After an Arch Linux update on my T430, the file picker in Firefox stopped working. The messages shown by Firefox in the terminal were vague, so I wasn't sure what exactly went wrong and it took me a while to figure this out.
I looked up some articles and forum posts and it looked like xdg-desktop-portal might be the issue. The command:
systemctl --user status xdg-desktop-portal
showed me that xdg-desktop-portal was indeed failing to start for some reason.
I also checked xdg-desktop-portal-gtk:
systemctl --user status xdg-desktop-portal-gtk
which complained about being unable to find a display. My first thought was to add some configs to set the DISPLAY environment variable, but that didn't help.
Finally, I found this thread on the Arch Linux forums: https://bbs.archlinux.org/viewtopic.php?id=308200, where the post by thrtx helped me finally fix the file picker.
That post contains a script to add to .xinitrc which runs all the scripts in /etc/X11/xinit/xinitrc.d when starting X. The particular script needed by xdg-desktop-portal in that directory is named "50-systemd-user.sh".
The thread was about X, but I use Wayland. I added a command to run 50-systemd-user.sh to my window manager startup script (~/.config/river/init for river) and it fixed the file picker!
I hope this will help anyone who had a similar issue after updating their Arch Linux system.