Examine source code of Twitter Cards Injector

{
  "manifest_version": 2,
  "name": "Twitter Cards Injector",
  "version": "1.0",
  "description": "Injects Twitter Cards metadata into the webpage for better sharing on Twitter.",
  "permissions": [
    "tabs",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "twitter_cards_injector_extension@gmail.com"
    }
  }
}