How can I make iframe create with bootstrap extension load dtds properly?

[117649]

Could I elevate the privilege of an iframe so it can load dtds to its src?
Just like this function did with the XMLHttpRequest()?

  fetchOverlay(srcUrl) {
    if (!srcUrl.startsWith("chrome://") && !srcUrl.startsWith("resource://")) {
      throw new Error(
        "May only load overlays from chrome:// or resource:// uris"
      );
    }

    let xhr = new this.window.XMLHttpRequest();
    xhr.overrideMimeType("application/xml");
    xhr.open("GET", srcUrl, false);

    // Elevate the request, so DTDs will work. Should not be a security issue since we
    // only load chrome, resource and file URLs, and that is our privileged chrome package.
    try {
      xhr.channel.owner = Services.scriptSecurityManager.getSystemPrincipal();
    } catch (ex) {
      oconsole.error(
        "Failed to set system principal while fetching overlay " + srcUrl
      );
      xhr.close();
      throw new Error("Failed to set system principal");
    }

    xhr.send(null);
    return xhr;
  }