Examine source code of YTB JP

{
  "manifest_version": 2,
  "name": "Context menu demo",
  "description": "Searches the selected text on Youtube Japanese",
  "version": "1.0",
  "icons": {
    "48": "icons/youjap.png",
    "96": "icons/youjap.png"
  },
  "permissions": [
    "contextMenus"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{403adba5-8111-4b61-9e75-4a26d357dda2}"
    }
  }
}