@barthalion – I am having the identical problem printing from Firefox 79.0/Flatpak 1.0 as Rich, and accumulating …cache/tmp/gtkprint* files just as he described. Attempting to Print To File (PDF) also defaults to saving the PDF in the …cache/tmp/ folder as well (but that, at least, can be changed to, say, ~/Downloads.) I require the Firefox 79.0 flatpak because version 68.11 currently available in the Debian repos does not support Zoom for job interviews. (I am a COVID-19 casualty.)
Printing seems to proceed normally, except the printer never sees the jobs (“No active jobs.”) Removing the printer and re-detecting and adding it back to the devices list has no effect.
In answer to your question, yes, I have the following xdg support modules installed.
~$ sudo apt list xdg-* | grep installed
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
xdg-dbus-proxy/stable,now 0.1.1-1 amd64 [installed,automatic]
xdg-desktop-portal-gtk/stable,now 1.2.0-1 amd64 [installed,automatic]
xdg-desktop-portal/stable,now 1.2.0-1 amd64 [installed,automatic]
xdg-user-dirs-gtk/stable,now 0.10-3 amd64 [installed]
xdg-user-dirs/stable,now 0.17-2 amd64 [installed,automatic]
xdg-utils/stable,now 1.1.3-1+deb10u1 all [installed,automatic]
If you can provide instructions for getting diagnostic information, I will be happy to comply.
:~$ lsb_release -ds ; uname -srm ; gnome-shell --version ; echo $XDG_SESSION_TYPE
Debian GNU/Linux 10 (buster)
Linux 4.19.0-10-amd64 x86_64
GNOME Shell 3.30.2