When browsing using a public computer, you want to prevent the next person who uses the computer from being able to access your web site accounts. After clicking the site's "log out" button, close the browser window so the next person can't click the Back button to view your information or resubmit your password (bug 155030).
If the site does not have a "log out" button, you should be able to log out using "Tools > Clear Private Data" -- ask it delete cookies, authenticated sessions, and cached web pages. You can usually also log out by closing all browser windows (not just windows containing that site -- see bug 117222).
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