Minor changes from the showheaders.js found at http://bgrins.github.io/devtools-snippets/ Discovering some nice features...

showheaders.js

// showheaders.js
// Little changes (for Firebug) from https://github.com/bgrins/devtools-snippets
// Print out response headers for current URL.

(function() 
{
  var request = new XMLHttpRequest();
  request.open('HEAD', window.location, false);
  request.send(null);

  var headers = request.getAllResponseHeaders();
  var tab = headers.split("\r\n").map(function(h) 
  {
    var hp = h.split(": ");
    return { "Key": hp[0], "Value": hp[1] }
  }).filter(function(h) { return h.Value !== undefined; });

  console.group("Request Headers");
  console.log(headers);
  console.table(tab);
  console.groupEnd("Request Headers");
})();