I got some help from the #jsapi IRC channel on this. Thanks to all
involved there.
I figured out the basics of embedding Spidermonkey ESR60 (i.e. the
version in Firefox 60), including adding a resolveHook for ESR6 module
resolution.
https://github.com/Chaircrusher/SpiderMonkeyES6Example.git
The module resolve hook is blindingly stupid; you'll want to add
something more sophisticated for actual use.