Probably you've seen examples of ES6 which is actively developed now, and many features from it are already available in firefox nightlies. ES4 was abandoned 3-

Probably you've seen examples of ES6 which is actively developed now, and many features from it are already available in firefox nightlies. ES4 was abandoned 3-4 years ago. It was partially implemented in firefox, and ff addons used it. One of it's features that didn't made into ES6 was inline xml inside javascript. It was recently removed from firefox.