There is also a way to make Firefox ignore any Windows 10 theme specifications, although I'm not sure if this would help in this particular case. To try this do the following:
First grab these two programs and install them.
Resource Hacker: http://www.angusj.com/resourcehacker/
Notepad++: https://notepad-plus-plus.org/download/v7.5.3.html
Now open firefox.exe in Resource Hacker and navigate to
24 --> 1 --> 1033
and you should see a bunch of text on the right side of Resource Hacker. Scroll all the way to the bottom till you see four lines starting with:
<supportedOS Id=
delete the first line so only three supportedOS Id= lines remain and then press on Compile Script. Then just save the file under a new name anywhere. After that open the new .exe file you saved with Notepad++, don't change anything but just go to "Save as..." and save it with yet another filename, then just rename that file to firefox.exe and replace your actual firefox.exe with it.
Start Firefox and enjoy
The last step with re-saving the file with Notepad++ may seem weird and unnecessary but for some reason saving the file like that forces the compiled manifest to be updated so Firefox "forgets" what Windows 10 is.