i will look into this.
Hey, thanks old friend ;-)
But I had a bright moment, and checked another pc, gave it the same W10 and Firefox updates, and that one behaves as it should with cleartype off.
So it seems to be a very specific issue with the installation of these updates on my main pc only.
So, please don't waste time on this on my behalf.
I'll see if I can revert some stuff and find out what happened exactly.