How to debug a script in Thunderbird's Quicktext extension

Question

I'm trying to debug a js script in Thunderbird's Quicktext extension, by modifying an existing script Script: Identity

if(action == "getCurrentIdentity") {
  let identityList = this.mWindow.document.getElementById("msgIdentity");
  let identities   = identityList.attributes;
  let result       = "";
  for(let identity of identities) {
      if(identity.getAttribute("selected") != "true") continue;
      result += identity.getAttribute("label") + "\r\n";
      console.log(identity)
  };
  return result;
}

When i try, i have this error :

So what's the best way to debug?