Same-origin policy..

The same origin policy requires that the protocol, port, and host are the same for both pages.  Otherwise, Firefox apparently "preflights" the HTTP request:

https://developer.mozilla.org/en/HTTP_access_control

https://developer.mozilla.org/en/Same_origin_policy_for_JavaScript

Mozilla considers two pages to have the same origin if the protocol, port (if one is specified), and host are the same for both pages. The following table gives examples of origin comparisons to the URL http://store.company.com/dir/page.html:

URL	Outcome	Reason
http://store.company.com/dir2/other.html	Success
http://store.company.com/dir/inner/another.html	Success
https://store.company.com/secure.html	Failure	Different protocol
http://store.company.com:81/dir/etc.html	Failure	Different port
http://news.company.com/dir/other.html	Failure	Different host