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.