Firefox 64 can use native KDE open/save dialogs!!!
To support this, two changes need to be made to the packaging:
1. Install the `xdg-desktop-portal` and `xdg-desktop-portal-kde` packages as dependencies
2. Set `GTK_USE_PORTAL=1` somewhere in the environment. One example is to do it in the desktop file, by making the `Exec=` read `Exec=/usr/lib/firefox/firefox --new-window %u`.
I can confirm that this works for me! I now have native KDE file dialogs in Firefox 64.