Err, your last sentence is a bit weird. First of all, 3.6 already has TraceMonkey; what's new in Fx 4 is JägerMonkey, and the integration between it and TM. Second, the Mozilla JS engine is largely developed separately (all of the actual development these days is in a separate repository, which merges semi-regularly with the main mozilla-central repo), but have you seen any other browser backporting anything other than security fixes?