NativeInterface2JSObject needs to return a special proxy rather than an XPCWrappedNative for the outer window.
Moving to beta6+, worrying (again) about mrbkap's workload ...
We have a large consolidated patch queue for this. This particular bug is already fixed in there. I will triage the compartment bugs today and post a new time estimate.
This is done in Blake's queue and working.
Please note that we have now created a branch for beta 7 work. In addition to landing your fix on mozilla-central default, please also land it on mozilla-central GECKO20b7pre_20101006_RELBRANCH (note: when landing on mozilla-central default, you will be given priority in the landing queue at https://wiki.mozilla.org/LandingQueue )