Examine source code of Ponify

{
  "manifest_version": 2,
  "name": "Ponify",
  "version": "1.1",
  "description": "A firefox extension that replaces slack's unread image tag's src with the Pony image.",
  "icons": {
    "48": "icons/48x48.ico",
    "96": "icons/96x96.ico"
  },
  "content_scripts": [
    {
      "matches": [
        "https://app.slack.com/client/*/unreads"
      ],
      "js": [
        "ponify.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "ponify@gmail.com"
    }
  }
}