I have a motu 2408 interface that’s pretty old that only takes rca inputs but I plug it into my computer via stereo cable. But whenever I use my monitor or even hit record nothing comes out.
An interface connects to a computer via some sort of digital cable (USB, FireWire, Thunderbolt, PCI, PCIe, etc.). What type of cable are you using to connect the interface to the computer?
I don't think it's even possible to have a USB-to-Firewire adapter. You need a computer that has a Firewire port. If you have a desktop PC you can install an expansion card with a Firewire port on it.
My understanding is that it is possible, but only in the opposite direction of what the OP is doing. You can connect a USB device to a FireWire input on a computer, but not the other way around. I might be completely wrong on that though.
USB to Firewire adapters don't work except in really rare cases....definitely not for an audio interface. If your interface is Firewire only and your computer doesn't have Firewire or Thunderbolt connections you are out of luck.