The president has every right to fire the FBI director at any point in his term, legally. Established norms say that he shouldn't, but norms do not have the force of law.
The Comey firing is only illegal if it was done to obstruct justice. Which seems likely, particularly because Trump's comments would make denial difficult, but proving it in an impeachment proceeding is another matter. Don't forget that even with all the chicanery around the Nixon administration (and there was a lot that gets lost in the narrative, particularly around money laundering) it was only the irrefutable evidence in the form of audio recordings that brought him down. The standard of evidence for impeachment is and should be very high- instability is worse for the country in the long term than bad leadership.
The solution is to keep digging, catch them in lies and inconsistencies, find crimes the lower level flunkies did and use the threat of prison to flip them, etc.
Watergate took THREE YEARS, during which time Nixon was doing Nixon shit all over the damn place. Trump has been in office four months and hasn't been able to name a post office. Find a paper bag and go breathe into it for a few minutes until you've settled down, because this is going to take some time.