Visual Editor has disappeared on all my sites

You mean only the html editor is there? Check settings under your user profile.
Visual Editor – Checking this box Disables the visual editor when writing, and uses the plain HTML editor.

Can you post the URL of any one of the sites so that we can have a look at what really the problem is?

Here’s the strange thing….

I thought that the only difference in my PC today was an Antivirus update, so I changed my antivirus client and re-started and cleared cache etc. No result.

So I tried in another browser ( I was in FF and then tried Chrome) and all is perfect in Chrome.

Why would that happen?

Can be markup errors.
Read: https://codex.www.ads-software.com/Validating_a_Website

I don’t think that the validation is necessary, as it is affecting all of my blogs and they are not all on the same server.

They all have different content and some are the latest WP and some have not been updated yet. So it is not that the code on one site is somehow broken.

I guess this is a Firefox problem. The visual editor displays as normal in Chrome and IE. Just not there in Firefox.

Can anyone understand why that would happen?

EDIT: Just checked in Firefox on another machine and all is good on there, so it is just on this machine somehow.

PROBLEM SOLVED:

I did a reset on Firefox and that cured the problem.

Thanks for all who contributed and helped.

You are welcome!