The Clear History dialog contains several individual private data components, each accompanied by a check box. The first option in this window deals with Browsing & Download History. Firefox 4 records and stores a list of all websites that you visit, as well as a record of all files that you have downloaded via the browser.
The second option deals with Form & Search History. Any time you enter information into a form on a website, Firefox 4 stores some of that data. For example, you may have noticed when filling out your name in a form that after typing the first letter or two your entire name becomes populated in the field. This is because Firefox has stored your name from entry in a previous form. Although this can be very convenient, it can also become an obvious privacy issue. Any time you perform a keyword search using the browser's Search Box, Firefox 4 stores that data as well. This data is defined as search history.
The third option deals with Cookies. When you visit certain websites, a text file is placed on your hard drive that is used by the site in question to store user specific settings and information. This text file, or cookie, is utilized by the respective site each time you return in order to provide a customized experience or to retrieve your login credentials.