What BRX says.
I have a Behringer desk with Firewire and USB2 multi channel outputs. Using a FW400 - FW800 adapter it used to work fine with my old iMac. When I got a new iMac I needed a FW - TB adaptor. The desk doesn't work over FW, but still does using USB.
Whilst FW was much better as it used dedicated controller chips at either end, so didn't suck processor cycles like USB. I've found there to be no real issue using the USB2 interface, even when capturing 10-12 channels of audio.
Unless you can afford a Thunderbolt interface (bearing in mind that TB3 uses a USB-C style connector, so you may need an adaptor for older TB interfaces), you may as well just look at USB2/3 ones.