There are a bunch of considerations for the native look-and-feel that go beyond the primary visual aesthetic. Using the OS-specific terms, shortcuts, font-sizes

There are a bunch of considerations for the native look-and-feel that go beyond the primary visual aesthetic. Using the OS-specific terms, shortcuts, font-sizes, button order, APIs. This maintenance effort is not trivial. This is all stuff people at Mozilla are trying to look out for. But additionally adopting the design language of the system is another burden altogether. It potentially triples your design- and implementation time and adds a lot of complexity to your code. And if you're unlucky the next OS version requires you to change it all again.