Images have disappeared

Installed the plugin and very happy with the seo reports until I tried to add a new image. All images have gone blank in the media preview, they don’t appear in the page editor and are blank on the live site. I’m using Firefox.

I deactivated, no change. Logged out and back in again, cleared browser history no change. Checked using Safari and Chrome and it’s looking fine and seems to work ok but I can’t get it back in Firefox and that makes me very nervous about what other people will see.

Why has it happened? How do I restore Firefox back to what it was? Is there any risk that other people using Firefox will see the same thing?!!!

https://wordpress.org/plugins/wp-fastest-cache/