With Google Chrome and IE 8 having Private Browsing Mode, Mozilla is feeling the heat and will include a private browsing mode in Firefox 3.1- the next release of Firefox.

In private mode all user activity, cookies, searches etc. are not saved to the disk and discarded once user leaves the private mode.

Besides using this private mode for nsfw sites, Mozilla states one of the valid uses of the private mode:

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.

Besides Private Browsing, Firefox 3.1 will include improved Awesome Bar, enhanced performance, better add-on manager and much more.

[via]