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.

He was around most of last week for the Comey fall out. If Trump was seriously considering firing him it was for his response in that first day after the Comey firing - not because of his Navy Reserve duties. He handled it horribly in the first 24 hours.
If Spicer gets fired it's because he's a crappy press secretary. Not because of his Reserve duties.

Comey got fired on 9 May. This is what happened that night after it was announced Comey was fired:

"Spicer had planned to email out a statement announcing Comey's termination, but as of about 5:40 p.m. EDT Tuesday, the system wasn't working well. Instead, he went to his doorway and yelled the news to the journalists gathered there, according to the Washington Post. Then he closed the door."

"At 5:41 p.m., reporters began to tweet the bombshell, and by 5:44 p.m. Spicer's statement explaining that Trump's decision came at the recommendation of Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein had arrived in inboxes. He followed up with a tweet at 5:50 p.m."

"Aides said he would speak to press about the termination, which was blamed on Comey's supposed mishandling of the investigation into Hillary Clinton's use of a private email server while secretary of state, then changed their answers and said he wouldn't. Outside, Spicer appeared on Fox Business with Lou Dobbs, at times stuttering and misspeaking, at one point saying the attorney general had been terminated before quickly correcting himself. Photographers could be seen at the edges of the frame taking pictures of Spicer as he spoke with Dobbs."

""What I can tell you is the president, when given these recommendations, made a decision to accept their conclusions and to remove director Comey and begin that restoration of confidence and leadership that needs to happen there," Spicer said.

The Post detailed also that "Spicer spent several minutes hidden in the bushes" while his press assistants negotiated the terms of a briefing. He stepped out, and asked for the lights on set to be turned off. They were.

He spoke with reporters for about 10 minutes."

So this was last Tues after the firing and that's how he handled it. Nothing to do with his Reserve commitments. He just handled that initial response horribly wrong. The deputy press secretary seems like she can handle this job better. Maybe she should be press secretary and he can just do communications director because right now he's both communications director and press secretary.

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.