Examine source code of Measure-it

{
  "manifest_version": 2,
  "name": "Measure-it",
  "description": "Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.",
  "version": "2.1resigned1",
  "icons": {
    "32": "icons/icon_32.png",
    "48": "icons/icon_48.png",
    "64": "icons/icon_64.png",
    "128": "icons/icon_128.png"
  },
  "browser_action": {
    "default_icon": "icons/icon_32.png",
    "default_title": "Measure-it"
  },
  "background": {
    "scripts": [
      "bg.js"
    ]
  },
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{edd380c6-9404-44aa-b704-dffa435246ee}"
    }
  }
}