He can be dismissed but no cause is needed. He is allowed one term of no more than 10 years, but it is not a “term”. It is a -maximum limit- designed to prevent a future J. Edgar staying for life.
But he is not granted a term under law. He can be fired at will. But 10 years is the max he can serve.
remember when the Clintons MAGICALLY had possession of all those FBI files?