Save WebP as PNG or JPEG for Firefox
You may have come across a "new" image format known as WebP. It has been around for about a decade, and was created by Google. Today it is used by many websites.
The annoyance you will discover (if you do not already know) in the form WebP is that it is not supported by many image editors. So when you download an image WebP, you will probably not be able to edit it. Of course you can not save it in a different format directly.
You can select the format you want to save the image to (PNG, JPG, GIF) but also the quality (100/92/85/80/75%) of the JPG format.