Network error 404 page not found on firebug
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I am using indexu deluxe,I have generated custom 404.php file on root of the site for the wrong link id and catgory name, this file called generic.html file of customized theme,Its working fine and displayed images headers and footer properly you can see it on following link

http://www.yippee.co/404.php
but when I have see the console of firebug there was following error occurs,

"NetworkError : 404 page not found - http://www.yippee.co/404.php"

can anybody help me that why this error occurs and what is solution for this issue..?

thank you