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."

Yes, the NexusMods site ignores the change made by this add-on. But for me, the .png file saved from Chrome is the same WEBP file -- with a .png extension.

If you hear of any way to fool the site, I can try to implement it. Until then, when you are viewing the image in its own tab, you can get Firefox to download it as a PNG by using the built-in screenshot tool. Either right-click the blank area above/below the image > Take Screenshot, or use the keyboard shortcut (on Windows, Ctrl+Shift+S). Then move the mouse toward the upper left of the image and a dashed rectangle should appear around the image. Click to select the image, then click the Download button. The file name is a mess, but it will be a true PNG file.