From https://support.mozilla.org/en-US/questions/1118394
hi, the ask anytime option for cookies was removed in firefox 44, which was supposed to happen for a long time (bug 606655).
i would recommend the following setup instead: in the custom history options choose "keep until i close firefox" as a default option & set exceptions for the few sites where you want stay logged-in across sessions. this way you'll have less work to do over all and a privacy-minded setup after all.
or as an alternative you could also take a look at an extension like https://addons.mozilla.org/firefox/addon/self-destructing-cookies/
Lots of people complain about this. Consider to keep the code for cookies "ask me every time" from Firefox 43 for IceCat 45.
-- http://gnuzilla.gnu.org
