mozjs38 is in Ubuntu because it was needed (briefly) for GNOME 3.24.
It's still in Ubuntu because cjs hasn't been ported to mozjs52 yet.
The packaging is at
https://anonscm.debian.org/git/pkg-gnome/mozjs38.git
Could 0ad use a distro mozjs38 if it were packaged in Debian?
Ideally, 0ad would switch to mozjs52 before Debian 10 "Buster" is released.
Thanks,
Jeremy Bicha