Purge Your Private Data
Firefox records where you've been browsing to help you retrace your steps, as well as to reopen the pages you've visited previously more quickly. But this feature may compromise your privacy by letting other people using your PC see what you've been up to.
The browser's default security settings may not strike the balance of convenience and privacy that's best for you. To customize those settings, select Tools, Options, and click the Privacy icon. The dialog box is divided into three sections: History, Cookies, and Private Data.
History: You might want to shorten the number of days Firefox saves your surfing history--the default is
Cookies: I've covered this before. See "Block Spying Cookies, but Keep the Helpful Ones."
Private Data: To clean things out regularly, check Always clear my private data when I close Firefox. Then click the Settings button to fine-tune the options.
What should you leave checked and unchecked in the Clear Private Data dialog box? Checking the 'Browsing History' and 'Cookies' sections will override any changes you made in the other sections of this dialog box, so leave them unchecked. Checking 'Cache' may slow Firefox down a bit, but probably not enough for you to notice if you have a fast Internet connection. Deleting 'Saved Passwords' is entirely pointless. You should check every other option. After you click OK to close the Clear Private Data dialog box, uncheck Ask me before clearing private data to avoid this annoying pop-up dialog box.