Do I need to upgrade the firmware on a Mac Pro with mismatched processor config and serial number?
The firmware had to have been updated before the newer processors were installed.

The differences between the 4,1 '09 &amp; the 5,1 '10 - '12 are minimal. What you're looking at is a hybrid, older machine [quad-core Nehalem originally] with upgraded firmware &amp; newer 6-core Westmere CPU.

In itself, there's nothing inherently wrong with that - I'm typing on one right now.
The fans are a little noisy - my older 3,1 you cannot even hear the fans until it's working hard - but it really has no issues, to all intents &amp; purposes it's now a 2012.

This is mine



The 5,1 upgrade does allow it to run Sierra too - I just don't like Sierra.