To sit. To wait. To do nothing is Fëanor's literal hell. Worse than choosing to do nothing -- which he couldn't fathom -- is the inability. He is the Greatest of the Noldor, constantly driven, constantly defined by his action, his ever-present motion forward -- weather that be in moving on to new projects and discoveries, or in the case of the flight of the Noldor, actually pushing his followers onward because to stop would be death for him. To stop would be to let the fires he'd lit under the other Noldor cool, and give them a moment to think and possibly to change their minds. To stop would be to loose his chance to do something. To take back what was stolen from him, to avenge his father, to act no matter how wrong that action might be, because atleast it is action.
It isn't nothing. Nothing is weakness, an admission of powerlessness. that he cannot fix things, that in this situation he cannot do anything. Nothing is bottled frustration and anger, restlessness and an energy that can't find an outlet. It's the gaping hole in his chest, that grew over his childhood seeing his mother's unmoving body in the gardens of Lorien.
And Fëanor would rather charge headfirst into battle, would rather fight an army of Balrogs, would face any torment Morgoth might put him under, then face that. The stillness, the waiting, the uncertainty, of sitting on the sidelines and letting someone else take command.
