Today in it's not #AlwaysDNS: it was some combination of #systemd and the incomprehensibly complicated dbus services underlying the Linux desktop architecture (from the people who brought you systemd, naturally).
But at least I found the answer: block yet another systemd --user service by creating a symlink to /dev/null in /etc/systemd/user, and now unsnapped #Firefox no longer stalls for 30 seconds every time it starts on #Ubuntu 22.04.