USB-to-Firewire interface.
To the best of my knowledge, such a beast does not exist. There are some USB-to-Firewire charging adaptors (which must necessarily fall short of Firewire's voltage and current ratings), but I've never heard of a functional USB-to-Firewire data transfer adaptor.
A Thunderbolt-to-Firewire adapter is a solution, since PCIe-to-Firewire adaptors are well established, and Thunderbolt is essentially an extension of the PC's internal PCIe bus. They'll be prohibitively expensive, though.