How do I let Firefox add the http post request id / name?

I can use the addObserver () function to add an "http-on-modify-request" observer and get the observer function when firefox is watching the post request. Then I can use the httpchannel variables or methods to get the cookies / originalURI / referrer of the post request. But how can I get the id / name of the form that is issuing the submit request? Thank!