Can Sean Spicer take advantage of USERRA if Trump decides to fire him?

It would end up depending on reason or possible perceived reason for termination. If the termination is for his inability to perform required duties due to military service then I would think it does apply. If the reason was poor performance or ineptitude in performance of duties than it would not apply. I would think one of the Presidents advisors would inform him that he cannot use military service as part of the reason for termination.

All he has to say is I don't like you; you're fired.

Thats a good question, that may need a SCOTUS review. The question would be is a political appointee considered an employee in the traditional sense? I would venture, the answer is no. A political appointee has the understanding that the tenure is limited also they serve at the pleasure of the POTUS. He can hire and fire at will usually. Then the question begs does Spicer want to serve in a post where is boss has lost confidence in him. I have to assume a political post is handled differently. Otherwise, lets say Trump loses in four years. Does this mean that Spicer could go on orders during the inauguration of the successor and be guaranteed to be the Press Sec for the new guy? I don't think so. I think that would hamper the intent of those posts.

And what kind of bullet does that firing generate for his OER?