Re: Firefox will not retain "use custom settings for history". It keeps reverting to "remember history".
Firefox shows the "(Never) Remember history" setting if all the history and cookie setting are the default.
Make at least one change to the default setting if you want "Use custom settings for history" to stay selected.
Otherwise Firefox will revert to "(Never) Remember history" if you return to the Privacy tab.
The choice "Use custom settings for history" will stay selected as long as not all settings are the default to make you aware that you made changes.