Currently, the settings for clearing cookies automatically when Firefox closes, and setting exceptions to that, are divided among two sections which can interfere with each other. This is very user unfriendly.
In Settings, under Privacy & Security, we have two groups:
1. 'Cookies and Site Data'
Both have an option to automatically clear data when Firefox closes, if you enable custom settings under History, but this allows you to set conflicting settings.
Now let's imagine the quite common situation where a user wants to automatically clear everything each time the browser closes, except one login or the cookies of one frequently used website such as google.com. This is very complex to set up when it shouldn't be. In this case you can't use the permanent private mode ('never remember history') because you can't set an exception to that. That will also clear the exception set under 'Cookies'. If you set 'Delete cookies and site data when Firefox is closed' and set an exception for google.com, but also use 'Clear history when Firefox closes', the exception also doesn't work. In fact, leaving only cookies unchecked under 'Clear history when Firefox closes' can still mess up your login or cause the European cookie wall to reappear. Apparently one must also uncheck some combination of 'Active logins' or 'Site settings'. But then this will probably not thoroughly clear data of other websites which we want to be cleared each time.
On top of that, you can, in some combination or another, have Firefox automatically clear 'Site data' (under Cookies), 'Site settings' (under History), and 'Offline website data' (under History). It is unclear what the difference is between these three things, because under History the two of them are grouped under 'Data' [=Site data?]. Please improve the clarity of what these three things clear. And isn't an 'Active login' saved in a cookie? Is this a fourth checkbox for possibly the same thing?
The best, easiest and clearest way to do this, would be to copy how Microsoft Edge does it: simply have one place called 'Choose what to clear every time you close the browser' or similar, with a check box for each category, and if you enable a category, only then show a button to add an exception to that category. Some clear, non overlapping descriptions for the categories would be a big help too.
PS Yes, I am aware there is a Cookie AutoDelete extension but it shouldn't be needed for something so simple as this.