Thanks,
tsialex, for all of the information.
Do any of these firmware upgrades cause issues if I want to boot into earlier OS's at some point? I use a variety of PCIe cards.
I successfully upgraded to High Sierra 10.13.6 and did that firmware upgrade, and am now ready to go to Mojave with an MSI Radeon RX-560 4GB installed, but am hesitant to do another firmware update, because I can't risk bricking it, and also want to occasionally boot into Mavericks 10.9 for some old projects.
I also don't mind so much not having the boot screen, but if that limits my troubleshooting ability, I think I will just stay on High Sierra and put the original ATI Radeon 5770 GPU back in. The only tested and approved GPU for my Pro Tools system is the Radeon RX-560, and it looks like that one will never get the boot screen.