No...it's not firing them. NFL players sign a contract for a length of time...when that contract is up, then that team or any other team can decide to hire them again...that is up to those TEAMS. I would even be ok with it if the NFL strongly suggested to a team that they not re-hire a certain player due to their off-field actions, but suspending and fining for something we have a legal system for is too much.
If a player kills someone, then the legal system takes care of that. If found guilty, they are put in prison and obviously can not play football. If the player is found not guilty (Ray Lewis), then he gets to continue to play.
Not the NFL's job to be anyone's moral compass, and it's a good thing because there are a lot of bad guys in the NFL.