Examine source code of TwitchPreview

{
  "manifest_version": 2,
  "name": "TwitchPreview",
  "version": "2.0.0",
  "description": "Shows a preview when hovering over a twitch livestream.",
  "icons": {
    "32": "icons/icon32.png",
    "64": "icons/icon64.png",
    "128": "icons/icon128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.twitch.tv/*"
      ],
      "run_at": "document_end",
      "js": [
        "inject.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "bundle.js"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{4fe7bf2a-8b37-432c-849a-d27d3f8bfb0e}"
    }
  }
}