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.
My only use for Reddit now is reading r/hfy stories, though I’m doing that through a local Redlib instance, hoping that those glitter sniffers don’t lock website content behind accounts like almost every other social medium does (but we all know that if something can get worse to make more money, it WILL get worse to make more money).