Firefox 52 no longer working on XP

[Dave-H]

I've had Firefox 52.9.1.6822 installed on the XP side of my machine for many years, as the last Firefox version which will run on XP.
I don't use it often now as I use a much more up-to-date version of Supermium.
However, I tried to run it today, and it won't run any more.

I just get a popup saying -

"The procedure entry point CreateThreadpoolWork could not be located in the dynamic link library KERNEL32.dll."

Anyone any idea why it would have stopped working?
I've not knowingly changed anything relevant since I last used it.
The Firefox files seem to be as they have always been, and no system files have been knowingly changed either.