I have for quite some time had this issue. The rich text editor in Wordpress went missing at some point (at least when using Firefox and Safari) and it's been bothering me like a popcorn kernel stuck behind one of my wisdom teeth. Solved. Both Firefox and Safari are using a userContent.css file to do a lot of ad blocking. I knew that the CSS file was the culprit and now I know exactly which line. The rich text editor returns if I comment out:

/*
img[src*="/adv"],
*/

Easy. Check that. Safari doesn't like the rich text editor regardless of the CSS used. Oh well. YMMV.