I think many users don't understand the concept Rated 5 out of 5 stars
Before installing 'Self-Destructing Cookies' one should do the following in order to have the add-on not remove cookies meant to be kept :
Translating from French layout ...
- Firefox Options / "Private life" / History / "Rules of preservation" : select "Use personalized settings for History"
- Under, "Accept cookies" checked, and on the right, choose "Exceptions", and in the "Exceptions" list popped up, choose the cookies/domains you wish to keep and grant them either "Allow" or "Session". "Allow" and "Session" status will prevent 'Self-Destructing Cookies' from deleting them. "Allowed" will remain after Firefox is closed, "Session" will be removed with Firefox exit.