I had always thought that Chrome's method of private browsing, which allows you to intermingle private and normal browsing in separate windows/tabs, was better than Safari or Firefox, which are an all-or-nothing proposition. Now Firefox, at least, can emulate Chrome's behavior.
With the Firefox add-on Private Browsing Window, the user can have private and normal browsing windows simultaneously. This can be used for things other than simply secure browsing, too. You could, for instance, log into two Gmail accounts simultaneously, or test a web app using two different accounts in one browser.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100205041324400