FireWire 400 to USB.

Hi,

The only direct adapter from Apple was a Thunderbolt-to-FireWire 800 adapter.

Apple Thunderbolt to FireWire Adapter - Apple

In order to connect a FireWire 400 device, a simple FireWire 800 to FireWire 400 cable would be needed.

However, a computer using something like Thunderbolt 3 or USB-C would require a combination of adapters, and that would be entirely experimental. It may or may not work. A better way could be to buy an old Mac computer with FireWire 800 or 400 ports, and then transfer the FireWire hard disk drive contents via a network, a USB flash drive, or a CompactFlash memory card (via a PC Card adapter for CF and/or other appropriate card readers).

Alternatively, depending on the FireWire enclosure and the contained hard drive, it may be possible to remove the hard drive and connect it to a USB universal bare drive adapter (similar to the example at https://eshop.macsales.com/item/NewerTech/U3NV2SPATA/).