Comey deserved to be fired long ago... by Obama. Problem was if Obama did that, it would have looked like a partisan play, and you can imagine the uproar if he did at that point. The current question is why Trump waited so long to fire Comey if the reason was due to his mishandling of the Clinton email issue, because it was a while ago.
It's this timing gap that I can understand why Wannatheater thinks the firing is not entirely due to email, since that issue was put to rest months ago. What has Comey been doing since the email investigation? Heading up the Russia investigations which involve Trump himself. The Russian influence is proven, and it has already cost many people in Washington their jobs. The last FBI director to be fired was 24 years ago due to ethical issues, an entirely different situation than Comey. For Comey it's a simple question of timing.
As to timing, last week Comey spoke out about his own possible involvement in the outcome of the election due to the email investigation. Trump is extremely sensitive about anyone casting doubt on the legitimacy of the election. It would be a simple matter of ego to fire a detractor like that. It took Trump less than 24 hours to fire Yates after she refused to enforce the travel ban without due process. While that too was an inevitable firing, Yates stance was later justified when the travel ban was overturned. Trump was overly quick to action with Yates, but the more-justified firings of Comey or Flynn were neither quick nor timely. Worse yet Comey found out about his firing from TV news, which means the handling was likely rushed and resulted in an awkward situation with no replacement identified.
Back to the Russian investigation... a more Trump-friendly FBI director would at a minimum delay the investigation while they come up to speed, and at a maximum be able to play a key role in clearing Trump from the investigation. It's not a stretch to consider that thought process wasn't involved in this action even if Trump is 100% innocent. Comey was unpredictable, mishandled press releases, and neither Democrats or Republicans felt they could predict his actions... increased predictability mitigates risk.