The developers of Firefox removed the option to disable third party cookies in firefox 2.0 stating the reason that it was not possible to block all third party cookies with this function. Third Party Cookies are mainly cookies that track user behavior, the big ad networks for instance like to use them a lot. There are basically two options to disable third party cookies in Firefox.
The first would be to disable it manually by opening about:config from the address bar. Search for network.cookie.cookieBehavior and take a look a the value. If it is set to 0 you accept all cookies, 1 means you only accept cookies from the same server, 2 means you disable all cookies. Setting it to 1 has the same effect that the option in the old firefox browsers had: it disables third party cookies.
I promised a second option. You could install a add-on as well that blocks third party cookies. One of the many extensions that does that is called CookieSafe. This one for instance makes it possible to disable all cookies and allow them only for specific sites (whitelist). There is no need that most sites store cookies when you are visiting them unless you have an account on that site.
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