Don't make head visible in fullscreen mode

chrome/show_window_title_in_menubar.css

@@ -1,7 +1,7 @@
 /* Shows window title in menubar */
 /* Enable menubar for correct behavior */
 @-moz-document url(chrome://browser/content/browser.xhtml){
-  head{
+  :root:not([sizemode="fullscreen"]) > head{
     display: block;
     position: fixed;
     right: 140px; /* Space for window controls */