This is firebug’s option:
You can read this article.
As can be seen from the article:
(1) If the cookie does not contain an expiration date, it can be regarded as a session cookie. Session cookie are stored in memory and never written to disk. When the browser closes, the cookie will be permanently lost.
(2) A cookie can be considered a persistent cookie if it contains an expiration date. On the specified expiration date, the cookie will be removed from the disk.