Quote:Firefox 65 will support the WebP image format that Google created for use on the Internet. Firefox 65 Stable will be released in January 2019.
Google states on the official WebP page on the company's Developers website that "WebP lossless images are 26% smaller in size compared to PNGs", and that "WebP lossy images are 25-34% smaller than comparable JPEG images".

Mozilla revealed plans in 2016 to add WebP support to the organization's Firefox web browser but nothing came out of it in the two years that followed. Microsoft introduced support for WebP in Microsoft Edge recently.

Chrome and Chromium-based browsers such as Opera or Vivaldi support the image format already.

Web servers that support WebP check whether the browser is capable of displaying the format. If that is the case, WebP images are provided and if that is not the case, fallback image formats, e.g. PNG or JPG are provided instead.

Firefox users can test this on Google Play where Chrome gets WebP images currently while current versions of Firefox traditional image formats.

Full reading: https://www.ghacks.net/2018/11/02/firefox-65-supports-googles-webp-image-format/