Examine source code of Today is

{
  "manifest_version": 2,
  "name": "Today is",
  "version": "0.2.0",
  "description": "A Firefox addon that displays 3 common Gregorian, Hijri and Jalaali calendars.",
  "icons": {
    "48": "icons/calendar-48.png"
  },
  "permissions": [
    "storage"
  ],
  "browser_action": {
    "default_icon": "icons/calendar-32.png",
    "default_title": "Today is",
    "default_popup": "popup/popup.html"
  },
  "options_ui": {
    "page": "settings/options.html"
  },
  "background": {
    "page": "background/options-handler.html"
  },
  "applications": {
    "gecko": {
      "id": "{2a76e59f-4bcd-4a19-b4a8-e13dcba1747a}",
      "strict_min_version": "57.0"
    }
  }
}