Use kdialog for file selection on firefox

Requirements

• firefox v64 or later
• plasma, version 5.15 or later
• xdg-desktop-portal and xdg-desktop-portal-kde packages installed

Steps

First set plasma's dolphin as default file manager.

Then just passing GTK_USE_PORTAL=1 environment variable when opening firefox. So in terminal, just type

$ GTK_USE_PORTAL=1 firefox

or by editing the firefox.desktop file, e.g in plasma:

1. open application launching and find firefox
2. right click and click edit application
   
3. in application tab add GTK_USE_PORTAL=1 at first on Command selection
   
4. save change and reopen firefox.

Reference

• Firefox v64 can now use the KDE file selection dialog when run with GTK_USE_PORTAL=1