Firefox 64 can use native KDE open/save dialogs!!!
To support this, two changes need to be made to the packaging:
- Install the xdg-desktop-portal and xdg-desktop-portal-kde packages as dependencies
- Set GTK_USE_PORTAL=1 in Firefox's desktop file, by making the Exec= line read Exec=GTK_USE_PORTAL=1 /usr/lib/firefox/firefox --new-window %u.
It can't be set in the global environment or else Thunderbird will crash on launch.
I can confirm that this works for me! I now have native KDE file dialogs in Firefox 64.