The extension should discourage servers from recompressing JPG and PNG images as WebP images -- typically without changing the file name/extension. Can you share a URL of a page that recompresses JPG and PNG images despite the extension? Perhaps I can figure out a workaround.

In case the problem is cached images from a previous visit to the page, you can clear Firefox's web cache: https://support.mozilla.org/kb/how-clear-firefox-cache

Note: the extension doesn't block pages like https://developers.google.com/speed/webp/gallery1 from using WebP images when they are part of the design.