The issue is not his termination but rather the timing of it.
Joel Pollack has suggested a very plausible reason for the timing. James Clapper's testimony on Monday that there was no evidence of collusion between the Trump campaign and Russia and that therefore Trump couldn't be reasonably accused of trying to cover up this conspiracy.
Of course it hasn't stopped Dems and their allies in the media unreasonably accusing Trump in their ever more histrionic pushing of this tired conspiracy theory but there's not much that can be done about that.
http://www.breitbart.com/big-government/2017/05/09/james-comey-simple-explanation/
The biggest problem Camp Trump is going to have trying to sell that narrative is that for months they praised Comey for how he was handling the investigation into Hillary's emails and now all of a sudden they use that as an excuse to fire him. Worse, in his firing letter, Trump goes out of his way to mention Comey telling Trump three times that he didn't yet have evidence of Trump doing anything illegal vis-a-vis Russia. It's the sort of mention that makes one think Trump doubted he'd get lucky a fourth time since Comey wouldn't bring the investigation into possible campaign collusion with Russia to a halt.
Actually, what I think sealed Comey's fate was for the last week or so, the narrative had been that Comey's admittedly inept handling of information into the investigation of Clinton's emails was what enabled Trump to win. Trump likes to believe he won the election all by himself. The idea that Russia or Comey was what pushed him over the top is simply unacceptable to the crybaby-in-chief.