I think GNOME’s filechooser is the GTK one (never used it so I’m not sure), mine looks like this:
It’s entirely possible that Firefox changed and now uses XDG portals by default, I configured it like this a long time ago.
As for how to configure it, I honestly don’t know.
It was a combination of messing with widget.use-xdg-desktop-portal
on about:config, and changing XDG envvars and dotfiles; both by following several conflicting Reddit and bbs.archlinux.org posts.
That is an extremely oddly specific cysec issue they’re choosing to target…