I came across this problem a week ago when I tried to develop an interface between an Iframe, or in Detail it’s content, and the iframes parent page. Turns out that the following code wouldn´t work in FireFox although all other browsers, yes even our friend the Internet Explorer, would behave as expected.

   function accessIframeJS(IframeName, methodName, methodParams)
   {
    var  IFrameObj  = window.frames[IframeName];
    var  methodCall = "IFrameObj."+methodName+"("+ methodParams +")";

    try{
       eval(methodCall);
    }catch(error){
       // some code here for error handling
    }
   }

Traced on Firebug it turns out that FireFox loses the IFrameObj on Drag & Drop while rewriting the DOM. Took me quite a while, and a long talk with uncle google until I took a look at bugzilla of mozzilla and the huge amount of iframe Bugs mentioned there. But I found the solution after reading alot of those posts,.. and yes as all ways it is an easy one, if you knew the solution.

function accessIframeJS(iframeId, methodName, methodParams)
 {
  var IFrameObj = document.getElementById(iframeId).contentWindow;
  var methodCall = "IFrameObj."+methodName+"("+ methodParams +")";

  try{
     eval(methodCall);
  }catch(error){
     // some code here for error handling
  }
 }

Ta ta,… this works everywhere,… but it took me a long search, until I found the post in Bugzilla.