You may have encountered a "new" image format known as WebP. It's been around for about a decade, and was created by Google. Today it is used by many websites.
The annoying thing you will find (if you do not already know) in WebP format is that it is not supported by many image editors. So when you download a WebP image, you probably won't be able to edit it. Of course you can not save it in a different format directly.
Recently, an add-on called Save WebP as PNG or JPEG (Converter) was released. The extension is very useful because it converts and saves WebP images to some editable format and resolution.
Once installed, you can find it in Firefox by right-clicking on a WebP image as shown below
Select Save WebP as or Save WebP as and many options will appear to the left of the image.
You can choose the format you want to save the image, as well as (PNG, JPG, GIF) but also the quality (100/92/85/80/75%) of the JPG format.