Are you on Firefox 2, or 3?
Firefox 3 should have this

As for the rest I only meant that as a "which" kind of question, to narrow down when the session is getting lost, if it keeps it for a while when you're idle we can rule out an expiring cookie as the problem, you say it happens when you make posts, or refresh, these are valid times to lose a cookie based on the way Fakku works,thing is you shouldn't be losing it, next time you get logged out, instead of logging back in go to your cookies, and click "Show Cookies" to see if an entry for Fakku still exists, trying to narrow down this problem is proving to be hard :(