One of the more popular new features in Firefox 3.1 is the Private Browsing feature. Private Browsing means the sites you visit (while in Private Browsing mode) will not appear in the browser history or search history, and won’t leave other traces, like cookies. This is already a feature (Incognito) with Google Chrome browser, however unlike Chrome, Firefox’s Private Browsing works a little lot (at least when it comes to activating).
“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.
So, yes, there is no way to have normal and private mode windows or tabs at the same time. As Ehsan Akhgari (who’s worked on the implementation for the last ten months) explains, it would require deeper architectural changes to Firefox that may or may not occur in a future release.”
At least with Chrome activating Incognito via CTRL+SHIFT+N opens a new Window all in that Window are Incognito, whilst you regular browsing is still sitting in the background. Not sure how private browsing would work in the future IE8.
Private Mode is now in the nightly Firefox 3.1 releases and can be obtained (at your own risk) via the Mozilla FTP site.