Q:

Is it any more or less secure to delete all my cookies vs. individually logging out?

So I've lost all of my Firefox bookmarks (well not since 3.0) or history (3.5?) or my current session enough times (yeah... this still happens) that I don't leave my precious Firefox profile to chance, so I zip it up as a backup weekly. About a year ago, I unzipped one of my very old profiles because I had lost a session or something, and noticed that it still had active logins to all of my websites!
It was a long time until I realized that Firefox wasn't clearing cookies on exit like I had configured it to ("Keep Until: I close Firefox"). I also tried setting up "Clear history when Firefox closes" with "Cookies" and "Active Logins" both selected, and Firefox wipes neither. Eventually, I posted a bug on Bugzilla for it.
Now, I use my Settings Buttons extension to quickly open the Cookies dialog and wipe all of my cookies before closing my browser for the day. My question: is this any more or less secure than individually logging out of everything I log into to deauthenticate those sessions, or am I fine wiping all of my cookies?

A:

Logging out is supposed to expire the cookies/sessions, but that depends on the server-side coding. 
If the cookies were stolen and you only deleted your cookies, then the stolen cookies might still be valid. 
The choice of deleting vs logging out has more to do with what you are trying to defend against. Cookies contain more data than just the session info.
The best method is to do both: log out to end the session and then delete cookies to remove any potentially sensitive data.

