@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Hide custom toolbars past the address/search bar,... */
#navigator-toolbox:not(:hover) > #customToolbars ~ * { display : none; }

/* ...unless the latter is hovered. Then, show only the toolbars that are not hidden. */
#navigator-toolbox:hover > *:not([hidden="true"]) { display : -moz-box; }

/* However, if the menu bar is hovered, re-hide the toolbars. */
#toolbar-menubar:hover ~ #customToolbars ~ *:not([hidden="true"]) { display : none; }