Need USB-C/Thunderbolt to Firewire 800 adapter

You need two Adapters to do that.

a USB-C to Thunderbolt 2 adapter.

https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter

and a Thunderbolt 2 the FireWire 800 adapter.

https://www.apple.com/shop/product/MD464LL/A/apple-thunderbolt-to-firewire-adapter

How much do you have to move or share?

Other than those expensive adapters for TDM, the other options are File Sharing over your local network,

Set up file sharing on Mac - Apple Support

Or using an external hard/flash drive.

Hello,

Please note that even if a combination of two adapters (Thunderbolt 3 to 2 to FireWire) may be OK for the connection of an external FireWire device, it does not guarantee that FireWire target disk mode will work. So, you may want to check whether someone actually has successfully used the exact adapters in question for this. Otherwise, consider this to be experimental.

The following adapter could perhaps be of interest, if you choose to use Ethernet instead:

The Belkin USB-C to Gigabit Ethernet Adapter - Apple (UK)

If somewhat lower speeds can be accepted, you should be able to carry out transfers via Wi-Fi as well.

There is no TB3 to FW adapter, only TB2 to FW adapter. Apple does have a TB3 to TB2 adapter.

FWIW, instead of bothering with the hassle of adapters

(and the cost) to interface via Firewire 800, just use

File Sharing via ethernet. It will likely be nearly as fast.

Glad to hear that it worked.