While I don't believe that Comey was fired for any of the reasons you mentioned, you might want to give your own theory a reality check.

Firing Comey changes nothing about the FBI's investigation. That's because the FBI's activities are under the jurisdiction of the Justice Department, not the president. (He just appoints or fires the FBI director.) The FBI also reports to the National Intelligence Committee, not the president. 
Additionally, there are a good number of other security agencies involved, such as the CIA, NSA, HS and so forth. They too report to the National Intelligence Committee.

It would require a mass conspiracy on the part of all the US security agencies to suppress the truth in this case. And I don't think any president has that kind of political power, let alone Trump. Firing Comey will make no difference to the outcome of the investigation. Aside from that, Comey deserved to be fired for his meddling in the US presidential election.