Just following-along on this thread about FF 400-800 (I have a FF 800) and modern Macs, I just want to say: mine works GREAT with an iMac Pro (Intel, yes, but keep reading) via an external Thunderbolt 3 PCI Chassis (with silent fan). This tiny external TB3-to-PCI chassis hosts a Texas Instruments' based Firewire 800 PCI-E card with ZERO ISSUES.
I've got perfect AD/DA (zero pops, perfect audio), and insane low-latency that essentially feels no different than if my FF800 was natively a "Thunderbolt Face" audio interface.
So I may have missed if anyone has tried the above "TB 3 external PCI card --> FF 800" chain with their M1 Apple Silicon Mac -- but here is my own hypothesis: since the Thunderbolt standard (and MacOS) allow for certain external PCI interfaces, it is entirely possible Apple migrated the standard built-in Firewire PCI card awareness (for TI based Firewire PCI-E, anyway). So this type of connection chain is well worth a try with an M1 Mac, if it hasn't been tried.
So my thought is that it is very possible / likely that M1 Macs may be able to leverage external PCI cards, so long as the Firewire card itself is extremely standardized with a solid chipset (like the TI based Firewire PCI cards) and the default MacOS driver exists for ARM Macs.
So, on the off-chance nobody has yet tried this (I got ADHD so I may have missed, lol) --- this weekend I will give it a try: I will swing my external PCI chassis (which connects to my FF 800, via the aforementioned TI Firewire PCI card) away from my iMac Pro, and over to my M1 Mac Mini and test. I will do this same test with my 2021 M1 Max MBP as well.
So I'll report back on how my M1 Mac Mini & 2021 M1 Max MBP do. I am running Monterey on both, with 3rd party Kernel drivers set to Allowed.
RME have been so absolutely AMAZING in their long, long-term support of FF 800 with driver & utility updates, that I'm actually excited to try. I really feel like that new 3.41 driver, if it sees my TB3 PCI-E Chassis & Firewire card (all up to Apple & MacOS) then I'm home-free.
And I don't really care if the audio mgmt app needs Rosetta, because that's all managed outside my DAW (Logic, usually) anyway.
Will be a few days before I get the lab time to test, but watch this space.