The biggest new feature in Firefox 86 is the enablement of AV1 Image File Format (AVIF) support by default. AVIF is a powerful, royalty-free and open-source image file format designed to encode AV1 bitstreams in the HEIF (High Efficiency Image File Format) container.
In Firefox 86, only basic support is offered, which means that features like animated images and colorspace support aren't supported. AVIF support was available in previous Firefox release, but not enabled by default. Therefore, you could enable it by setting the image.avif.enable option in about:config to true.