Hello. i'd like an extension that does the following:

I have some folder of bookmarks, and my firefox refuses to go to those sites, OR (even better) I can specify times of the day that it will refuse to go to those sites. I figure, atrios, boingboing, and chromewaves are going to get along just fine without me, but dissertation will not. You know what I mean?

Has anyone heard of such a thing? Or has a suggestion for an alternative (barring the obvious "have more self-discipline.")