Yes..it can be
The House Judiciary Committee can continue any FBI or Mueller investigations that they want and they can draw up articles of impeachment if they find any wrong doing by the president.
That's the Constitutional way. To take powers away from the Chief Executive, by saying that he can't fire his FBI director, isn't the Constitutional way.
Trump needs to take back the executive branch from the Deep State.
There are con. Lawyers and con. professors who disagree with you. So no...its not set in stone that a potus can fire an fbi director no matter the circumstances with no repercussions