Examine source code of Simple Volume Override

{
  "manifest_version": 2,
  "name": "Simple Volume Override",
  "version": "1.0",
  "description": "Set video volume per site",
  "permissions": [
    "storage",
    "activeTab",
    "<all_urls>"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "icons": {
    "128": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e68c4c82-bb71-4a63-b2ca-806165afdbc5}"
    }
  }
}