How can an extension listen to a WebSocket? (and what if the WebSocket is within an iframe?)

I am writing a Firefox extension that needs to listen to communication between browser and server. For HTTP communication, I use the webRequest library in a background script. But according to this Mozilla bug report, I cannot use webRequest to intercept WebSocket messages. How can my extension listen in on WebSocket communication?

Update:

I've injected my WebSocket wrapper, but it's never called! The WebSocket I am trying to listen to is inside an iframe. Does that matter?

The only way to listen to WebSocket communication is to inject a WebSocket wrapper into the site's code and have it relay the messages to you. You're code should look something like this:

manifest.json

{
  ...
  "content_scripts": [
    {
      "matches": [
        "<website-url>",    
      ],
      "js": ["content/syringe.js"]
    }
  ],
  "web_accessible_resources": ["lib/socket-sniffer.js"]
}

content/syringe.js

var s = document.createElement('script');
s.src = browser.runtime.getURL('lib/socket-sniffer.js');
s.onload = function() {
    this.remove();
};

lib/socket-sniffer.js

(function() {
  var OrigWebSocket = window.WebSocket;
  var callWebSocket = OrigWebSocket.apply.bind(OrigWebSocket);
  var wsAddListener = OrigWebSocket.prototype.addEventListener;
  wsAddListener = wsAddListener.call.bind(wsAddListener);
  window.WebSocket = function WebSocket(url, protocols) {
    var ws;
    if (!(this instanceof WebSocket)) {
      // Called without 'new' (browsers will throw an error).
      ws = callWebSocket(this, arguments);
    } else if (arguments.length === 1) {
      ws = new OrigWebSocket(url);
    } else if (arguments.length >= 2) {
      ws = new OrigWebSocket(url, protocols);
    } else { // No arguments (browsers will throw an error)
      ws = new OrigWebSocket();
    }

    wsAddListener(ws, 'message', function(event) {
      console.log("Received:", event);
    });
    return ws;
  }.bind();
  window.WebSocket.prototype = OrigWebSocket.prototype;
  window.WebSocket.prototype.constructor = window.WebSocket;

  var wsSend = OrigWebSocket.prototype.send;
  wsSend = wsSend.apply.bind(wsSend);
  OrigWebSocket.prototype.send = function(data) {
    console.log("Sent:", data);
    return wsSend(this, arguments);
  };
})();

(document.head || document.documentElement).appendChild(s);

All credit for the above code obviously goes to the posters linked above. I have only copied tt here for convenience' sake.

Update:

Yes, it does matter that the WebSocket is within an iframe! By default, extensions are only loaded in the top-most frame. For it to load into the iframe, you must add "all_frames": true to your manifest.json:

manifest.json

{
  ...
  "content_scripts": [
    {
      "matches": [
        "<website-url>",    
      ],
      "all_frames": true,      
      "js": ["content/syringe.js"]
    }
  ],
  "web_accessible_resources": ["lib/socket-sniffer.js"]
}

If you'd like to read more, here's the documentation for all_frames.