gistfile1.js

Components.utils.import("resource:///modules/devtools/gcli.jsm");

//from http://stackoverflow.com/questions/9996986/reading-writing-preferences - need to check over this in here
var prefsBranch = Components.classes["@mozilla.org/preferences-service;1"].
                getService(Components.interfaces.nsIPrefService).
                getBranch("general.useragent.");

var mdnCommandParams = [
    {
      name: 'property',
      type: 'string',
      description: 'property name',
    }
  ];

function execMDNCommand(args, context, ref) {
  var $this = this;
  var promise = context.createPromise();
  
  var locale = prefsBranch.getComplexValue("locale", Components.interfaces.nsIPrefLocalizedString).data; //haven't tested - https://developer.mozilla.org/en-US/docs/Code_snippets/Preferences
  var url = "https://developer.mozilla.org/" + locale + "/docs/" + ref + "/" + 
            encodeURIComponent(args.property);
  var xhr = Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"];

  let req = xhr.createInstance();
  req.open('GET', url + "?raw&macros&section=Summary", true);
  req.onreadystatechange = function () {
      if (4==this.readyState) {
          var content = this.responseText;
          let req = xhr.createInstance();
          req.open('GET', url + "$json", true);
          req.onreadystatechange = function () {
              if (4==this.readyState) {
                  var meta = JSON.parse(this.responseText);
                  promise.resolve([
                      "<div>",
                      "<h1><a href=\"", url, "\">", meta.title, "</a></h1>",
                      "<div>", content, "</div>",
                      "</div>"
                  ].join(""));
              }
          };
          req.send();
      }
  };
  req.send();

  return promise;
}

gcli.addCommand({
  name: 'cssdoc',
  description: 'Documentation for CSS properties',
  returnType: 'html',
  params: mdnCommandParams,
  exec: function(args, context) {
    return execMDNCommand(args, context, 'CSS');
  }
});

gcli.addCommand({
  name: 'domdoc',
  description: 'Documentation for DOM methods and properties',
  returnType: 'html',
  params: mdnCommandParams,
  exec: function(args, context) {
    return execMDNCommand(args, context, 'DOM');
  }
});