One of the nicest features on OmniWeb (Mac OS X) is that I can set a 'default action for new cookies'. I can tell it to keep all new cookies, and I go log on to all my favorite web sites and set up my preferences on them and have them store the appropriate cookies in my browser; then I change the setting to reject all new cookies. The cookies that were already set will stay; no new cookies will be accepted. I'd like to be able to do this with Mozilla, too. But right now the best I can do with Mozilla is to tell it to accept all cookies, then I have to periodically go into my cookies list and delete the dozens of cookies set by every two-bit web site I've wandered past recently, while I leave the cookies set by Slashdot and LiveJournal.
This is very similar to what mbrubeck suggested in bug 82734 comment 2.
Based on the second paragraph of comment 0, I think bug 75915, "whitelist for sites allowed to store (permanent) cookies", would allow you to do what you want. IMO, changing the "default action" for new cookies and then visiting all your favorite sites is an odd and unreliable way to build up a whitelist.
marking WONTFIX whitelist should be sufficient.