Add autocomplete="off" attribute to all specifically billing related input fields for online contributions/membership signup and for online registration. Affected fields are Credit Card #, Security Code (CVV2), Billing First/Middle/Last Name, Billing Address fields.
This attribute should prevent browsers for auto-completing these fields with previously entered values.
— Original Post ----------------
Just noticed that Firefox remembers credit card numbers that have been entered in CiviEvent. There is some way to disable this behaviour for specific fields, I believe, which would improve user security.
I imagine the same problem exists for IE and for other e-commerce components like CiviContribute and CiviMember.