Dropdown menus "break" when optimise CSS is enabled and Firefox Browser is used to view site.

When I enable "optimize CSS" to improve site performance (and stop IE breaking completely) the drop down menu breaks in Firefox (tested OK in Chrome, IE8, Safari, Opera). Child menus items are displayed, but the parent menu item list is pushed down below level of child items.

Probably a simple css tweak required to address this but I'm stumped.

Image of the problem and the specific CSS for the primary links menu that was added to nice_menus_default.css are attached in case anyone can help.

/******************************
 Primary Navigation
******************************/
#nav {
background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0;
}

#primary {
background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0;
}
#primary ul.nice-menu-down {
  float: left;
  border: 0;
background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0;
border-top: 1px solid transparent;
 color:#FFF;
		font-weight:bold;
		padding:0 12px;
		line-height: 30px;
		text-decoration:none;
		display: block;
border-style: none none none none;
}

#primary ul.nice-menu-down a {
color:#FFF;
		font-weight:bold;
		padding:0 12px;
		line-height: 30px;
		text-decoration:none;
		display: block;

}

#primary ul.nice-menu-down li {
  border-top: 1px solid transparent;
 background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0;
color:#FFF;
		font-weight:bold;
		padding:0 5px;
		line-height: 30px;
		text-decoration:none;
		display: block;
border-style:none none none none;

}

#primary ul.nice-menu-down li li {
  border-top: 0;
}

#primary ul.nice-menu-down ul {
  left: 0;
color:#FFF;
		font-weight:bold;
		padding:0 5px;
		line-height: 30px;
		text-decoration:none;
		display: block;
  
}

#primary ul.nice-menu-down ul li {
  clear: both;
color:#FFF;
		font-weight:bold;
		padding:0 5px;
		line-height: 30px;
		text-decoration:none;
		display: block;
}

#primary ul.nice-menu-down li ul li ul,
/* Repeat for Garland header. */
#primary ul.nice-menu-down li ul li ul {
  left: 12.5em;
  top: -1px;
}

#primary ul.nice-menu-down .menuparent a {
  color:#FFF;
		font-weight:bold;
		padding:0 5px;
		line-height: 30px;
		text-decoration:none;
		display: block;
}

#primary ul.nice-menu-down li.menuparent,
/* Repeat for Garland header. */
#primary ul.nice-menu-down li.menuparent {
 background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0 ;
}

#primary ul.nice-menu-down li.menuparent:hover,
#primary ul.nice-menu-down li.over,
/* Repeat for Garland header. */
#primary ul.nice-menu-down li.menuparent:hover,
#primary ul.nice-menu-down li.over {
background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0 ;
}

#primary ul.nice-menu-down li li.menuparent,
/* Repeat for Garland header. */
#primary ul.nice-menu-down li li.menuparent {
background:transparent url(../images/top_menu_bg.gif) repeat-x scroll 0 0;
}

#primary ul.nice-menu-down li li.menuparent:hover,
#primary ul.nice-menu-down li li.over,
/* Repeat for Garland header. */
#primary ul.nice-menu-down li li.menuparent:hover,
#primary ul.nice-menu-down li li.over {
  background: #003456 url(../images/top_menu_bg.gif right center no-repeat;
}