Navbar slightly detached

When I load the page: https://developer.mozilla.org/en-US/docs/Web/HTTP/MIME_types in the browser, between the navbar of MDN and the bar of the browser there are few pixels of distance. As you can see, when scrolling the page content, it shows up a bit behind the navbar.
I report it because on Chrome those few pixels of distance are not present. So, I guess it’s not a choice of the web developers of the page (?)