In previous versions, MPlayerX could play the videos in Youtube.com directly.
However, you have to copy the URL → open MPlayerX → open URL panel → paste the URL, kind of annoying and even stop myself using this feature.
Here is how to:
1. Drag the link directly
2. Open the URL in Services menu
In case you don’t see “MPlayerX: Open URL” in the contextual menu, please go to System Preference to enable it.
Note: if there is no entry in System Preference, you could log off and on once. Sometimes OS needs to re-login to find the necessary info from application’s package.