gistfile1.js

let {WebRequest} = Cu.import("resource://gre/modules/WebRequest.jsm", {});

let pattern = /https:\/\/mdn\.mozillademos\.org\/.*/;

function listen(event) {
  event.addListener(redirect, {urls: [pattern], types: ["image"]},
                                                 ["blocking"]);
}

function redirect(e) {
  console.log("Redirecting: " + e.url);
  return {redirectUrl: "https://38.media.tumblr.com/tumblr_ldbj01lZiP1qe0eclo1_500.gif"};
} 

function unlisten(event) {
  event.removeListener(redirect);
}