WhiteSur-gtk-theme/src/other/firefox/common/titlebuttons/titlebutton-close-backdrop-alt.svg

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><g fill-rule="evenodd"><path fill="#bababa" d="M8 15A7 7 0 1 0 8 1a7 7 0 0 0 0 14"/><path fill="#cecece" d="M8 14.5a6.5 6.5 0 1 0 0-13 6.5 6.5 0 0 0 0 13"/><path d="M8 11a3 3 0 1 0 0-6 3 3 0 0 0 0 6" opacity=".5"/></g></svg>