Examine source code of Pixel Ruler

{
  "manifest_version": 2,
  "name": "Pixel Ruler",
  "version": "1.0",
  "description": "Draw a pixel ruler overlay on the page.",
  "icons": {
    "64": "icon64.png",
    "128": "icon128.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Pixel Ruler",
    "default_icon": "icon64.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ed786b92-7b96-4d59-b04d-3e7396d36ae1}",
      "strict_min_version": "111.0"
    }
  }
}