Hi pjcamp, thank you for your report.

This extension makes a change to how Firefox requests images, but many sites ignore the change. If there is a specific site you want to mention, you can contact me by creating a new Github issue on https://github.com/jscher2000/dont-accept-webp/issues or emailing jscher2000@outlook.com.

On the other hand, if it fails on every single site, check for this issue: https://support.mozilla.org/kb/extensions-private-browsing

There also are some pages that give Firefox multiple format options (picture tag/source tags) and it selects WebP for some reason. I don't have a fix for that yet.