"Complicating matters, however, is the problem that the Cyles implementation in Poser 11/12 doesn't handle HDRIs well. They quickly create fireflies depending on the image. Depending on the HDR material, you may not be able to get rid of them. No matter how many bounces you render with."

I have determined that low-resolution HDRis do create Firefly artifacts.  And I stress low-resolution images, 1 to 2k.  Replace the same HDRI scene at a higher resolution, 3 to 5k, and the Fireflies are resolved at the expense of memory.  Just using the luminance of an HDRI appears to also resolve Firefly artifacts, something to consider when using EZDome.  

This is what a Convoluted HDRI will look like when applied to the background:

This is just pure luminance from an HDRI.  Unfortunately, this needs to be created in a 3rd party application.  However, If you are only interested in providing lighting, and not an environmental background, this type of image will suffice.