You can enable XDG portals by setting `widget.use-xdg-desktop-portal` to `true` in `about:config` inside PWA profile (you can open `about:config` by pressing F6 and then typing it into the popup).

It seems Firefox by default never uses XDG portals, but distros with KDE instead set some environment variable (`GTK_USE_PORTAL=1`) that makes Firefox use portals. However, because FirefoxPWA launches the default Mozilla-provided Firefox without setting any additional environment variables, Firefox won't use portals unless you manually enable them in `about:config`.