Document.layers has no properties error

I have the following code that works OK in IE, but I have just tried it in Firefox and I get the following error:
"Document.layers has no properties"
and the line is:
document.layers[i].visibility="hide";

I do not understand Javascript.  Could anyone explain if and how I can get the Javascript to run?


function closeAllMenusExcept(MenuID)
{
var NoOfMenus=10
      for (i=0; i<NoOfMenus; i++ )
      {
        if (navigator.appName=="Netscape"){
        if (i != MenuID){
          document.layers[i].visibility="hide";
        }else{
          document.layers[i].visibility="show";
        }         
      }else{
          var what="menu" + i
        if (i != MenuID){
//              alert(document.all.item(what).innerHTML)
          document.all.item(what).style.visibility="hidden";
        }else{
          document.all.item(what).style.visibility="visible";
        }
        }
      }
}