Examine source code of Fixed Ruler Mode

{
  "manifest_version": 3,
  "name": "__MSG_extensionName__",
  "description": "__MSG_extensionDescription__",
  "version": "1.0",
  "icons": {
    "128": "icons/icon-128.png"
  },
  "action": {
    "default_popup": "popup.html",
    "default_icon": "icons/icon-128.png"
  },
  "permissions": [
    "storage",
    "activeTab",
    "scripting"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{b31a3a1c-6e53-4b2a-9a9e-57a2fd37c59e}"
    }
  }
}