Port to SpiderMonkey 60
Mozilla SpiderMonkey 60 is now out. "Pre-release": http://ftp.mozilla.org/pub/spidermonkey/prereleases/60/pre2/ (they still haven't worked out a release process for tarballs, but it really is coming...)
The porting effort required is relatively small. However there also aren't that many exciting JS features. The most exciting for us will probably be the performance improvements introduced in Firefox Quantum (57), and for the future, the improved SpiderMonkey Rust bindings that are now in-tree.