I'm not an American, but by the way I followed the news and my personal opinion on why he was fired was:
1. Comey was too soft on Clinton - Comey is fired because of his handling of the email investigation on the Clinton. And he had exonerated Clinton from criminal wrongdoing.
2. Trump colluding with Russia to win the election- Comey even admitted in front of the Senate that the FBI was investigating Trump's campaign and his Russian connection.
3. Comey didn't support Trump's claim that Obama tapped his phone in Trump Tower.
So basically, he dig up his own hole leading to his firing and dismissal as FBI director.
That's all true.
But in the United States people are taught to try to respect fairness over power.
Nobody really doubts that Trump used power to derail an investigation and repopulate the FBI with people who would support him. Nobody doubts that. The question is whether or not it is acceptable to do that.
I have the impression that Trump accepted the challenge and prepared to fight. He deliberately goes to the aggravation of the situation to show that he is the master of the situation. This is a very exciting fight. I wonder whether it will end with impeachment?