Examine source code of Otto Numbers for Firefox

{
  "manifest_version": 2,
  "name": "Otto Numbers for Firefox",
  "version": "1.0.1",
  "description": "Adds ranking indicators next to each streamer's name on Twitch to display their revenue standings over the last two years. A Firefox port of Ottomated's original Otto Numbers extension",
  "content_scripts": [
    {
      "matches": [
        "https://*.twitch.tv/*"
      ],
      "css": [
        "styles.css"
      ],
      "js": [
        "script.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{0600b03e-03a8-461f-bbfe-a0e32ca75c31}"
    }
  }
}