This issue came up not long ago in one of the Hyprland threads. My hunch is that the OpenSUSE patches built in to Firedragon (for enhanced KDE integration) are somehow responsible for this issue
I found this clue in the ArchWiki:
To use the KDE file selection and print dialogs in Firefox 64 or newer, install xdg-desktop-portal and xdg-desktop-portal-kde, then set widget.use-xdg-desktop-portal.mime-handler to 1 in about:config. You should also change widget.use-xdg-desktop-portal.file-picker from 2 to 1.
I compared the value of widget.use-xdg-desktop-portal.file-picker in Firedragon and Librewolf, and sure enough it’s 1 on Firedragon, but 2 on Librewolf. Unfortunately, this is sort of a “baked-in” change–it can’t be set back to 2 on Firedragon because the value is greyed-out and “locked”.
Firedragon on the left, Librewolf on the right:
Some part of applying the OpenSUSE patches appears to permanently change this setting. I’m afraid the simplest solution may be to simply use Librewolf instead.