How can I log off

From PFSenseDocs

There is no log off function in pfSense 1.2.x because it uses HTTP basic authentication, which is retained and controlled by your web browser and cannot be controlled by the firewall. Some browsers have the ability to clear stored credentials for HTTP basic authentication, while others you must close the browser for it to stop using the credentials you have entered. Refer to your web browser's documentation for information.

In pfSense 2.0, a different type of authentication is used that does allow logging off from the firewall.