About this Add-on
Note: at the moment it is not possible to write this Addon in webextensions for Firefox 57+ because the new API does not support to show this Clock in the Toolbar, I hope that it will be possible later on.
when you need a Clock you must use a Icon in the Toolbar that open a Popup, as well as this Addon
https://addons.mozilla.org/de/firefox/addon/date-today/