XDG portal filechooser for Firefox: the KDE implementation uses Dolphin, which is full of features and I use most of them; the default GTK one is mildly infuriating to use and looks ugly too, but getting the browser to use the portal I want was a nightmare - especially since GTK discontinued the GTK_USE_PORTAL envvar.
The related Firefox config entries make no sense either.
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.