One of things I disliked about using Firefox was the lack of “Private Window”. Whenever you wanted a private session it would reuse the current window. This is contrary to how Chrome has always done it.
Now, in Firefix Nightly (version 20 onwards), you get a private window instead of replacing the tabs. This is a neat little feature that lets you keep your current tabs open and browse privately in the new window. Anonymity online just got a little easier!