No, it is not possible to connect a FireWire interface to your computer's USB port via a FireWire to USB adapter since this connection is not sufficient to run a FireWire audio interface.

If your computer does not have a built in FireWire port you should check if it has a PCI or PCIe slot (desktop computer); or a PCMCIA or Expresscard slot (laptop computer).

This article here explains about which cards work the best with Focusrite FireWire interfaces.