Firefox Add-on: Form History Control

Firefox, like most modern browsers, has a handy feature which remembers what you type into form fields on web pages.  Unfortunately, sometimes it remembers things it shouldn't (oh, like your credit-card number, SSN, etc.), because the fields aren't labelled correctly on the web page.  In order to protect yourself against someone obtaining this information from your browser profile, I recommend the Form History Control add-on (available here).  It allows you to view saved form information, and set criteria for deleting fields you don't want saved.  You should review what the browser is storing, looking for sensitive data (pretty easy to glance through the list).

If you use another web browser, find out how to do something similar, or, for the really paranoid (or lazy, lol), disable the automatic form-filling feature to be on the safer side.