Comment # 1 on bug 1226112 from Frank Krüger

(In reply to Fabian Vogt from comment #0)
> 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?

JFYI: Removing kmozillahelper solved bug #1215852.