I haven't, but feel I may be on the verge of it. I find everything so difficult to do, get easily distracted, forgetful, very disorganised and my work has definitely taken a nose dive since hitting perimenopause. I feel paralysed sometimes just trying to send an email.
Were you fired or did you hand your notice in before they fired you? How long were you at the company for?
Re what to say to a new employer, if you were there for a long time, say that your were stagnating/hit a ceiling for development opportunities and were no longer challenged by the role.
If you were there for a short amount of time, say that the job was not as it had been advertised and you did not feel that it was the right fit for your career progression.
If you were there for a medium amount of time, perhaps say you wanted to explore other avenues.
You can also give non-work related reasons for leaving a post, like wanting to find something with a shorter commute, something to fit around caring responsibilities, something to fit around studying, better work-life balance etc.
I think a new employer can ask you in an interview why you left a job, but I was once told that a previous employer can't say anything negative about a former employee. If asked for references they will only confirm your job title and the dates you started and left the company (not the reason for leaving). Perhaps someone with more HR experience can confirm this though...