I think it's useful to allow users to have the possibility to enable cookies for some domain even if in Private Browsing mode. See my personal experience: http://forums.mozillazine.org/viewtopic.php?t=2036003 Doing this is very simple: you can add the option "browser.privatebrowsing.allowCookieWhitelist" - False by default. If True, domains stored in permissions.sqlite that are explicitly allowed to set cookies can set them in Private Browsing too.
Having such a preference will potentially weaken the guarantees made by the private browsing mode, and therefore, we cannot incorporate such a feature into core Firefox. -> WONTFIX.
Excuse me, but you can weaken other security behaviours with prefs. See capability.policy.<policyname>.sites for example.
(In reply to comment #2) > Excuse me, but you can weaken other security behaviours with prefs. See > capability.policy.<policyname>.sites for example. That may be true, but it's not a justification to make the problem worse. :-)