@IceWolf I don't use Firefox but everything I do use seems to respect my fontconfig settings.
@alva So this is a Fontconfig thing perhaps? Hmm... wonder why it's using this font in particular.
It's not the font I'm using for the rest of my system.
I'd expect that to be what is set in about:preferences -> Language and Appearance -> Fonts and Colours
Under Advanced you could also uncheck the Allow pages to choose their own fonts box if the page ignores your settings there