The idea of her being fired makes absolutely no sense. It's fan fiction to support a specific narrative.
There was nothing about her specific job performance that was being criticized externally, except some PR gaffs. Even today some circles are still hailing her as a hero and savior for the company.
If there was any cause for concern in her performance, Disney wouldn't be highlighting the fact that shes staying on. They're doing that to single consistency, but specifically consistency with the way McCarthy managed finances at Disney.
The idea that she somehow fell out of favor with Iger because he wanted to spend more money than she was comfortable with is reliant on two supremely false ideas: that Iger didn't want to take advice to save money, and that Iger would be dumb enough to try to fire someone in the midst of the already shaky C-Suite. She could be up there making fun of Iger's mother and I doubt he would dare try to oust her. He worked with her for how many years before he retired and somehow now we think that the last six months has just been TOO MUCH to deal with? That seems unlikely as well.
The far more likely scenario, the simplest one based on the facts, is the one that was posted by Disney: she needed to take time off to spend with her family during a medically sensitive time. It actually makes far more sense when you piece together the history of her actions over the last few months. She went over Chapek and straight to the board out of desperation to have the CEO question resolved in her favor, knowing that she wasn't planning to be around much longer herself. Those irregular events and actions now take a different context knowing she was planning to exit.
They might have a plan for bringing in a new CEO that would now necessitate having the new CEO pick a CFO from their team to come in and run finances. That doesn't seem all that unusual and perhaps that the reason why Lansberry is being pinned as interrim (for now).