Examine source code of Rhombus - Epoch converter

{
  "description": "Adds an icon on toolbar which allows conversion from epoch timestamp to date",
  "manifest_version": 2,
  "name": "Rhombus",
  "version": "2.2resigned1",
  "icons": {
    "48": "icons/default-icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/default-icon.png",
    "default_title": "Rhombus",
    "default_popup": "popup/rhombus.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{c8a68c13-2f68-4325-b963-9f08afbde63e}"
    }
  }
}