i got this css stylesheet code

#nav li ul {
    position: absolute;
    visibility: hidden;
    float: none;
    top: 42px;
    left: 0px;
    width: 150px;
    margin: 0;
    padding: 5px 10px 6px 10px;
    z-index: 10000;
    border: 1px solid #C0ACB2;
    border-bottom-width: 2px;
    border-bottom-color: #AF9DA3;
    border-top: 0;
    background-color: #fff;
    opacity: 0.97;
}

#nav li:hover ul,
#nav li.iehover ul {
    visibility: visible;
}

i want to make every #nav to be hidden and then displayed when cursor hover around it. But the problem is everytime is show, it's overlapped by other div. it's seem z-index is not working to make the #nav li ul becoming the front container.

i'm testing it in firefox and flock.