Examine source code of Language Immersion

{
  "manifest_version": 2,
  "name": "Language Immersion",
  "version": "1.0",
  "description": "Replaces some words on webpages with your target language vocabulary.",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "icons": {
    "48": "icons/icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icons/icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{8e6907fc-f191-41e7-b2be-83471ccb5aec}"
    }
  }
}