Examine source code of Simple Calendar

{
  "manifest_version": 2,
  "name": "Simple Calendar",
  "version": "1.0",
  "description": "A lightweight calendar addon for Firefox.",
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "permissions": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "{3564d776-454e-4a74-9f08-4c02d8762eaa}"
    }
  }
}