For security reasons we allow URLs that begin only with:
http://backend.deviantart.com/
http://ivan.vucica.net/
http://tornado.zrs.hr/~dcihlar/
http://feedproxy.google.com/
http://otfans.net/
http://www.zrs.hr/
http://bitbucket.org/
This is mostly because of the proxying using PHP that we need to do in order to bypass Firefox security measures. You can get the source and adapt to your needs, without these limitations.