When Firefox 65 introduced the WebP image support,
image/webp was added to the HTTP
Accept request header for both default navigation (normal HTML page load) and images. Then, Firefox 66 removed it from the default
Accept header, because the value didn’t align with the Fetch spec.
While the spec hasn’t changed yet, given that Google Chrome continues sending
image/webp and various sites are using it for server-side WebP support detection, Firefox 72 has added
image/webp again to the default
Accept header. The new value will look like this: