What is wrong with a 2001 Ford Expedition 4.6l that will only make a clicking sound at the firewall but nothing at the starter if the battery is fine?

That sounds to me like the solenoid is stuck. That's the part of the starter that engages the gear to the flywheel in order to turn the engine and thus start it. Try tapping the starter a few times with a hammer to knock it loose (I swear this works), then if it keeps happening to you it's time to buy a new starter yo.