You posted this question in the Mac mini group and no Mac mini made so far has a USB-C connection.
However assuming you are referring to a different model Mac e.g. a MacBook Pro which does have USB-C then the answer would be as follows.
- Get the Apple Thunderbolt3 to Thunderbolt2 adapter
- Get the Apple Thunderbolt2 to FireWire800 adapter
- Connect the FireWire cable to the second adapter
Note: Most if not all old FireWire audio interfaces are not supported under Sierra or High Sierra. You should consider getting a USB audio interface to replace it. Furthermore by the time you have paid for both the above Apple adapters you will be well on the way to the cost of a brand new USB audio interface.
Obviously you should first check to see if your existing ancient FireWire interface is compatible with Sierra or High Sierra before buying anything.