RE: Firewire card for XPS8500
I think the least cost - most likely to work well option for the XPS 8500 is to add a PCIe 1x card to the 8500.
Here is a review of some choices
A couple of the cards support line-level power for devices that need power from firewire. The gotcha is the power is 4 pin Molex, so if you need this, then you also need a SATA power to molex adapter ( they cost a few bucks) and a spare SATA power connection. The 8500 has I think five SATA power ports so you likely have one available.
I'd read the reviews of the cards since some use firewire for storage, and some use it for video capture and make sure the Win10 support is there with the drivers.
I don't think the USB to firewire adapters work well for video. They barely work for storage...
good luck, and let us know what you go with. I have a laptop with firewire, but it won't be around forever.