I'm not sure about Firefox's implementation of it, but throughout computing I'm seeing more and more need for this kind of thing, not to mention something softe

I'm not sure about Firefox's implementation of it, but throughout computing I'm seeing more and more need for this kind of thing, not to mention something softer than full user-account switching for handing a device between family members or teammates. As everything gets more personalized and more tightly bound to the user by learning their habits and typing and voice and all that, and simultaneously in a social networking context we broadcast stuff about ourselves incidentally (like Youtube learning your viewing preferences and likes) that the ability to switch context neatly and quickly is becoming more important.