Examine source code of Editable Countdown

{
  "manifest_version": 2,
  "name": "Tab Countdown Timer",
  "version": "3.0",
  "description": "Shows a countdown timer in the toolbar.",
  "permissions": [
    "storage"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon128.png"
  },
  "icons": {
    "128": "icon128.png"
  },
  "options_ui": {
    "page": "options.html",
    "open_in_tab": true
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "tab-countdowner@example.com"
    }
  }
}