Examine source code of MessPostage

{
  "version": "0.0.8resigned1",
  "manifest_version": 2,
  "name": "MessPostage",
  "description": "Notify user of postMessage messages and handlers",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "all_frames": true,
      "run_at": "document_start"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "browser_style": true,
    "default_title": "No messages detected",
    "default_popup": "popup.html",
    "default_icon": {
      "48": "no_message_48.png"
    }
  },
  "icons": {
    "48": "no_message_48.png"
  },
  "permissions": [
    "webNavigation"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{cd982b7a-ece2-4476-a2bc-5b0d085ccbcd}"
    }
  }
}