Previously it could only be enabled in nightly, but today in Librewolf 119.0-5 (based on firefox 119 stable) I found that you could also go to about:config and set image.jxl.enabled to true and enable support for jpeg-xl. Is it only a librewolf thing? I am asking here cause I don’t have time to personally test.
The option has been there for awhile now but it has no functionality
It actually works for me on 119 (except animations) - https://sh.itjust.works/comment/4930703