He fired her because she intimidated Ukrainians by forcing them to never investigate a list of Obama people. She created a list and ordered, like if she was running Ukraine, to never investigate her own colleagues. Giuliani found that and informed Trump. Trump rightfully fired her.
Ukrainian Prosecutor General Yuriy Lutsenko :
“Unfortunately, from the first meeting with the U.S. ambassador in Kiev, [Yovanovitch] gave me a list of people whom we should not prosecute,” Lutsenko said. “My response of that is it is inadmissible. Nobody in this country, neither our president nor our parliament nor our ambassador, will stop me from prosecuting whether there is a crime.”
Source: the hill
“The current United States ambassador Marie Yovanovitch has bad mouthed the President of the United States to Ukrainian officials and has told them not to listen or worry about Trump policy because he’s going to be impeached,” diGenova said.