Nico, AreYouLoco has already posted the output of `sudo lspci -vvxxx -s 0d:00.3` (when he was still running with the vendor firmware).
Here it is. https://del.dog/raw/firewire_lspci

On Wed, Apr 8, 2020 at 6:41 PM Nico Huber <nico.h@gmx.de> wrote:
Hi,

On 08.04.20 13:45, AreYouLoco? wrote:
> I can confirm that my Firewire controller with coreboot is detected as
> MMC/SD Controller. The same situation as yours so seems like a bug.
>
> I tried Debian and Ubuntu Studio both detect it as SD/MMC Controller.
> It gets detected correctly on stock BIOS. Devs?

what you show in the lspci output with coreboot just looks like a
different controller. The output is not enough to figure where it
is connected. The controller is not "detected" as SD/MMC, it identifies
itself as such. In theory, this identification could change, e.g. when
the BIOS uploads some special firmware. But it seems unlikely.

To know more, we'd need to see the PCIe RootPort allocation. It's
the 1c.* devices, but the firmware can change the numbers. Both

  lspci -nnvvv -s 1c
  lspci -tv

with the vendor firmware would be helpful. Or just -nnvvv for all
devices (just -v might also be enough if you have that at hand).

Nico