Steps to reproduce:
- Install JW Player Macro using the Extension Manager
- Go to http://localhost:8080/xwiki/bin/view/Macros/JWPlayer
- Click Play for the second video, which is using a Youtube URL
Tested on IE11, Edge 38, Chrome 57 and Firefox 52.
Youtube videos work only on Firefox, but the display for the JW Player is also different on this browser.