For some time now, Firefox (and Thunderbird I hope) can use xdg-desktop-portal
APIs for DE integration. We should try to migrate away from kmozillahelper in
favor of using the builtin xdg-desktop-portal integration.

For that, we need to make sure that on Plasma it uses xdg-desktop-portal
instead of GTK file dialogs out of the box. Currently this needs to be done
manually by setting

widget.use-xdg-desktop-portal.file-picker=1
widget.use-xdg-desktop-portal.mime-handler=1

I'm not sure about widget.use-xdg-desktop-portal.open-uri.

Can we get FF and TB configured such that it uses xdg-desktop-portal by default
if it detects a Plasma session?