By default, Firefox keeps track of your browsing history by storing content and links of visited Web sites, cookies, downloads, passwords, search terms and formula data. Collecting and storing this data makes browsing faster and more convenient. However, when you use a public terminal or a friend's computer, for example, you could turn this behavior off. In Private Browsing mode Firefox will not keep track of your browsing history nor will it cache the content of pages you have visited.
To enable the Private Browsing mode, in the menu bar, click File › New Private Window. The current Web site and all open tabs will be replaced by the Private Browsing information screen. As long as you will browse in private mode, the string (Private Browsing) will be displayed in the titlebar of the window.
Disable Private Browsing by closing the private window.
To make Private Browsing the default mode, open the Privacy tab in the Preference window as described in Section 13.7.1, “Preferences”, set the option Firefox will: to Use custom settings for history and then choose Always use private browsing mode.
Warning: Bookmarks and Downloads
Downloads and bookmarks you made during Private Browsing mode will be kept.