> The addon does something, but it's not resulting in refusal of webp images. ...
> Furthermore, websites such as Youtube (Google in general) just plain refuse to offer anything but webp, even if Firefox does not indicate support for it in the "Accept" header.

Hi Bjorn, the purpose of this extension is to discourage server farms from re-compressing PNG and JPEG images as WebP images on the fly to save bandwidth, since this complicates saving in the original formats. It is not a goal of this extension to prevent sites designed to serve WebP images from doing so.

> You may need to fool these websites in assuming you're using an old browser by changing or emptying the "general.useragent.override" key.

That might work, but some sites might serve less useful pages, so you'll have to test and see what is the best compromise.