being an owner of a vintage firewire audio interface Focusrite Saffire Pro40, I'd like to share my experiences with this device and Windows 10.
The Saffire Pro40 was launched by Focusrite in 2009. It features 18 physical inputs (2 analog mic/line/inst + 6 analog mic/line + 8 digital via optical ADAT + 2 digital via SP/DIF) and 20 physical outputs (10 analog line + 8 digital via optical ADAT + 2 digital via SP/DIF) in a 19" case. Furthermore, there are 2 virtual loopback inputs to record audio from software sources. To connect to a computer, the device provides a firewire (IEEE 1394) interface.
The interface is of excellent audio quality, which is the reason why I try to use it as long as possible. Unfortunately, Focusrite announced in 2019, that their firewire interfaces won't support Windows 10 newer than 1809. Fortunately, this isn't true, at least regarding certain hardware configurations
This is how I got my Saffire Pro40 to work:
Firewire is an obsolete standard and most newer computers aren't equipped with a firewire controller. However, at least desktop computers can be retrofitted by a firewire expansion card. According to Focusrite, firewire controllers based on Texas Instruments XIO2213B (B is important) or LSI/Agere FW643 chipsets are the only ones which work properly with Windows 10 and the Saffire. So I purchased a PCIe controller card equipped with the TI XIO2213B chip.
Usually, Windows 10 detects firewire controllers automatically and installs the driver it provides. However, Focusrite does not recommend to use this driver. Instead, one should use a legacy driver which came with Windows 8. You can download this driver from Microsoft from here. The page also explains how to install it. When installed, you have to change the firewire controller settings in device manager to use the legacy driver instead of the one provided by Windows 10.
Download the newest version (3.7) of the software package for the Saffire Pro40 (Saffire Mix Control) from here and install it. The package also contains an ASIO driver for Saffire interfaces, which will be installed automatically. If necessary, the package also updates the interface's firmware.
You might have to assign the Focusrite driver manually to the interface. This can be done using the device manager.
This is how I got my Saffire to run flawlessly with Windows 10 2004. Amazingly the interface continued to work after I updated to 20H2 and 21H1 later. The only thing I had to do after installing a new feature update was to change the firewire driver setting to legacy according to #2 above.