Mac Pro 5,1 firmware update for High Sierra will it effect Snow Leopard?

To the best of the accumulated knowledge here, these firmware updates are all backward-compatible at least to 10.6.8 snow leopard.

The firmware updates make changes to allow the use of nVME Boot disks, allow AFPS boot drives, and patch the microcode for improved robustness against obscure Meltdown and similar cache-reading malware.

@HWTech--

You may be correct about certain firmware causing some diagnostics issues, but most of the issues I have seen have been caused by trying to skip ahead without installing the proper firmware. There seem to be a lot of "cockpit error" problems and trouble with users who tried to replace the Apple SSD in a 2013 MacPro at around the same time they needed to update the firmware, but the firmware would not install with a non-Apple built-in SSD.

I still can not recall seeing anyone whose system was PRECLUDED from returning to an older version by updating firmware. I think Apple tries hard to make it backward-compatible.

@Grant is probably correct for the High Sierra firmware.

I have seen issues with the firmware Mojave installs on MBPros and MacPro 2013 systems. I don't know if a later High Sierra update will install the Mojave firmware. I do know updating with the Mojave firmware has caused some systems not being able to boot into the Apple Diagnostics and this applies to the consumer diagnostics and the Apple service diagnostics. I have seen some people report issues with older versions of OSX after updating, but I believe they had updated to the Mojave firmware and not High Sierra. I believe most of these issues are related to the firmware update Mojave installs as I have not encountered any issues with High Sierra systems though not all of them have been fully patched/updated. Of course I haven't tried to use an OS as old as Snow Leopard either.