Examine source code of Screen Width Indicator

{
  "manifest_version": 2,
  "name": "Screen Width Indicator",
  "version": "1.1",
  "permissions": [
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{9d193077-247a-4a93-8e6e-77ec871ff5ab}"
    }
  }
}