I hit this bug as well, it seems to only affect firefox, but I hit it with "the australian", and a few other online news sites.
I'm not completely sure about the correct fix for /etc/fonts/conf.d/90-fonts- baekmuk.conf but I ended up changing qual="any" to qual="all", assuming that means it has to pass every test and not just one of them, and it did seem to fix it. I suspect the correct answer is just to make it look like /etc/fonts/conf.d/ 20-unhint-small-dejavu-sans.conf.
