Hi,
EDIT, I should have put this first: If your MBP cannot start in TDM, I believe the only thing you can do is put the HDD in an external enclosure: if it's USB, this adapter should work: http://www.apple.com/shop/product/MJ1M2AM/A/usb-c-to-usb-adapter, if it's FW, use those adapters, if it's Thunderbolt use that adapter.
Obviously the $9 USB-C to USB cable is the least expensive, so I'd try that route first; if you don't have USB external enclosures they're only $10-15, so I think that would be worth a try to get your data. Again, I haven't heard of anyone using this method yet, so I can't say if it will work for sure.
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First, see if your MBP will boot into Target Disk Mode by holding down the T key while booting. If you have a logic board failure, I don't think this is going to work.
If TDM doesn't work like this, it may by connecting the two Macs first, but that's another story.
I don't believe there is an adapter to go directly from USB-C to FireWire, as they are different protocols. I've never tried it nor heard about anybody trying it yet, so I have no idea if it will work, but an Apple support document says it should, and if it does, you're going to need two adapters, one from Thunderbolt 3/USB-C to Thunderbolt and the other from Thunderbolt to FireWire 800 (plus a FW cable). You shouldn't need to do it that way though, as your MBP has a Thunderbolt port on it. So all you should need is the T3/USB-C to T adapter (plus a Thunderbolt cable). The adapter is here: http://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2 -adapter (Note this is a special price until Dec. 31, 2016; I believe the regular price is $49.)
NOTE: Your Mac must be running macOS Sierra to work with the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
