The jxl-oxide dev is a jxl-rs dev. jxl-oxide is decode only while jxl-rs is a full encode/decode library.

zune also uses jxl-oxide for decode. zune has an encoder and they are doing great work but their encoder is not threading safe so it's not viable for Mozilla's need.

And there's work already being done for properly integrating jxl implementations with firefox but frankly things take time.

If you are seriously passionate about seeing JPEG-XL in firefox there's a really easy solution. Contribute. More engineering hours put towards a FOSS project tends to see it come to fruition faster.