Specifically for wikia and some sites with content negotiation, i set up my firefox this way: in about:config, i put "*/*;q=0.8,image.webp;q=0.0" as the value for key "image.http.accept".
It resulted in FF telling the sites that it doesn't like getting webp formatted images. It won't work everywhere, but it's enough for most cases i encountered.
Just sharing this outside of the comment section, in case if anyone ever finds it useful to have in their browser's config.