I think Comey will be hard to get rid ot. He has a 10 year contract and would probably have to get charged and convicted of something to be removed.
The FBI Director has a ten-year term, indicating his job is not permanent (see J. Edgar Hoover).
Director is not automatically replaced with a new administration, but still serves at the pleasure of the president. Director can be dismissed for ‘cause’, but ‘cause’ is undefined and broad.
See William Sessions and Bill Clinton.