Examine source code of Screenshot.rocks

{
  "name": "Screenshot.rocks",
  "version": "1.1",
  "description": "Create beautiful browser and mobile mockups in one-click.",
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_icon": "button.png",
    "default_title": "Capture a screenshot"
  },
  "permissions": [
    "activeTab"
  ],
  "manifest_version": 2,
  "icons": {
    "16": "16.png",
    "48": "48.png",
    "128": "128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{7d134911-f49d-478c-b219-7072c64b8307}"
    }
  }
}