how would a modify headers add-on be able to let the server think that you are coming from a different address? It's not as if the browser sends the origin addr

how would a modify headers add-on be able to let the server think that you are coming from a different address? It's not as if the browser sends the origin address as a HTTP header. It doesn't have to. It's part of the IP packet which contains the TCP packet which contains the request headers. Are you sure that you are not using a proxy server at the address Wolfram gives you? Edit: On second thought, you could try to fool server-side detection by setting a non-standard X-Forwarded-For header, but a what is my ip service shouldn't trust that and just report the real remote address.