Examine source code of ResponsiveHelper

{
  "manifest_version": 2,
  "name": "ResponsiveHelper",
  "version": "1.0.4resigned1",
  "description": "A Firefox add-on to show the screen width and height when resizing the window",
  "icons": {
    "48": "icons/rh-icon-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/"
      ],
      "js": [
        "responsive-helper.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c2faf9a0-1a3d-429b-a67a-08d298486bfe}"
    }
  }
}