Sep 22 06:50:39 lubuntu dbus-daemon[1698]: [session uid=1000 pid=1698] Activating via systemd: service name='org.freedesktop.portal.Desktop' unit='xdg-desktop-portal.service' requested by ':1.170' (uid=1000 pid=7686 comm="/snap/firefox/3145/usr/lib/firefox/firefox" label="snap.firefox.firefox (enforce)")
Sep 22 06:50:39 lubuntu systemd[1670]: Starting xdg-desktop-portal.service - Portal service...
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/home/lubuntu/.config/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/home/lubuntu/.config/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg/xdg-Lubuntu/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg/xdg-Lubuntu/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/etc/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/home/lubuntu/.local/share/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/home/lubuntu/.local/share/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/Lubuntu/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/Lubuntu/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/local/share/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/local/share/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/var/lib/snapd/desktop/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/var/lib/snapd/desktop/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/xdg-desktop-portal/lubuntu-portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: Looking for portals configuration in '/usr/share/xdg-desktop-portal/portals.conf'
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: load portals from /usr/share/xdg-desktop-portal/portals
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: loading /usr/share/xdg-desktop-portal/portals/gnome-keyring.portal
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Secret
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: loading /usr/share/xdg-desktop-portal/portals/kde.portal
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Access
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Account
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.AppChooser
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Background
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Email
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.FileChooser
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Inhibit
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Notification
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Print
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.ScreenCast
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Screenshot
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.RemoteDesktop
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.Settings
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.DynamicLauncher
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.GlobalShortcuts
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: loading /usr/share/xdg-desktop-portal/portals/lxqt.portal
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: portal implementation supports org.freedesktop.impl.portal.FileChooser
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.MemoryMonitor
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.PowerProfileMonitor
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.NetworkMonitor
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.ProxyResolver
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.Trash
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.GameMode
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: providing portal org.freedesktop.portal.Realtime
Sep 22 06:50:39 lubuntu xdg-desktop-por[8799]: No skeleton to export
Sep 22 06:50:39 lubuntu dbus-daemon[1698]: [session uid=1000 pid=1698] Successfully activated service 'org.freedesktop.portal.Desktop'
Sep 22 06:50:39 lubuntu systemd[1670]: Started xdg-desktop-portal.service - Portal service.
Sep 22 06:50:39 lubuntu xdg-desktop-portal[8799]: XDP: org.freedesktop.portal.Desktop acquired

Extracted from a test VM by guiverc after fighting with the systemd unit file for xdg-desktop-portal to get it to go into verbose mode.

Some observations:

1. Firefox is trying to call org.freedesktop.portal.FileChooser. The provided "method" in xdg (is that what you call it, a method?) is org.freedesktop.impl.portal.FileChooser. Dunno if the .impl missing in one of those means anything.

2. Running "/usr/libexec/xdg-desktop-portal --verbose --replace" in a terminal fixes the problem while that command is left running. The problem immediately resurfaces when the process is terminated.

3. XDG_CURRENT_DESKTOP appears to be set correctly, all files appear to be where they should, and you can even see xdg-desktop-portal loading the right file, so I think smcv's hunch is right about LXQt not copying things into the "activation environment" correctly.