What this add-on does is remove a signal that Firefox normally sends to the server that your browser can handle WebP images. The idea is to discourage the server from sending WebP in place of JPEG or PNG.

I'm not sure what problem you ran into with saving an image not working, but do you mean you still got a WebP image? That definitely can happen when the site insists on sending you one, or when the page is using a "picture" tag, but this add-on should help on most sites.

If you need an add-on to convert a WebP image to something else, check out https://addons.mozilla.org/firefox/addon/save-webp-as-png-or-jpeg/