Found the answer myself in case anyone is interested (source is blogs.telerik.com)
WebSockets
Fiddler’s HTML5 WebSockets support continues to grow; Fiddler extensions can now capture and manipulate WebSocket messages by handling the FiddlerApplication.OnWebSocketMessage event.
In preparation for a full-featured WebSockets UI, Fiddler no longer spews WebSocket messages to the Log tab. If you’d like to re-enable that behavior until the full UI is available, you can do so using FiddlerScript. Simply click Rules > Customize Rules and add the following function inside your Handlers class.
static function OnWebSocketMessage(oMsg: WebSocketMessage) { // Log Message to the LOG tab FiddlerApplication.Log.LogString(oMsg.ToString()); /* // Modify a message's content var sPayload = oMsg.PayloadAsString(); if (sPayload.Contains("time")) { oMsg.SetPayload(sPayload + "... bazinga!"); } */ }