It used to be relatively easy to override the filechooser by forcing portal mode, but Gnome actually disabled that because people aren't supposed to use it.
But while they did remove that option, they didn't implement an option to do it properly. So now even though I use KDE, Firefox drops me in the shitty Gnome filechooser which is entirely useless.
(Luckily firefox ended up making that an option on their side instead: widget.use-xdg-desktop-portal.file-picker)
(See https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4829 and related discussions)
But while they did remove that option, they didn't implement an option to do it properly. So now even though I use KDE, Firefox drops me in the shitty Gnome filechooser which is entirely useless.
(Luckily firefox ended up making that an option on their side instead: widget.use-xdg-desktop-portal.file-picker)