Put on a Performance Improvement Plan (PIP). Should I quit before getting fired?

I was put on a 30 day Performance Improvement Plan about 3 weeks ago, and based on my last conversation with my supervisor it sounds like I will be fired. I've been doing interviews and applying to jobs. Should I resign before the 30 days? If I do, are they still legally able to fire me? I'm concerned about future employers contacting them and asking about my employment status. Would they say I resigned or I was technically terminated (because I was still under the PIP when resignation letter was given)?

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