This issue has come up in a few of our other Wayland-based spins, such as Sway and Hyprland. I believe the crux of the issue is Firedragon has a hardcoded value of widget.use-xdg-desktop-portal.mime-handler=1, see here:
If you install xdg-desktop-portal-kde and reboot it should work.
If you don’t want to pull in a bunch of KDE stuff, you can try using the Librewolf browser which is very similar feature-wise to Firedragon.