I cannot connect to the web UI because it says someone is already logged in. How can I disconnect the current session of the admin user?
The admin user that is logged in will time out after 10-15 minutes of no activity, and you'll be allowed to log in.
The firebox does this to protect itself from multiple admins making changes at the same time.
If you need to access it right now, rebooting the firewall will force the other session to disconnect, and you'll be able to access it as soon as it comes back online.
WatchGuard Customer Support
You can set the idle timeout for admin sessions in Firewall Authentication -> Management Session.
The default is 15 mins.