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

#navigator-toolbox:not(:hover) > toolbar:first-child + toolbar ~ toolbar {
    display: none;
}

/*

#navigator-toolbox:not(:hover) ~ #browser-stack .tabbrowser-tabs {
    display: none;
}

 * If you want you can copy the code above and paste after
 * this line if you wanted to hide the tab bar when the mouse is not over. */