I've seen this:
As you can see Firefox is using the Ubuntu notification to notify the user.
How to do that?
I just want to get notifications for anything in general: for example, if a download is completed...
UPDATE: 08-01-2020
As the GNotifier extension is dead, you can use the Firefox Gnome Download Notifications.
Add the extension from here.
Ok found it:
GNotifier is a Firefox extension that makes the browser use native notifications on the Linux desktop. The extension currently supports GNOME (Shell), Unity, KDE and Xfce.
Besides regular notifications, like the one displayed when a file is downloaded, the extension also supports the standard webkitNotification API (for instance, receiving notifications on new emails in Gmail)
