You seem to not understand that Apple defines the platform; consistency has to start somewhere, and it's not going to emerge by committee.
Apple and 3rd-party developers extends conventions by exploring coherent and consistent extensions to the platform.
Mozilla choosing to use a font that nobody else uses, for the purposes of consistency across their browser, not the platform, has nothing to do with what benefits the rest of the ecosystem, or by extension, the users, and everything to do with what Mozilla wants.
Apple and 3rd-party developers extends conventions by exploring coherent and consistent extensions to the platform.
Mozilla choosing to use a font that nobody else uses, for the purposes of consistency across their browser, not the platform, has nothing to do with what benefits the rest of the ecosystem, or by extension, the users, and everything to do with what Mozilla wants.