Hmm, what do you want to sign out from? Usually when you view a website in Firefox, the website will provide a method to end your session. This protects you against someone else accessing your account on the site if they happen to have access to your computer. If you have Firefox save logins for you, you can use a Master Password to protect against other people using those if they get access to your computer. See: [[Use a Master Password to protect stored logins and passwords]].

Some sites hide it. Look for your user name and click it.