Firefox's file chooser window does not open in non-GNOME desktops
In desktops that not GNOME — tested in Budgie, Icewm and others —, Firefox is incapable of spawning the file chooser — e.g. by clicking Ctrl+S or by trying to add an attachment to this very message.
Uninstalling xdg-desktop-portal-gnome and restarting the session solves the issue.
% apt list --installed|grep xdg-desktop-portal
xdg-desktop-portal-gnome/mantic,now 44.1-1 amd64 [installed]
xdg-desktop-portal-gtk/mantic,now 1.14.1-1 amd64 [installed]
xdg-desktop-portal/mantic,now 1.16.0-2ubuntu1 amd64 [installed]
% lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu Mantic Minotaur (development branch)
Release: 23.10
Codename: mantic