Examine source code of StereoToMono

{
  "manifest_version": 2,
  "name": "StereoToMono",
  "description": "Mixes stereo audio together to create mono output in both channels.",
  "version": "1.2",
  "author": "no9org@gmail.com",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "stereotomono.js"
      ],
      "run_at": "document_end"
    }
  ],
  "permissions": [
    "activeTab",
    "contextMenus",
    "storage"
  ],
  "browser_action": {
    "default_icon": "images/icon19.png"
  },
  "icons": {
    "16": "images/icon16.png",
    "32": "images/icon32.png",
    "48": "images/icon48.png"
  },
  "applications": {
    "gecko": {
      "id": "{2d34c8eb-3be3-46de-9a8a-3e99e95feeb6}",
      "strict_min_version": "42.0"
    }
  }
}