How to make Firefox use native notification (notify-osd)?

Install the GNotifier addon to get firefox notifications to integrate with the current desktop you are using, either unity, gnome-shell, kde or xfce.

https://addons.mozilla.org/en-US/firefox/addon/gnotifier/

UPDATE: 08-01-2020

As the GNotifier extension is dead, you can use the Firefox Gnome Download Notifications. Add the extension from here.

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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)

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