Thanks! I had to apply one change as the keepassxc server socket has a different name now:
$XDG_RUNTIME_DIR/app/org.keepassxc.KeePassXC/org.keepassxc.KeePassXC.BrowserServer
Therefore my ~/.local/share/user-tmpfiles.d/kpxc_server.conf now goes
L %t/kpxc_server - - - - %t/app/org.keepassxc.KeePassXC/org.keepassxc.KeePassXC.BrowserServer
Also, there is no need to reboot, just run
systemd-tmpfiles --user --create
P.S.: After a few tests, I have to say that this doesn’t work reliably. Keepassxc and firefox seem to be connected, but in most instances, keepassxc-browser says “no entries found”.