Firstly, it's the President's Constitutional authority to fire the FBI director...regardless of reasons.  So, impeachment on this basis would be seen by future historians for what it would be:  a coup.  Not a Trump fan, but VERY concerned about the establishment using these tactics to supplant law and the ballot box.  It's a proverbial Leftist theme.  

Next....two incontrovertible facts.  Obama got a record amount of campaign cash from BP.  Later during the Deep Water Horizon hearings, it was learned Obama's administration gave BP special concessions on environmental and safety regulations.  Had he been a Republican; the press corps would have screamed "CORRUPTION!!!", sparking all manner of investigations.  Indeed, like Hillary being able to get away with destroying her electronic devices (under subpoena), the 'objective' media will say doodly about it...while they exaggerate and fabricate the flimsiest items into 'atrocities' on the other side.  Journalists are propagandists!