In the past several months James Comey has stepped on his dick more than a few times. However, unless proven otherwise I'll believe his firing is because he was getting a little too close to the truth in investigating Trump's Russia connections.
House Republicans never even pretended to investigate; during the Yates testimony all the Senate Rs wanted to do was steer the conversation elsewhere. And now Comey's gone. So if we're ever going to learn about Trump and Russia, it'll be up to old-fashion reporting by those "failing" and "fake" news organizations. It might have to be the ones in Europe that are on the case.
For now, though, Trump, his Attorney General, and his Congressional lackeys are doing everything in their power to shut it all down.
But who knows? Maybe there'll be people in the FBI willing to keep at it. Or leak. Hard to believe anyone Trump appoints to replace Comey will carry out an impartial search for truth.