I have Win 10 Pro and a PCIe Firewire card with a T.I. chipset.  I am using the default driver that Windows installed and NOT the legacy driver.  My MOTU 896Mk3 Hybrid works perfectly with Firewire (or USB as it has both USB and Firewire capability).

I don't have any great answers, but because you are using outboard adapters, my guess would be that the adapters are changing the FireWire protocol before it reaches your PC.  So in that case, maybe your PC should be using a Thunderbolt driver internally that recognizes the protocol from the converter plugged directly into the PC.  I would think Focusrite support should be able to explain that part.
 
If the protocol is Thunderbolt 3 when it reaches the PC, then the PC needs to support Thunderbolt 3, rather than FireWire. Hopefully your USB-C port is not carrying USB3.1, as that will not work.  You will need to have Thunderbolt 3 on the motherboard.
 
What Jim R. has stated about Thunderbolt 3 on PC:
http://forum.cakewalk.com/FindPost/3643037
 

Don't assume that because a motherboard has USB-C port/s that it has Thunderbolt-3.
USB-C can carry USB-3.1 or Thunderbolt-3.
Had a guy call last week... and was actually arguing with me saying that Gigabyte had X299 motherboards (actually available) with Thunderbolt-3 on the motherboard.  This is not correct.  
Gigabyte X299 motherboards have a USB-C port... but it's carrying USB-3.1.
Gigabyte X299 motherboards can provide Thunderbolt-3 via an optional (not included) Thunderbolt-3 add-in-card.
 

Another thought, is that if your PC has Thunderbolt 3 over USB-C, and it can see the adapters correctly, then the Focusrite software is probably not completing the connection.
 
Maybe the suggestion to roll back to an earlier version is worth trying!

I hate to bring this up. I really hope it is not your problem. Please check the pro 24 again on another computer with a firewire port. I crapped out a Pro 24 DSP by inserting the 400 cable the wrong way. It is really easy to do. I feel stupid every time I think about it.
 
edit- to add
I replaced the Pro 24 DSP with a pro 24 and it does work with Thunderbolt through the Apple adapter without any additional drivers. 
 
I have since upgraded to an Antelope interface.

Apple have their ways of locking down even the wiring connections. 

The problem is almost surely the adapters.
 
We've had good luck with the Apple Thunderbolt-3 (USB-C) to Thunderbolt adapter.
In many different desktop and laptop configurations, it's always worked fine.
That said, we haven't tested it with a Thunderbolt to Firewire adapter.
 
One other thing to keep in mind:
Audio interfaces can be finicky about a particular Firewire controller.
Some combinations work fine, others work with less than ideal performance, and some don't work at all (or cause complete instability)
 
If you have the means, I'd test each stage.
Using the Apple Thunderbolt-3 (USB-C) to Thunderbolt adapter, test the Thunderbolt-3 port with a Thunderbolt-2 peripheral.  It's always worked for us... but there are reports of the Apple adapter not working properly with PCs.
Once you've established Thunderbolt is actually working (peripherals are recognized and functioning as expected), then I'd try a Firewire peripheral.  A Firewire HD is a good choice... as they'll typically work with most Firewire controllers.

Thanks Jim. I really appreciate your expertise. Is there a way to check to see whether a Firewire driver is in the laptop and if so which one? I guess another more important question is whether I need a Firewire driver or whether the thunderbolt 3 is backward compatible. This is all way beyond technical knowledge.

Fireberd, I thought Windows dropped support for Firewire. Did they add it back? I just read an article that said you had to install a legacy driver for FW support in Win 10.

Firewire drivers are stock Microsoft/Windows. 
The one Windows 10 Pro installed for my PCI firewire card as reported in the Device Manager is:
Texas Instruments 1394 OHCI Compliant Host Controller
Driver date is 6/21/2006 and Version 10.0.15.063.0
 
It doesn't state "legacy" driver as I've seen in Win 7 and Win 8, but given the 2006 date it may still be the legacy driver.
 

tonyzub999
Fireberd, I thought Windows dropped support for Firewire. Did they add it back? I just read an article that said you had to install a legacy driver for FW support in Win 10.

I'm running FireWire in Windows 10 Pro x64 just fine here.  Just needed to add in a PCIe adapter for the port.  The driver is supplied by Microsoft for this version of Windows.
 
But I really wonder if your PC is going to need to handle FireWire at all, at the end of that converter device chain.  It looks like you are trying to convert a FireWire audio interface to Thunderbolt 3 at the PC end.
 
FireWire Audio <=====> Converters <=====> Thunderbolt 3 over USB-C (no FireWire port on PC)