In case you don't want to install an extension, search in about:config for "image.webp.enabled" and set it to false. Worth mentioning, there's also a "image.avif.enabled" entry.
That is not the same thing. This addon does not modify "image.webp.enabled" or "image.avif.enabled", it modifies the accept header. If you modify the preferences you mention you will still be served AVIF and WebP, but viewing will break...
