By whom?
It is a presidential appt. it can be undone
Impeached for cause? Someone watching-the-watchers has a dossier on Comey.
According to a 2014 report by the Congressional Research Service, there are no statutory conditions on the Presidents authority to remove the FBI Director. In other words, the President can indeed fire the FBI director. An FBI director can also be removed by congress, through the impeachment process.
Because firing an FBI director is certain to bring accusations of playing politics with law enforcement against any president who chooses to do so, Clinton repeatedly asked William Sessions to resign his post. But Sessions refused to step down.
So in July of 1993, saying that he had been advised by then-attorney general Janet Reno that Sessions can no longer effectively lead the bureau and law enforcement community, Clinton fired the FBI director eventually appointing Freeh in his place.
https://fas.org/sgp/crs/misc/R41850.pdf