what can you do when your employer fires one person and not another person for the same thing?

what can you do when your employer fires one person and not another person for the same thing? I was expected to do another persons job since she did not want to do it herself, when i told her that she needed to do this herself she went and told the boss i was bullying her because i did not do it. th boss then came to me and yelled at me and called me a bully and fired me. She told me i was fired for "code of ethics policies" stating that i did not share the work load , and be a team player and had no respect for the other co-worker. She then send me a copy of the policy which the boss has changed to suit her purpose of firing me. This other co-worker has been known as not being a team player, has never shared the workload with any co-worker she has ever worked with and has never treated any co-workers with respect, she has also broken other work poliy and rules, all of which the boss knows about but has not taken any action against her.