Examine source code of MondlyPress

{
  "manifest_version": 2,
  "name": "MondlyPress",
  "version": "1.21",
  "description": "Use keyboard in Mondly.",
  "icons": {
    "16": "icon-16.png",
    "32": "icon-32.png",
    "48": "icon-48.png",
    "64": "icon-64.png",
    "128": "icon-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://app.mondly.com/*"
      ],
      "js": [
        "mondlypress.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{04ced123-2a7a-4139-a04d-21d0b05ecae2}"
    }
  }
}