FBI Director term of service:

[elleng]

The FBI Director is appointed by the President and confirmed by the Senate.[2][3] J. Edgar Hoover, appointed by Calvin Coolidge to the predecessor office of Director of the Bureau of Investigation in 1924, was by far the longest-serving director; he held the position from its establishment under the current title in 1935 until his death in 1972. In 1976, in response to Hoover's lengthy tenure, Congress imposed a term limit of ten years for future directors, which was waived by the Senate for Robert Mueller on July 27, 2011 due to serious security concerns at that time.[4] Since the 1976 law, Directors serve a ten-year term unless they resign, die, or are removed, but in practice, since J. Edgar Hoover, none have served a full ten years, except Robert Mueller, who served twelve years. >>>

https://en.wikipedia.org/wiki/Director_of_the_Federal_Bureau_of_Investigation

[MrPurple]

1. J. Edgar's incriminating files on everyone scared them from firing him

He totally abused the powers of his office to maintain his own power.

[flotsam]

2. This looks familiar:

"But only one of those directors was ever fired by a president — William S. Sessions who had been appointed by President Ronald Reagan in 1987, but who was beset by allegations of ethics violations, such as disguising private vacations as law enforcement business, and using a chauffeured government limousine for personal transportation.
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 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"