Works with Waterfox on macOS but curiously not Firefox. I wonder if their search deal with Google included keeping the image.jxl.enabled setting off.

That’s an interesting speculation, but I’m inclined to believe their official reasoning. (That being they just didn’t really care about the format and/or went with whatever Chrome said at first. A year or so later they changed their mind and said they wanted an implementation in a memory-safe language, which prompted the JXL team to work on it.)

Works on Zen as well.