Indeed, Windows and Linux have options to enlarge fonts globally which will also affect Firefox. But not macOS. “Theme and Font Size Changer” was really useful mostly on macOS. I guess we should create a feature request for built-in alternative to the aforementioned add-on.
Just wondering, how do you use macOS after all? From my experience, it’s the worst OS from the perspective of visually impaired (not the completely blind though). All it have is global zoom (Control+Mouse Wheel) and an ability to emulate higher display DPI which causes all UI to be rendered bigger. Same situation with colors: there is no real high contrast theme (only full invert which is, emm, probably better than nothing but it’s still awful).