lets say a website is at
http://www.pligg.com/pligg
and the website tries to use xmlhttprequest to get data of a different website
http://www.evil.com/pligg
then firefox won't allow that, sort of a security feature. i think
http://www.captain.at/howto-ajax-per...ttprequest.php
just check you config file has the same url as the way users use it
like pixelpump said with the www.
Does this error come up when you access the website with or without a www.