I also sometimes get the Poser 12 render freeze. It's weird because the application looks like it's working, but nothing responds to clicking. So the visual cues that I clicked something or moused over something are there, but nothing happens when I click. No spinning wheel, nothing. I'm running Poser 12 under Big Sur on my 2019 iMac. It doesn't always happen when I render with Firefly, just sometimes.