Go to [https://bugzilla.mozilla.org/userprefs.cgi?tab=email email preferences]
Watch the user general@spidermonkey.bugs
Or watch one of the JS components in the Core product: JavaScript Engine, JavaScript Engine: JIT, JavaScript: GC, or JavaScript: Standard Library
SpiderMonkey contributors generally hang out in the very active [irc://irc.mozilla.org/jsapi #jsapi IRC channel].