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 the global environment (this used to make Thunderbird will crash on launch, but this got fixed in Thunderbird 68: https://bugzilla.mozilla.org/show_bug.cgi?id=1514605) or Firefox's desktop file, by making the Exec= line read Exec=GTK_USE_PORTAL=1 /usr/lib/firefox/firefox --new-window %u, or.
I can confirm that this works for me! I now have native KDE file dialogs in Firefox 64.