If Trump's reasons for firing the FBI director were to impede an investigation, then no, he can't just fire him.  Based on the article and other things I have read, the firing may or may not have risen to the level of obstruction, but other actions may have.  We got to watch years of investigations that resulted in the blue dress, years of investigations that turned up the email server, and years of investigations that led to Nixon resigning.  Be patient, nothing may come of the investigation, or something may actually be there.  As I have said in numerous other posts, I'm betting that Trump had little to nothing to do with any collusion with Russia, but his actions in relation to the investigation are what will bring him down.  These things take time.