In 2008 the Firefox browser added a new feature called "private browsing," which enables you to surf the Web without leaving behind information on where you've been. Although private browsing doesn't create a cache on your computer's hard disk, your browsing history may be available either from your own computer or from routers and servers along your Internet path.
Firefox 3.1 and later offer a "Start Private Browsing" option under the "Tools" menu. When you choose this option, Firefox closes any open tabs and starts a new private browsing session for you. During private browsing, Firefox doesn't track your history, save usernames or passwords, or write cached files or cookies to disk. This means that when you're done with a private browsing session, Firefox itself will contain no trace of your behavior, with only a couple of exceptions.
While Firefox doesn't record your path while you browse privately and erases any files downloaded to your system's temporary folder, any files you download to other locations on your computer will remain, as will any bookmarks you make. In addition, older versions of Adobe Flash will retain any Local Shared Objects, also known as Flash cookies, that you receive while browsing. You can view the LSO files stored on your computer by visiting Adobe's website. To stop Flash from retaining these cookies, either update Flash to the latest version and set its preferences to disallow cookies, change your preferences at the Adobe Flash website or use a Firefox plugin such as Better Privacy.
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Your computer handles the job of associating domain names with IP addresses, and speeds up the process by storing these associations in a temporary file called the DNS cache. While the DNS cache is normally flushed when you sign out of your user account or shut down your computer, the information may be available during an active session. You can flush the DNS cache in Mac OS X or Windows using the command line.
Even if you browse privately and delete all remaining traces from your computer, your website visits are likely to be recorded in log files at your home or office router, your Internet service provider, and the Web sites you visit. In addition, some offices and parents block certain sites to prevent time-wasting or unsafe browsing. The only way to prevent this from happening is to use a proxy service that sends your browsing session to a dedicated server for redirection to the site. However, an administrator may also block access to such a proxy service.