Is what the url SHOULD look like, if it were only that simple! It seems it has to be either referring from the original page or actively requires a submission. The frigot.php form is also the page that returns the results once it's been submitted. Is this possible with GM_xmlhttpRequest? I know you can GET with it, but haven't seen any code with regards to posting.
You can definitely do a POST request with GM_xmlhttpRequest. Here's an example:
Note the "Content-type" header and the data field. This example should turn the URL above into a POST instead of a GET.