Examine source code of YouTubeRSS

{
  "description": "Adds browser action icon to toolbar for getting rss feed url from channel url.",
  "manifest_version": 2,
  "name": "YouTubeRSS",
  "version": "2.1",
  "homepage_url": "https://github.com/rredford/YouTubeRSS",
  "icons": {
    "48": "icons/page-disabled-48.png"
  },
  "background": {
    "scripts": [
      "youtuberss.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "http://*.youtube.com/*",
        "https://*.youtube.com/*"
      ],
      "js": [
        "channelId.js"
      ]
    }
  ],
  "browser_action": {
    "default_icon": "icons/page-disabled-32.png"
  },
  "permissions": [
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{a8eca5f0-0e71-4892-abe1-6f310e35388a}"
    }
  }
}