I have the same problem after upgrading. Firefox screenshots save without issue to the "Downloads" directory (probably because they save directly to the specified directory and never prompt the user) and Thunar can be launched from the "Downloads panel" to see the downloaded file. I can also open files in Firefox from Thunar without issue.
The issue seems to be limited to the save/open file dialog.
journalctl reports the following when I try to save something:
systemd[709]: Starting xdg-desktop-portal-gtk.service - Portal service (GTK/GNOME implementation)...
xdg-desktop-por[8136]: cannot open display:
systemd[709]: xdg-desktop-portal-gtk.service: Main process exited, code=exited, status=1/FAILURE
systemd[709]: xdg-desktop-portal-gtk.service: Failed with result 'exit-code'.
systemd[709]: Failed to start xdg-desktop-portal-gtk.service - Portal service (GTK/GNOME implementation).
xdg-desktop-por[1137]: Backend call failed: Could not activate remote peer: startup job failed.