Examine source code of Visited Link Marker

{
  "manifest_version": 2,
  "name": "Visited Link Marker",
  "version": "1.2.4",
  "description": "Keep your own record of the links you've followed. Any link you click is remembered and shown struck through and faded the next time you encounter it on any page, making revisits obvious.",
  "icons": {
    "32": "assets/icon-32.png",
    "48": "assets/icon-48.png",
    "64": "assets/icon-64.png",
    "96": "assets/icon-96.png",
    "128": "assets/icon-128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{91ac3e4f-1874-409d-b01f-aeb2409a23b8}",
      "strict_min_version": "101.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "permissions": [
    "storage"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "app/content.js"
      ],
      "run_at": "document_idle",
      "all_frames": false
    }
  ],
  "browser_action": {
    "default_title": "Visited Link Marker",
    "default_popup": "app/popup.html",
    "default_icon": {
      "32": "assets/icon-32.png",
      "48": "assets/icon-48.png",
      "64": "assets/icon-64.png"
    }
  }
}