This add-on is just what was needed. My cookies.sqlite had somehow become corrupt and Firefox was deleting all of my cookies every time I restarted. I could restore from a previous file, but still FF was resetting the cookies (that I very much rely on) every time I shut down.
Cookie Import Export saved the day. I restored an old cookies.sqlite file and exported it to a .json file using the add-on. The next time FF reset my cookies I simply imported the .json file I exported and the cookies have stayed permanent since. While some people actually want to remove cookies after each session, I don't recommend it unless you want to have to log into each site that remembers you, or tweak the location settings on every Weather site that you visit.
It always helps to have a Backup. Thank you to the developer.