Custom 404 error page does not render in Firefox and Chrome but works in IE

Had an issue last week where our custom 404 error page rendered fine in Internet Explorer but did not render in Firefox and Chrome.

Thanks to a colleague of mine, he stumbled across this awesome post by Brian farm hill.

http://blog.brianfarnhill.com/2012/12/issues-with-a-custom-404-page-in-sharepoint-2010