Safari/Webkit has added JPEG XL support already.

Firefox is "neutral", which I understand as meaning they'll do whatever Chrome does.

All the code has been written, patches to add JPEG XL support to Firefox and Chromium are available and some of the forks (Waterfox, Pale Moon, Thorium, Cromite) do have JPEG XL support.

I believe they were referring to that WebSocket issue, not JXL.