As announced some time ago, the latest Firefox 3.1 nightlies (Shiretoko) now feature a Private Browsing mode, to enter Private Browsing, select Private Browsing in the Tools menu. Firefox will ask for confirmation which you can disable for future uses. Once confirmed, Firefox will save and close your current session and restart in Private Browsing mode. The only hint of this are the words (Private Browsing) displayed next to the window title. Once you’ve finished, just click back on Private Browsing in the Tools menu. Your session will be restored as it was before entering private mode immediately or, if you exit Firefox (or crash), the next time you start it.