My concern with that was that I don't have a corresponding `start` event for `last-pb-context-exited` (see [bug 826782](https://bugzilla.mozilla.org/show_bug.cgi?id=826782)).  So if there is a private hidden window then I'm not certain multiple `start` events won't fire.  This might be ok, but even if it is, there could be a new pb context added at some point in the future which would wreck my start event, and it's hard to write tests for that situation.