function listener(details) {
  let filter = browser.webRequest.filterResponseData("12345");

  filter.onerror = event => {
    console.log(`Error: ${filter.error}`);
  }

  //return {}; // not needed
}

browser.webRequest.onBeforeRequest.addListener(
  listener,
  {urls: ["<all_urls>"], types: ["main_frame"]},
  ["blocking"]
);