We can now use the energy of the electrons on the dome to create sparks, attract pith balls, make hair stand on end . . .
Where did this energy come from?
As it moved the belt, the motor had to work harder and harder to pull the electrons on the outer surface of the belt up to the increasingly charged dome. This was done against the repulsive electrostatic force trying to push these negative charges apart. In fact, if you listen closely as the generator is first turned on, you can hear the motor working harder and harder.
So energy that went into the motor is converted to the energy of charge separation.