When is being fired from my job illegal? When is it unfair?

Getting Fired: When is getting fired from your job illegal? When is it unfair? 

I am a good worker, but my boss fired me because of a personality conflict.

I was fired because I am gay, a woman, disabled, etc. 

I was fired because my boss wants to give my job to his girlfriend.

I was fired because my boss only wants women working for him.

I was fired because I was the senior and highest paid employee and they found someone they could pay less.

I was fired because I was the senior and highest paid employee and my boss said they need to get younger people.