Hmm, are you trying to emulate "start" and "stop" private browsing events using the per-window PB APIs here?  This is unnecessary I think since:

* For global PB builds, the private-browsing-transition-complete notification will continue to be dispatched.
* For per-window PB builds, the start and stop events don't mean anything any more, so add-ons depending on these events will misbehave.