Examine source code of Auto Volume Leveler

{
  "manifest_version": 2,
  "name": "Auto Volume Leveler",
  "version": "1.0",
  "description": "Automatically levels volume across all tabs.",
  "permissions": [
    "tabs"
  ],
  "icons": {
    "48": "icons/icon48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c7e4f123-9306-4670-acff-e013e2fd1fcb}"
    }
  }
}