The author of the website should not want to prevent copying in the first place. It's the 21st century, copying is trivial.
> Ok, so Firefox will prevent websites from modifying a clipboard. Then, such websites will show you the text in PNG images. What is next?
What happens next is those sites will immediately lose a ton of users due to their poor usability. Site translation services will be completely broken. Search engines will fail to index the text contained in the images. People will still be able to save, copy, screenshot, crop, edit and share the image itself; the authors just made their own content look worse on purpose by making it impossible to scale the text properly. If developers care enough about the content, they will eventually create and use an optical character recognition browser extension that extracts the text and overlays it on top of the original image.
The only thing that isn't going to happen next is users going "aw shucks the author's will is absolute so I guess I can't copy the text".