Setting Browser Prefs about:config

I would like to stop stock browser from accepting 3rd party cookies by setting network.cookie.cookieBehavior to 1 in about:config. Default is 0. If I enter about:config, set it to 1 and confirm, this seems to work at first glance, but when you come back to check, ist only to find out that it’s still set to 0. So my question is, why can’t I change this or what am I doing wrong and what can I do to set network.cookie.cookieBehavior to 1 and save.