var myLoader:Loader = new Loader();
var myRequest:URLRequest = new URLRequest("http://www.sentientbeings.com/deadDomain.jpg");
//var myRequest:URLRequest = new URLRequest("http://www.xxxyyyzzzabcdefg.com/deadDomain.jpg");
 
myLoader.load(myRequest);
myLoader.contentLoaderInfo.addEventListener (Event.COMPLETE, function():void{trace("load complete")});
myLoader.contentLoaderInfo.addEventListener (IOErrorEvent.NETWORK_ERROR, function():void{trace("network failed")});
myLoader.contentLoaderInfo.addEventListener (IOErrorEvent.IO_ERROR, function():void{trace("IO failed")});
 

When you run this code, you'll get an IO_ERROR. But if you comment the first myRequest and then uncomment the second myRequest, you'll get nothing when testing on a Mac with Firefox. With "nothing", I mean the code does nothing. Somehow, the way Firefox on a Mac instantiates requests keeps Flash from firing the IO_ERROR event that you should be getting when trying to load a resource from a domain that does not exist.