This 4 year old cookie issue is fixed now with v154
This has been an issue for me for years, and now its fixed!
I'd add a list of sites I wanted to keep cookies for, so when I closed firefox, only those cookies would be kept. Except it would also keep other cross-site cookies, since by adding them to the exception list it also disabled total cookie protection for those sites.
So I'd spend time going through my cookies and manually deleting stuff that shouldn't be there.
Now finally, there are 2 lists, and you can have an exception list for clearing history on shutdown without bypassing total cookie protection.
"Add a way for cookie-clearing exceptions to not also affect cookie partitioning" https://bugzilla.mozilla.org/show_bug.cgi?id=1767271