I'm a tad confused too; I've been working on a bug/project that involved setting a cookie 90 days into the future. This cookie was closely identified with sessions, and both Chrome and "The Fox" were expiring the cookie on browser close instead of holding it in storage. I found a setting in Chrome, e voila, the cookie remained. No such luck yet with FFox (but I'm working on Y.A.B. ['YetAnotherBug'] today) ...
It could be, of course, that it has more to do with the software that's producing the cookie in the first place than the browser ... still more work to decipher that one, though.