In the past we've highlighted that many popular websites use flash to track your browser behaviour because it's not as easy for users to block or remove — for example, even when you go into private browsing mode in some browsers, Flash cookies persist.
Flash now supports InPrivate browsing in IE, and as soon as other browsers take advantage of the new private browsing options in Flash, the same should be true for the rest of the browsers on the market. (From what we can tell, IE is the only browser that currently supports Flash's private browsing features.)
In the meantime, Firefox users can address Flash cookies with the previously mentioned BetterPrivacy extension. If you're really serious about your browsing privacy, check out our guide to really browse without leaving a trace.