gistfile1.txt

const {Cc, Ci, Cu, Cr} = require("chrome");

Cu.import("resource://gre/modules/Services.jsm");
Cu.import("resource://gre/modules/XPCOMUtils.jsm");

var observer = {
  QueryInterface: XPCOMUtils.generateQI([
    Ci.nsIObserver,
    Ci.nsISupportsWeakReference
  ]),

  observe: function(subject, topic, data) {
    if (topic == "http-on-modify-request" && subject instanceof Ci.nsIHttpChannel) {
        var channel = subject.QueryInterface(Ci.nsIHttpChannel);
        var requestUrl = channel.originalURI.spec;

        if ("WE CHECK THE PATTERN HERE AGAINST $requestUrl") {
            channel.cancel(Cr.NS_BINDING_ABORTED);
        }

      }
    }
  };

Services.obs.addObserver(observer, "http-on-modify-request", true);