How can I connect firewire HD to USB C?

You can't, not directly, not if the drive only has a Firewire interface. You'd need Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter, Apple's Thunderbolt 2 to Firewire 800 adapter, and then if your drives have only Firewire 400, a Firewire 800 to Firewire 400 port adapter or cable. And then there's no guarantee it will work. It might be cheaper, and probably more reliable, to get a USB-C drive enclosure and put the drives in that.

Or find a friend or computer store that has an old Mac with Firewire that you could use temporarily.

Regards.

What brand and model are those older FireWire external hard drives...?

Many of those also had USB connections and supplied USB cables.

You probably have in mind that you paid a lot for such drive. And you likely did. Just like older ones of us did with SCSI, SATA, drives.

keep in mind that newer drives will likely cost you less (in money, reliability and required adapters) than trying to leep that dinosaur.

Frustrating? Yes. But at some point we all need to chose our battles.

Unless it’s a matter of recovering data which would only be available on that drive, move forward with newer drives. These won’t be slower or worst than the FW ones.