Chrome extension

content.js

var iframeContentBody = document.getElementById('contentBody');
var innerDocContentBody = iframeContentBody.contentWindow.document;

var iframeRawContent = innerDocContentBody.getElementById('rawContent');
var innerDocRawContent = iframeRawContent.contentWindow.document;

console.log(iframeRawContent.messages);

manifest.json

{
  "manifest_version": 2,

  "name": "Read Comments",
  "description": "Read all comments from the current forum",
  "version": "1.0",

  "content_scripts": [{
    "matches": ["*://*.forum.net/*", "*://localhost/*"],
    "js": ["content.js"]
  }],
                                                                                                                                                                                                                   
  "browser_action": {
    "default_title": "Read Comments"
  },
                                                                                                                                                                               
  "permissions": ["activeTab","tabs"]
}