William Sessions.

Sessions's dismissal was a long time coming. Sessions was accused of serious ethical violations by the outgoing Republican Attorney General before Clinton was sworn in, including tax evasion, using FBI planes for private travel and using taxpayer money to install a security system at his house. http://www.nytimes.com/1993/01/19/us/fbi-chief-plans-to-fight-for-job.html?n=Top%2fReference%2fTimes%20Topics%2fSubjects%2fT%2fTax%20Evasion

Clinton asked him to resign in January 1993, giving him the chance to resign without being fired. Sessions refused. As you point out, Clinton fired him before Foster killed himself months later. You seem to imply this would affect the investigation in Vince Foster, but the FBI director of course, would not be on the ground gathering evidence.

It's noteworthy to know that Foster right before his death, talked with his wife about leaving Washington and talked with his sister about depression and getting medication. He called his doctor and got a prescription for the anti-depressant, Deseryl the day before he killed himself. He took 50 mg about a third of a regular dose.