Examine source code of e621 Subscriptions

{
  "manifest_version": 3,
  "name": "e621 Subscriptions",
  "version": "2.3.0",
  "description": "Adds Watched/Subscribe feature to e621.net",
  "browser_specific_settings": {
    "gecko": {
      "id": "{7e0fb8e6-3552-4999-85a8-5c5f4b473d49}"
    }
  },
  "icons": {
    "96": "graphics/e621.png"
  },
  "permissions": [
    "storage"
  ],
  "host_permissions": [
    "*://*.e621.net/*"
  ],
  "action": {
    "default_icon": "graphics/e621.png",
    "default_title": "E621 Watched",
    "default_popup": "popup/popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.e621.net/*"
      ],
      "js": [
        "shared.js",
        "combatfox.js"
      ]
    }
  ]
}