Firmware Upgrade of Apple Mac Pro (Early 2009) from 4.1 to 5.1
I ran an ethernet cable and was able to progress through the list of tasks. It took two attempts to pass step 5. The trick was to keep the power button pressed well past the flashing of the power light and only releasing it again after the continuous tone has sounded. Long story short, I now have the 5.1 firmware installed, and have upgraded to Sierra 10.12.5.