The Mozilla Firefox dev team is looking to add “private browsing” to Firefox version 3.1 by the end of the year.
The goal will be to store as much data from private browsing sessions
as possible in memory to avoid writing to the hard drive. Information
that users save explicitly – bookmarks, for example – will still be
written to the disk.
For users interested in being able to add private browsing functionality to Firefox for now, there is Stealthier.