A great blog post on this by one of the creators of the format, Jon Sneyers:
https://cloudinary.com/blog/time_for_next_gen_codecs_to_deth...
I wouldn't be too surprised if every big CDN already does something like that internally as a competitive advantage, and if so then the incentives switch around and there's no benefit publically switching to JPEG XL - when everyone has it, there's no easy way to do better than everyone else.
For Firefox you need the nightlies I believe - I had no luck with release Firefox 93.
In Yinglish (Yiddish-English) jizzle would mean something like 'a drop of seamen'.
At least, JPEG-XL offers a solution on this side, like store as JPEG-XL and serve as old JPEG to IE11 users. Now, if only there was a JPEG-XL implementation for Java...