Examine source code of FF Highlighter

{
  "manifest_version": 3,
  "name": "FF Highlighter",
  "version": "0.1.0",
  "description": "Highlights other instances of selected text when the selection is at least three characters.",
  "browser_specific_settings": {
    "gecko": {
      "id": "ff-highlighter@example.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "background.js"
      ],
      "css": [
        "highlight.css"
      ]
    }
  ]
}