For example, let's take the website github.com

If you don't want to have to log in everytime you restart the browser, you have to whitelist the entire website as follows

But you would only need to whitelist the following cookies to do so.

on .github.com
logged_in

on github.com
_device_id
user_session
__Host-user_session_same_site
_gh_sess

I found an add-on called "Cookie Quick Manager" which is a great way to consult your cookies on a per-site basis, is container aware, it's great.

In that add-on there was a "protect cookie" function

Unfortunately, it would only only protect cookies from getting deleted by "Cookie Quick Manager" and not firefox's "delete data when Firefox is closed" but that would have been a fantastically convenient way to handle this

I think what would make sense would be the ability to append cookie name and container names to the "delete data when Firefox is closed" exception list

So instead of just

https://github.com

You might be able to specify containername!cookiename@https://github.com

With both the containername part and the cookiename part being optional limits to the whitelisting

Here is a mockup of what that might look like