Phew for macOS is an open-source FLIF image viewer.
FLIF, or Free Lossless Image Format, is a new lossless image format that surpasses PNG, lossless WebP and lossless BPG in terms of compression ratio.
Phew can open FLIF images and export them to PNG or TIFF. FLIF images can be viewed in the Finder and Spotlight with thumbnails and previews if QuickLook Plug-In has been installed. Animated FLIF images cannot be supported at the moment.