I was using FF800 fine using a FW400 ->FW800 cable on older macbooks for many years. 
I love RME products they are very good quality.
Once I had a power supply capacitor go but I replaced the PS unit for about £30 no problem.

Recently I connected the FF800 via FW400->FW800 cable. FW800-> TB2 adapter. TB2->TB3 (USB-C) adapter . to the macbook 12" (which has no TB3) USB-C port (this is also the laptop's charger port).

The outcome I have is:
          FW800 with red host light is permanently on (its my last fw400 port so I can't check for other live ports)

I suspect that the above action has fried my last remaining FW port on the FF800 (others were damaged mechanically over time)
e.g. Is there some power coming back across that USB-C from the macbook 12" along the FW connections?

Can you please confirm that the USB-C standard as implemented by Apple is safe in this instance or not for Firewire connection.
I suspect that others may well fall into the same trap as I did.
In retrospect I should probably have fully understood how the standards interact before I tried to connect things together.
Could you please publish something on your website with your advice about how (and how not) to connect to various Apple devices?

It could save you time investigating many red light issues that are arising as a consequence of Firewire's demise at Apple.

If its some other reason then I have tried a second old macbook - still red light, so my other option is to wait for a driver update. 

Ronaldo