This is basically the same argument that caused the libreboot vs gnuboot thing and I just personally don't get it. It seems to me like the FSF is letting perfect be the enemy of the good. Having a FOSS driver isn't something to be celebrated it's something to be punished if the firmware isn't also FOSS. Yes, ofc, FOSS firmware is better than closed firmware, but when almost no modern hardware has that as an option, it's not even something you can really vote on with your wallet unless you just run ancient hardware all the time.
It matters because for me, a good chunk of the FOSS benefit is the auditability of code. Being able to make changes is nice and that's the freedom bit, but being able to audit it is also a huge benefit. If the code is not running on the main CPU then the driver on the main CPU can contain possible exploits of firmware using the IOMMU etc so it becomes more tolerable than a closed source driver. Basically a firmware vulnerability effectively becomes a hardware vulnerability as opposed to a driver running with full kernel privileges and no oversight or containment.
Virt manager does but only during creation. You have to check the option for "customize VM before creation" or whatever that option is on the last screen. Then there's a firmware drop down where you can select OVMF