KDE: How to make firefox to open kde's file manager (Dolphin) when saving or open file in KDE

 Make sure your Mozilla Firefox is version > 78. Mine is version 93 as of 12 Oct 2021.

1. Open firefox and type in the address bar : about:config

2. Ignore the warning by clicking "Accept the risk and Continue" button

3. type in the search bar: widget.use-xdg-desktop-portal and change the value to true

4. Restart the browser

 Have fun!