Examine source code of CF GetRating

{
  "name": "CF GetRating",
  "description": "Get rating for Codeforces problems even with problem tags off.",
  "version": "1.0.4",
  "manifest_version": 2,
  "icons": {
    "16": "images/icon_16.png",
    "32": "images/icon_32.png",
    "48": "images/icon_48.png",
    "128": "images/icon_128.png"
  },
  "browser_action": {
    "default_icon": "images/icon.png",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "*://codeforces.com/problemset/problem/*/*",
        "*://codeforces.com/contest/*/problem/*"
      ],
      "js": [
        "background.js"
      ],
      "css": [
        "background.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e683b888-616b-40fa-b425-a47dc6058a52}"
    }
  }
}