We knew it was coming. After chrome's incognito mode, everyone knew that people like to watch pr0n without worrying about the sites showing up in the awesome/whatever-the-fuck-is-the-name-bar! I have a shortcut (chrome -incognito) just for that reason but now it seems like I can do all that right from Firefox!
It's good but, IMHO, not perfect though. The private browsing mode saves+hides your current tabs and starts a new session in the new mode, and when you switch it off, it loads all those tabs all over again! It's perfect for casual users but for freaks like me who have close to 100 tabs open on an average, it's not the right way to go. I would have preferred it to be done the way chrome handles it - open the private mode in a completely new window and run both the sessions simultaneously depending upon which window the user is browsing on. I hope they tweak it soon or someone makes an extension to run it that way.