Examine source code of Detect Language

{
  "manifest_version": 2,
  "name": "Detect Language",
  "version": "1.0",
  "description": "Detect the language of selected text on a page.",
  "permissions": [
    "activeTab",
    "contextMenus"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Detect Language",
    "default_icon": {
      "128": "icon-128.png"
    }
  },
  "icons": {
    "128": "icon-128.png"
  },
  "applications": {
    "gecko": {
      "id": "{303768fe-a7ee-47da-9d27-70aae3f3a354}",
      "strict_min_version": "109.0"
    }
  }
}