Unless something changed recently, Firefox never dropped support. It's always been behind a feature flag though. Google gave some reasons on their issue tracker.

https://bugs.chromium.org/p/chromium/issues/detail?id=117805...

For Google, focusing on one next gen image codec is the basic reason.

A perfectly reasonable decision if made by a committee of disinterested parties deciding the future of the web for the benefit of humanity, but a bad look when pulled by a megacorp.

For Firefox, it's about hoping to get to that latter situation. They want the web to move forward in step as much as possible rather than it being some wild west where everyone just does their own thing. They've lived through that already. And it sucked. Hence the various standardisation and community interoperability measures which have radically improved the web platform over the last decade or so.

Might have to do with https://flak.tedunangst.com/post/on-building-jpeg-xl-for-was... libjxl being a big C++ pain to build and maybe needing some serious audit/fuzzing. Still, AVIF isn't that much better except libavif being C (libheif is C++ though, and it's probably what's in use).

From what I remember, Firefox was basically neutral on it, and didn't want to extend resources to it if it wasn't adopted at wide scale. Chrome basically said that there wasn't much interest so they weren't going to put resources towards it.

Firefox still supports it - there are even pending patches for animation support. Unless it has recently changed, it’s only available on nightly builds + a preference toggle.

I pulled all the patches in to Waterfox and have had full support for JPEG XL for nearly a year now.