The Targeted Advertising Cookie Opt-Out (get it? TACO) extension sets up permanent opt-out cookies for many different advertising networks. 

Here is a little more about the extension from the developer:

Unlike other earlier opt-out solutions, this tool will make the cookies completely persistent. That is, clearing the browser’s cookies will delete all other installed cookies, except these.  Users wishing to delete these 40+ cookies (some advertising networks require multiple cookies, for different domain names) must first remove the add-on, before then deleting all installed cookies from the preferences window.

For a list of the 33 advertising networks that this tool help you hide from, check out the developer’s official web site.  So if you are more than a little concerned about who is watching you, this is a great security related add-on to add to your Firefox install.  Who said tacos aren’t good for you?  This one will help protect your privacy!