Who has the power/rank to fire a Fighter Fighter ?
I will assume most in the U.S. are covered by a local collective bargaining agreement with a no termination clause except for just cause.
The procedure for termination is outlined in it.
If we have a situation where the FF is employed "at will" meaning he or she can be discharged at any time, for any reason, no contract, etc., it depends on how the chain of command/handbook is written.
If a one level termination is permissable, so be it.
By that I mean if an upper wishes to terminate a subordinate, s/he, by policy can either do it himself or if 2 are needed, s/he needs an upper approval to sustain it.
The secretary of Department of Interior.