The feature allows great applications:

For example: Friends at a party in my flat, drinking beer, watching Youtube Videos. In the future they can also make jokes on my internet usage habits!

And: Finally i can control what my girlfriend does on the internet! (Of course i won't tell her about the feature - she wouldn't be interested anyway...)

My suggestion for the next FF-Version: Send about:me data directly to boss. Weekly. Hidden, please.

(The concept of "One Browser == Private Area of One User" was always wrong on desktop-PCs. Why does everyone insist on it?)