I've set Firefox to default private browsing. Of course, now that the default window state is private the purple mask icon doesn't show. I'm sure someone decided to do this for a reason but it seams like a serious flaw to me. While trying to find out how to get Firefox to load private windows as default I've seen more than one thread where this default private mode was set without the users input (maybe during an update or something) or maybe the user mistakenly changed the default. In either case I think it is very dangerous to have no indication that private browsing is activated as then the user will continue browsing under the assumption that it is in a case where the default reverts back to normal browsing without their knowledge.

You may have noticed that this is more of a statement than a question and if anyone knows of a solution that would be great. If there is no solution I'd be happy to post a bug report