I'm in Firefox 21. I tried to delete my cookies, but when I close and reopen the "Show Cookies" window, all cookies have respawned, including many third party advertising cookies from sites I have never visited, sites such as 2o7.net, interclick.com, adlegend.com, and about a hundred others. This is in spite of the fact that I have "Accept third-party cookies" unclicked. How can I delete these advertising tracking cookies and ensure that they never come back?