UPDATE: SOLVED
First I tried updating the bios (build 9/8/2014), which involves booting TinyCore Linux on a thumb drive, which I did with no trouble. (Still using Win32DiskImager)
I was still unable to boot pfSense on a thumb drive.
Then I tired writing the same pfSense image to a new thumb drive using 'dd' on MacOS. That finally worked.
So the combination of my 5 year old desktop, its USB plumbing, Win32DiskImager, and certain memory sticks results in unbootable media.