From the article: "...replacing JPEG with something better has been a frequent topic of discussion. The major downside to moving away from JPEG is that it would require going through a multi-year period of relatively poor compatibility with the world’s deployed software. We (at Mozilla) don’t doubt that algorithmic improvements will make this worthwhile at some point, possibly soon. Even after a transition begins in earnest though, JPEG will continue to be used widely."

This is Mozilla's roundabout way of saying that they want to put off starting on WebP or JPEG 2000 as long as possible.