I know this is going to piss people off a lot, so let me preface it by writing that my concern is for the sanctity of justice, not for the sanctity of Trump, if you allow yourself to believe that.
This Russia news all started when Hillary was extremely embarrassed by leaks about how her campaign was handled. Suddenly, and almost arbitrarily, she blurted out: “The Russians did it. The Russians hacked my email and gave the dirt on me to Trump's campaign.” Without any prodding, except from my familiarity with the Clintons and with their degree of perverse dishonesty, my first response was that this was total garbage, spewed to deflect charges against her, and to get revenge against Trump.
And any time the news media reported on in as if it had credibility, I already had in my mind an awareness that most of the news corporations had not only been on Hillary's side throughout the campaign, but had been actively campaigning for her.
If my first instinct was correct, and if Comey was doing a favor for friends by “investigating” fabricated charges, then firing him is not obstructing justice. Firing him is getting rid of someone who is abusing his power to a malicious end.
Let's put this in the form of an analogy. Right now, no one considers you to be a child molester. There is no evidence of it, hence there is no reason to believe it. But suppose someone who hates you for whatever reason wants to ruin your life, and they throw it out there, “He is a child molester.” Or, to avoid a claim of defamation of character, they use the Fox News tactic of posing it as a question, “Is that person a child molester?” Or putting a face on it, “Are your children safe around that person?”
This country loves, and clings to, gossip. None of us has any idea what's going on in the world except that the news media says it. Which technically is all gossip, since we have no direct access to the evidence of it.
So as soon as someone puts it out there that you are a child molester, two-thirds of people are ready to believe that you are a child molester. Some aren't so quick to judge, but even so, maybe another 25% will say, “Well, if there's a charge that he's a child molester, we should investigate.” But no, you shouldn't investigate. There should not be an investigation of every rumor that comes along, or we're all a target for having our lives destroyed because someone wants to get even with us.
Now suppose the person who suggested you are a child molester has a friend in the DA's office, and they bring an investigation against you, knowing that all there is is a rumor. If the governor steps in and fires the DA because he's misusing his position, is that obstructing justice? No.
This is a bit of a long post, so I'm leaving out my actual opinion on Trump and the Russians, or why a reasonable person might not be so quick to impeach. But this seems to be what Trump's statement on the firing is, and if he's being honest (it may come down to who are you going to believe: Donald or Hillary?) then it is not obstruction of justice.