I think it's a great opportunity for Firefox to stand out by supporting JPEG XL before any other browser.
Imagine images served by Cloudflare and Cloudinary load faster and look better only with Firefox. Firefox was a pioneer of web technologies and it should win the title back, if Firefox just keep following Chrome without any differentiation, why would people choose Firefox?
If the decoder memory safety is a concern, maybe Mozilla can start a crowd funding campaign to sponsor a Rust decoder, even the campaign itself will attract reports and attentions for Firefox.
Mozilla argued AVIF was already supported as a same generation but clearly JPEG XL has many advantages: