If I am fired do I have basis for a human rights case for discrimination on the basis of gender?
No.
Can what I did at my previous firm be held against me even though I have not done anything at my current firm? Can they use unsubstantiated rumors and gossip to fire me?
You can call them unsubstantiated all you want, but it's not like any of it is untrue. Yes, the numerous bad acts relating to your prior employment would be legal grounds for firing you from your current job. These are predictable consequences of the acts you described.