Examine source code of selection allow

{
  "manifest_version": 2,
  "name": "User-Select to Auto",
  "version": "1.0",
  "description": "Changes user-select to auto on all websites",
  "permissions": [],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "selection.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{6d0b913e-34b7-4682-8097-e1a05e7ad97f}"
    }
  }
}