Examine source code of Volume by Mouse Position

{
  "manifest_version": 2,
  "name": "Volume by Mouse Position",
  "version": "1.0",
  "description": "Adjust the volume of playing media by moving the mouse vertically while holding Alt.",
  "permissions": [
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{9a8d5830-02ea-448f-bf33-0d1a10d15da9}"
    }
  }
}