It's yahoo doing this, not Firefox. Sites that keep you signed in usually do it by setting a cookie. If you delete your yahoo cookies it will probably force you to sign in again - and at that point you can say you don't want to kept signed in. When in yahoo mail, click on the icon to the left of the site address, select More Information then View Cookies - then delete them all. (There are other ways that sites remember you, but that's the most common.)