The president has the right to appoint and dismiss the FBI Director at his own discretion. While one can agree or disagree with his decision, exercising this power is completely within his rights and should not be reason for impeachment.
It is not a question whether it was legal or not, it is absolutely within the power of the president to make this decision. But the circumstances matter to deem whether Trump abused his powers. Just because the president has the power to launch nuclear missiles, does not mean that he may not be impeached for abusing this power.
In this situation, Trump did not just fire an individual because he did poorly in his job and there was a lot of pressure in political and civil circles to dismiss the FBI Director, no the Director actually seems to have done nothing wrong other than pressing on an investigation into possible collusion with Russia during Trump's campaign. As such, Trump's interference is a highly questionable move