Examine source code of OpLeagueOPGG

{
  "manifest_version": 2,
  "name": "OpLeagueOPGG",
  "version": "1.0",
  "description": "Adds a OP.gg link to OpLeague team-preview.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://www.opleague.pro/match/*",
        "https://www.opleague.pro/team/*"
      ],
      "js": [
        "OpLeagueOPGG.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2f2a8a69-e5b3-4381-9c71-8041804d60cc}"
    }
  }
}