<IE>
var iframeDocument = iframe.contentWindow.document;
<FireFox>
var iframeDocument = iframe.contentDocument;
<둘다 동작하게 하기>
if(iframe.contentWindow) {
var iframeDocument = iframe.contentWindow.document;
} else if (iframe.contentDocument) {
var iframeDocument = iframe.contentDocument;
}
[출처] firefox에서 iframe document 객체 얻기|작성자 Diego