Examine source code of Flatline

{
  "manifest_version": 2,
  "name": "Flatline",
  "version": "1.6",
  "description": "Select apps directly from your favorite software center by clicking Install from Flathub, apps.gnome and AppCenter.",
  "icons": {
    "48": "icons/flatline-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://flathub.org/*",
        "https://apps.gnome.org/*",
        "https://appcenter.elementary.io/*"
      ],
      "js": [
        "main.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e65646b0-2abf-4adb-a748-f7908da651c3}"
    }
  }
}