window.document[movieName]      // (on Mozilla browsers such as Netscape)
window[movieName]               // (on Internet Explorer as of ver 5)
document.embeds[movieName]      // Mozilla Netscape, Firefox or Opera

===================================================

function getFlashMovieObject(movieName){
 if (window.document[movieName]){
  return window.document[movieName];
 }else if (navigator.appName.indexOf("Microsoft Internet")==-1){
  if (document.embeds && document.embeds[movieName])
  return document.embeds[movieName];
 }else{
  return document.getElementById(movieName);
 }
}

===================================================

引自：http://www.permadi.com/tutorial/flashjscommand/