Remembered that "Fantasy" is a valid generic font family for CSS and was curious to see what the default fantasy font is.
I don't know about you, but for me it's Impact which I find extremely funny
Firefox defaults to a very non-fantastic sans-serif font, and I had to do some digging to find out how to change it. Turns out the only way is to add a font.name.fantasy.x-western preference in about:config.
Anyway, my fantasy font is now the Cookie Run font.