You can already switch on private browsing in Firefox with Stealther, but in the wake of IE8’s InPrivate Browsing and Google Chrome’s Incognito mode, Mozilla is set to incorporate their own private browsing mode with Firefox 3.1.
In private mode, all user activity involving data such as searches, visited web pages, downloads, cached pages, credentials passed to restricted sites, etc. is discarded once the user returns to normal mode. Despite being infamously well known as “porn mode” there are many other valid use cases. As stated in Mozilla Wiki:
For instance, users may wish to begin a private browsing session to research a medical condition, or plan a surprise vacation or birthday party for a loved one. Use cases will range from users cheating on their spouse, to users buying engagement rings. Given the breadth of our user base, specific use cases are likely to be extremely varied.
With Chrome’s InCognito and Internet Explorer 8 Beta 2’s InPrivate, introduced in the last couple of weeks, lacking a private mode option by targeted year’s end release, would put Firefox in one obvious disadvantage.