When i place the cursor in Workshops option in Firefox the submenus appears behind forum menu of ActiveForums.
See:http://www.dotnetwork.it/Forums.aspx

Can you give me a solution?

for your code you need to add z-index to the .submenu class:

.submenu {
    border: 1px #ffffff solid;
    padding: 0px;
    background: #dee9f3;
    margin-left: 0px;
    margin-top: 2px;
z-index: 1000;
}

I found this by using the techniques here:
Troubleshooting DotNetNuke Skins

 It works, thanks!