Examine source code of Dnevnik.ru average mark

{
  "manifest_version": 2,
  "name": "Dnevnik.ru average mark",
  "version": "1.0.2",
  "description": "Adds a new cell into marks table at schools.dnevnik.ru/marks.aspx - \"Average mark\"",
  "icons": {
    "48": "icons/48.png"
  },
  "applications": {
    "gecko": {
      "id": "{F9D5d5D7-b04a-B809-beF1-c4E9BF53B949}"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://schools.dnevnik.ru/marks.aspx*"
      ],
      "js": [
        "app.js"
      ]
    }
  ]
}