For years I have set my preferences to block cookies (Privacy/custom settings for history/accept cookies unchecked (i.e., don't accept)) and used an exceptions list to allow cookies from trusted sites. This no longer works for me. Firefox appears to ignore the exceptions list and block all cookies so that there is no way to get many logins to work without accepting all cookies. Windows users on the other thread do not seem to experience the problem.
I am not sure if this is a general bug or something particular to my installation. I am posting here in case someone else might want to confirm the problem. There was a long discussion elsewhere, I went through all the create a new profile stuff etc., but I never solved the problem directly. I finally got a workaround via an add-on. If it is a bug, it may be specific to Mac. It seems to have started with the upgrade that came out in late December 2015, but my comments here are based on Firefox 50.0.2.