I think it will be useful to explain a little what's diff between previous private browsing mode and per window. Probably tell that previously whole firefox was in private browsing mode or not. While now specific windows for private browsing can be created. That's also why start and end events will never be emitted neither attribute value will be `false`, since firefox will no longer go into private borwiseing mode.

I'll leave proper phrasing of this to you & @wbamberg and would encourage to do this in separate pull request, no need to block this.