There is function "changeInitRunUser", which take new user name as an agrument:

changeInitRunUser() {
    NewUser=$1

    InitFile=`getInitScriptLocation`
    if [ -f $InitFile ]
    then
        editFile $InitFile FBRunUser "FBRunUser=$NewUser"
chmod u=rwx,g=rx,o= $InitFile
    fi
}

But it's always called without any agruments. So user name will be empty.

In the second place editFile uses awk to change user name. It works for classic and xinetd. init.d script in superserver declares run user as:
FBRunUser=firebird
Awk try to find field "FBRunUser", but "=" is not default field separator. Therefore awk couldn't find and change this variable.