Examine source code of Pixel Ruler Overlay

{
  "manifest_version": 2,
  "name": "Pixel Ruler Overlay",
  "version": "1.1.2",
  "description": "Inject a measurement ruler overlay onto any webpage",
  "browser_specific_settings": {
    "gecko": {
      "id": "{fe111e04-1119-41b9-a5d3-17adf0fe13bb}",
      "strict_min_version": "96.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "icons": {
    "16": "img/i16.png",
    "32": "img/i32.png",
    "48": "img/i48.png",
    "96": "img/i96.png",
    "128": "img/i128.png"
  },
  "browser_action": {
    "default_popup": "dialog/dialog.html",
    "default_icon": {
      "16": "img/i16.png",
      "32": "img/i32.png",
      "48": "img/i48.png",
      "96": "img/i96.png",
      "128": "img/i128.png"
    }
  },
  "background": {
    "scripts": [
      "ext/service.js"
    ]
  },
  "permissions": [
    "tabs",
    "activeTab"
  ]
}