Firebug won't work in the drop down menus in the editor (right click does nothing).
I did however find the exact same default code in /clientscript/ckeditor/skins/kama/panel.css (why on earth it's in 2 different files I have no idea). According to Firebug both css files (editor.css and panel.css) are called (I didn't know the ckeditor was in an iframe

).
It still doesn't answer why additional.css isn't working.