What method were you using? Was it through an add-on, or a web-based service? Firefox's Screenshot feature saves in PNG format by default, unless the image is very large, in which case it might switch to JPG. See: [[Take screenshots in Firefox]] ''(if you hover the image and click it, Firefox usually will select it)''. If you don't find any other ideas, you can use my add-on to save any image as a PNG: [https://addons.mozilla.org/firefox/addon/save-webp-as-png-or-jpeg/ Save webP as PNG or JPEG (Converter)]