The reason I get worked up about discoverability in user interfaces is because that is how I learned nearly everything I know about computers up until I was in highschool. I was lightyears ahead of my parents, relatives, and most adults I came into contact with ever since I bought my parents' old IBM PC-XT for $75 when I was around eight years old. The only way I could keep learning was by exploring and discovering (and library books).
And you're right that just because something is deep in advanced settings doesn't mean that the design has to be shit. But in this case, and in other browsers, 99.99% of users don't even know it exists, so Mozilla likely feels justified in leaving it as is.