Ok, so the error you are seeing is in Firefox 3.6.13. Which OS? Can you direct me to the page on the open web or send me a copy to test locally?
I can augment the line to test for WINDOW.onload's existence before setting the property like that, but I'm not sure if that's the cause as I have not seen this issue myself.
Another quick question: the paths to your CSS files in the pastebin are full URLs. Are those URLs on the same domain?