O’Hare wasn’t fired by Straczynski, he was fired by Warner Bros. A little known fact is that Babylon 5 had a GUARANTEED 22 episode run for the first season – meaning it couldn’t be cancelled. It was part of the deal set up between Warner and the stations that signed up to PTEN.
At the end of season 1 the show was generally being panned by TV critics as being to slow and talky – with O’Hare being singled out as a wooden actor. So what happens – season two is much more action orientated and O’Hare is replaced by a known name to act as the new commander. All the other writers involved with the previous story arc that was building up during season are gone, the story takes a major right turn and Straczynski starts writing all the episodes.
It was for the better, the show improved immensely, but from season 2 onwards, it was not the show which Straczynski had originally planed out – as can be read in one of his memo’s to Warner - during season 1 - who wanted to know where the thing was going.