LSO cookies are sometimes called Flash cookies. They are put on your computer by various websites and stay there. They are not deleted when you clear cookies, or use popular programs such as CCleaner. They are also not limited in size like regular cookies, and some will be 100k. But there is a convenient way to delete them whenever you close Firefox. Get the addon BetterPrivacy. The first time I ran it, it found and deleted 150 supercookies. On a friend's machine, there were more than 1500, because she uses YouTube a lot. I do not know of a similarly easy way to delete them, so even if you don't use Firefox, it's worth installing on your computer just to be able to easily remove these privacy-stealing cookies.