Did they actually say that? All the statements i've seen them have been much more guarded and vauge. More of a, maybe we will think about it if that happens.
That's what they said a year ago. And a couple of Mozilla devs have been in regular contact with the JXL devs ever since then, helping with the integration. The patches to use jxl-rs with Firefox already exist, and will be merged as soon as a couple of prerequisite issues in Gecko are fixed.
They're looking at the same signals as Chrome of a format that's actually getting use, has a memory safe implementation, and that will stick around for decades to justify adding it to the web platform, all of which seem more and more positive since 2022.