Firefox 57.0 (64 bit) I'm getting this problem also. There is a website that I don't even visit and I noticed that site kept getting a cookie set. So I added an entry in the 'cookies exceptions', and marked it 'block'. Then checked to see it was in the exception list, and it was. Then exited firefox, opened again, checked cookies exceptions and it was gone ?? Closed firefox and opened firefox again. Went to http://facebook.com , read a few posts and didn't go to any links or other sites. Exited facebook and I see 2 cookies. One for facebook and one for the site that keeps getting added. What could be causing a cookie to be set when I'm staying on the Facebook site and not going to any other sites. No following of links from facebook.