Recently I discovered how to make GTK apps use KDE file dialogs under KDE
plasma. The GTK file dialog is really annoying if one's used to dolphin like
features in KDE plasma file dialogs, it was disturbing me especially using
Firefox. The solution is very simple: Just add "GTK_USE_PORTAL=1" to /etc/
environment:

as root do:
    echo "GTK_USE_PORTAL=1" >> /etc/environment

... or edit the file directly. PAM reads that file upon login and sets the
variable to your environment. This tells GTK apps to use XDG portal. If XDG
portal did not get installed, do

    apt install xdg-desktop-portal-kde

and you're good to go. Log out and back in to take effect. After that Firefox
and all newer GTK apps show KDE file dialogs.

Regards, Adrian.


PS: There must be a way for GNOME users that prefer GTK's dialog over KDE's
for KDE apps since there is a packet xdg-desktop-portal-gtk.