Mozilla/IE compatibility problem JavaScript

Can someone please explain why the following code works in Internet Explorer, but doesn't work in Firefox/Mozilla:
parent.frames["ctl00_ContentPlaceHolder1_LibraryFrame"].document.forms['form1'].id
or
document.frames["ctl00_ContentPlaceHolder1_LibraryFrame"].document.forms['form1'].id
when I run the code in Venkman, I keep getting:
Caught exception: TypeError: parent.frames.ctl00_ContentPlaceHolder1_LibraryFrame has no properties
when I execute the line in the console.
(The code definitely runs in Internet Explorer so I can't imagine it's a simple spelling mistake)Start Free Trial