Examine source code of Timer and addTask

{
  "manifest_version": 2,
  "name": "Timer",
  "version": "1.0",
  "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#beastify",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/beastify",
  "icons": {
    "48": "icons/icon.png"
  },
  "browser_action": {
    "default_icon": "icons/icon.png",
    "default_title": "timer",
    "default_popup": "popup/timerify.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{cc968705-e005-4fb4-a984-e36dbc2e318b}"
    }
  }
}