It was informally called that from the very beginning; see e.g. https://www.theregister.com/2008/11/05/firefox_market_share_private_browsing/ https://www.thereg

It was informally called that from the very beginning; see e.g. https://www.theregister.com/2008/11/05/firefox_market_share_private_browsing/ https://www.theregister.com/2008/11/05/firefox_market_share_... (I think the usual term among the people developing the feature was actually "pr0n mode", but I may be wrong and/or misremembering. For the avoidance of doubt, I was not one of those people, I'm just describing what I think I remember seeing on the interwebs at the time.)