Do you mean Firefox doesn't have a way to return the zoom level to its default? (It has two; you can do ctrl-0 or you can click on the zoom-level indicator in the URL bar.)
Or do you mean you can't tell it to default to a zoom level that isn't 100%? (That seems to be true.) I confess that's never a thing it's occurred to me to want; it feels like if I wanted every website's text to look larger or smaller, I'd probably want everything to look larger or smaller and would adjust my OS's scaling level. But I know some OSes handle that better than others, and maybe there are good reasons for wanting different scalings for websites than for everything else. Still, it's not clear to me that this is so obviously important a thing that "doesn't even have" is fair.
Or do you mean you can't tell it to default to a zoom level that isn't 100%? (That seems to be true.) I confess that's never a thing it's occurred to me to want; it feels like if I wanted every website's text to look larger or smaller, I'd probably want everything to look larger or smaller and would adjust my OS's scaling level. But I know some OSes handle that better than others, and maybe there are good reasons for wanting different scalings for websites than for everything else. Still, it's not clear to me that this is so obviously important a thing that "doesn't even have" is fair.