Using 52.4.0 (64 bit) ESR on windows 10: if I open "page info" and de-select use default on set cookie, then "block"; remove the existing cookie, and then exit Firefox the setting is not retained upon opening a new session. This worked previously, and went away recently. Note that I also lost my "sync" account login someplace. I have refreshed the install and tried manual entry using the about: preferences. I note that any setting for exceptions for cookies is long gone on entering a new session. History is: use custom settings uncheck private mode, uncheck remember browsing and download history, uncheck remember search and form history, check "accept cookies from sites (exceptions are there after I use the page/info method), accept third-party is "never", keep until "they expire, check clear history when Firefox closes..."