Firefox actually silently and automatically converts images to webp in some cases when you Save As them, with no option to revert to the format you see in the DOM. It’s fucking infuriating. I hate webp because the support is so spotty.

I highly doubt that is what's happening. More likely the server is serving a webp under a .png filename or something.

I don't think that's firefox doing it? I'd guess it's some CDNs doing the conversion. Or the URL says .jpg but when you look at the mimetype sent by the server it's actually webp. Extentions are meaningless for browsers.

It's not Firefox, it's the server. Edit and resend the request with header "Accept: image/jpeg" only and you'll most likely get a JPEG file.