Unfortunately, this add-ons is only effective on sites that pay attention to the "Accept" header. Other users have identified certain sites that ignore the change made by this add-on, such as NexusMods and PlayStation. These sites either no longer support legacy browsers, or require a legacy user agent (such as Internet Explorer 11) before sending legacy media formats. For cases like that, you could consider using a converter. I have one here on the Add-ons site called "Save webP as PNG or JPEG."